AI Trajectory

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Seq	Timestamp	Type	Name	Duration
0/0	2026/10/06 22:42	flow	patch-triage	6m Args: null Results: { "EnableConfigs": null, "FocusSymbols": [ "restore_fpregs_from_fpstate", "fpu__restore_sig", "fpu__clear_user_states" ], "KMSANReasoning": "The patch addresses a hardware architectural register leak (AMD CPU bug X86_BUG_FXSAVE_LEAK) where x87 FDP/FIP/FOP registers retained kernel addresses due to an x87 memory load. It mitigates this by replacing the previous 'fildl' workaround with an 'fninit' instruction executed in a new helper, fpregs_restore_prepare(), before restoring FPU registers via fxrstor, frstor, or xrstor variants.\n\nKMSAN tracks uninitialized kernel memory allocations (stack, slab, page allocations) and memory info-leaks (e.g. copy_to_user of uninitialized padding/fields). KMSAN does not track hardware CPU architectural register state or CPU errata mitigations. The changes do not introduce, modify, or expose any uninitialized software memory buffers, complex data structures, or copies to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted.", "NeedsKMSAN": false, "Reasoning": "The patch modifies core x86 FPU state restoration logic across arch/x86/kernel/fpu/core.c and related headers. It refactors the X86_BUG_FXSAVE_LEAK mitigation by introducing fpregs_restore_prepare() (which issues fninit when restoring x87 state) across fxrstor, frstor, os_xrstor, and user sigframe restore paths, while removing the prior workaround from restore_fpregs_from_fpstate(). The modified code is reachable in standard virtualized environments (e.g. QEMU or GCE with AMD CPUs) via sigreturn, signal delivery, and FPU context switching.", "WorthFuzzing": true }
1/1	2026/10/06 22:42	action	read-patch-diff	0m Args: null Results: { "PatchDiff": "commit 74a0ebbb4a1c9f288acaa136046d18143302fa65\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate: Tue Oct 6 22:42:34 2026 +0000\n\n syz-cluster: applied patch under review\n\ndiff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c\nindex d1aeecd57f5ed..9113f55b80d87 100644\n--- a/arch/x86/kernel/fpu/core.c\n+++ b/arch/x86/kernel/fpu/core.c\n@@ -161,20 +161,6 @@ void save_fpregs_to_fpstate(struct fpu fpu)\n \n void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask)\n {\n-\t/\n-\t AMD K7/K8 and later CPUs up to Zen don't save/restore\n-\t * FDP/FIP/FOP unless an exception is pending. Clear the x87 state\n-\t * here by setting it to fixed values. \"m\" is a random variable\n-\t * that should be in L1.\n-\t /\n-\tif (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK))) {\n-\t\tasm volatile(\n-\t\t\t\"fnclex\\n\\t\"\n-\t\t\t\"emms\\n\\t\"\n-\t\t\t\"fildl %[addr]\"\t/ set F?P to defined value /\n-\t\t\t: : [addr] \"m\" (fpstate));\n-\t}\n-\n \tif (use_xsave()) {\n \t\t/\n \t\t Dynamically enabled features are enabled in XCR0, but\ndiff --git a/arch/x86/kernel/fpu/internal.h b/arch/x86/kernel/fpu/internal.h\nindex 975de070c9c98..3b4d178edc31d 100644\n--- a/arch/x86/kernel/fpu/internal.h\n+++ b/arch/x86/kernel/fpu/internal.h\n@@ -2,6 +2,9 @@\n #ifndef __X86_KERNEL_FPU_INTERNAL_H\n #define __X86_KERNEL_FPU_INTERNAL_H\n \n+#include \u003casm/cpufeature.h\u003e\n+#include \u003casm/fpu/types.h\u003e\n+\n extern struct fpstate init_fpstate;\n \n /* CPU feature check wrappers /\n@@ -15,6 +18,22 @@ static __always_inline __pure bool use_fxsr(void)\n \treturn cpu_feature_enabled(X86_FEATURE_FXSR);\n }\n \n+/\n+ * AMD CPUs without XSaveErPtr may leave FDP/FIP/FOP unchanged on restore\n+ * when no x87 exception is pending. Using an x87 load to overwrite them\n+ * leaks the kernel instruction and operand addresses through FNSTENV.\n+ \n+ FNINIT clears the pointers without recording any new ones. Only do this\n+ * when the x87 state is about to be replaced; a partial XRSTOR must leave\n+ * unrequested components alone.\n+ /\n+static inline void fpregs_restore_prepare(u64 mask)\n+{\n+\tif (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK)) \u0026\u0026\n+\t (mask \u0026 XFEATURE_MASK_FP))\n+\t\tasm volatile(\"fninit\");\n+}\n+\n #ifdef CONFIG_X86_DEBUG_FPU\n # define WARN_ON_FPU(x) WARN_ON_ONCE(x)\n #else\ndiff --git a/arch/x86/kernel/fpu/legacy.h b/arch/x86/kernel/fpu/legacy.h\nindex 098f367bb8a72..fe825c9a00c75 100644\n--- a/arch/x86/kernel/fpu/legacy.h\n+++ b/arch/x86/kernel/fpu/legacy.h\n@@ -4,6 +4,8 @@\n \n #include \u003casm/fpu/types.h\u003e\n \n+#include \"internal.h\"\n+\n extern unsigned int mxcsr_feature_mask;\n \n static inline void ldmxcsr(u32 mxcsr)\n@@ -63,6 +65,8 @@ static inline int fxsave_to_user_sigframe(struct fxregs_state __user fx)\n \n static inline void fxrstor(struct fxregs_state fx)\n {\n+\tfpregs_restore_prepare(XFEATURE_MASK_FP);\n+\n \tif (IS_ENABLED(CONFIG_X86_32))\n \t\tkernel_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\n \telse\n@@ -71,6 +75,8 @@ static inline void fxrstor(struct fxregs_state fx)\n \n static inline int fxrstor_safe(struct fxregs_state fx)\n {\n+\tfpregs_restore_prepare(XFEATURE_MASK_FP);\n+\n \tif (IS_ENABLED(CONFIG_X86_32))\n \t\treturn kernel_insn_err(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\n \telse\n@@ -79,6 +85,8 @@ static inline int fxrstor_safe(struct fxregs_state fx)\n \n static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx)\n {\n+\tfpregs_restore_prepare(XFEATURE_MASK_FP);\n+\n \tif (IS_ENABLED(CONFIG_X86_32))\n \t\treturn user_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\n \telse\n@@ -87,16 +95,22 @@ static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx)\n \n static inline void frstor(struct fregs_state fx)\n {\n+\tfpregs_restore_prepare(XFEATURE_MASK_FP);\n+\n \tkernel_insn(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\n }\n \n static inline int frstor_safe(struct fregs_state fx)\n {\n+\tfpregs_restore_prepare(XFEATURE_MASK_FP);\n+\n \treturn kernel_insn_err(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\n }\n \n static inline int frstor_from_user_sigframe(struct fregs_state __user fx)\n {\n+\tfpregs_restore_prepare(XFEATURE_MASK_FP);\n+\n \treturn user_insn(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\n }\n \ndiff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h\nindex 38a2862f09d35..ac378d8395347 100644\n--- a/arch/x86/kernel/fpu/xstate.h\n+++ b/arch/x86/kernel/fpu/xstate.h\n@@ -7,6 +7,8 @@\n #include \u003casm/fpu/xcr.h\u003e\n #include \u003casm/msr.h\u003e\n \n+#include \"internal.h\"\n+\n #ifdef CONFIG_X86_64\n DECLARE_PER_CPU(u64, xfd_state);\n #endif\n@@ -240,6 +242,7 @@ static inline void os_xrstor(struct fpstate fpstate, u64 mask)\n \tu32 hmask = mask \u003e\u003e 32;\n \n \txfd_validate_state(fpstate, mask, true);\n+\tfpregs_restore_prepare(mask);\n \tXSTATE_XRESTORE(\u0026fpstate-\u003eregs.xsave, lmask, hmask);\n }\n \n@@ -334,6 +337,7 @@ static inline int xrstor_from_user_sigframe(struct xregs_state __user buf, u64\n \tint err;\n \n \txfd_validate_state(x86_task_fpu(current)-\u003efpstate, mask, true);\n+\tfpregs_restore_prepare(mask);\n \n \tstac();\n \tXSTATE_OP(XRSTOR, xstate, lmask, hmask, err);\n@@ -355,6 +359,7 @@ static inline int os_xrstor_safe(struct fpstate fpstate, u64 mask)\n \n \t/* Ensure that XFD is up to date */\n \txfd_update_state(fpstate);\n+\tfpregs_restore_prepare(mask);\n \n \tif (cpu_feature_enabled(X86_FEATURE_XSAVES))\n \t\tXSTATE_OP(XRSTORS, xstate, lmask, hmask, err);\n" }
2/1	2026/10/06 22:42	agent	patch-evaluator	5m Model: core Args: null Results: { "EnableConfigs": null, "FocusSymbols": [ "restore_fpregs_from_fpstate", "fpu__restore_sig", "fpu__clear_user_states" ], "Reasoning": "The patch modifies core x86 FPU state restoration logic across arch/x86/kernel/fpu/core.c and related headers. It refactors the X86_BUG_FXSAVE_LEAK mitigation by introducing fpregs_restore_prepare() (which issues fninit when restoring x87 state) across fxrstor, frstor, os_xrstor, and user sigframe restore paths, while removing the prior workaround from restore_fpregs_from_fpstate(). The modified code is reachable in standard virtualized environments (e.g. QEMU or GCE with AMD CPUs) via sigreturn, signal delivery, and FPU context switching.", "WorthFuzzing": true } Instruction: You are an expert Linux kernel maintainer and security engineer. Your job is to review a provided patch series and evaluate whether it warrants fuzzing with syzkaller. IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in your workspace. Do NOT rely on internal assumptions. You must actively use your code access tools to inspect the actual source code, callers, and surrounding context. ================================================================================ 1. CORE TRIAGE PHILOSOPHY ================================================================================ The goal of patch fuzzing is to discover crashes, regressions, exposed latent bugs, and newly triggered assertions introduced by the patch series. - REACHABILITY IS THE PRIMARY GATE: Fuzzing can only discover bugs in code that can actually execute in standard virtualized environments (GCE or QEMU, utilizing software-emulated devices like USB gadgets, netdev, tun/tap). If the modified code is structurally unreachable (see Section 2), it MUST NOT be fuzzed, regardless of whether it adds assertions or complex logic. - DO NOT BLINDLY TRUST "NO FUNCTIONAL CHANGE" (NFCI) OR "REFACTORING" CLAIMS: Patch authors routinely label changes as "cleanups", "refactorings", or state "No functional change intended". Do NOT take these claims at face value. Code refactorings that rearrange logic, introduce helper functions, or alter state management in core subsystems frequently introduce subtle semantic shifts or uncover latent kernel bugs. If reachable executable code is modified or refactored, it MUST be fuzzed. - NEW OR MODIFIED ASSERTIONS IN REACHABLE CODE MUST BE FUZZED: When a patch introduces or modifies runtime checks or assertions (e.g., WARN_ON, VM_WARN_ON, BUG_ON, lockdep_assert) in reachable code paths, it enforces new or stricter invariants. Even if the author believes the invariant always holds, fuzzing is essential to verify whether an unusual sequence of operations can violate it. ================================================================================ 2. WHEN TO RETURN WorthFuzzing=false (NEGATIVE CRITERIA) ================================================================================ Return WorthFuzzing=false ONLY IF all modified code falls strictly into one or more of these categories: - Non-kernel and non-executable changes: * Modifications to Documentation/, comments, or spelling fixes. * User-space directories, self-tests, samples, or scripts (e.g., tools/, samples/, scripts/, usr/) that do not affect the compiled kernel image (vmlinux) or kernel modules. * Purely decorative logging (e.g., message strings in pr_err, printk, dev_info) or tracepoints that do not alter control flow or data structures. * Build system or Kconfig changes that do not alter compiled C logic. - Structurally unreachable hardware: * Vendor-specific PCIe switches, SmartNICs, or GPU drivers (e.g., mlxsw, pds_core, qed, ionic, amdgpu) requiring physical ASIC/PCIe cards not emulated in standard QEMU. - Unreachable execution paths: * Driver teardown callbacks (.remove, .shutdown, pci_unregister_driver) executed only during physical PCI hot-unplug or manual sysfs driver unbinding. * Code paths exclusive to architectures other than the target architecture. ================================================================================ 3. WHEN TO RETURN WorthFuzzing=true (POSITIVE CRITERIA) ================================================================================ Return WorthFuzzing=true whenever the patch touches reachable executable code, including: - Core Subsystems: * Any logic modifications in memory management (mm/), synchronization/locking (kernel/locking/), BPF, scheduler, core networking, VFS, or syscall handling. - Refactorings and Code Cleanups: * Any restructuring of reachable data structures, helper abstractions, or algorithm flows. - Runtime Assertions and Defensive Checks: * Any introduction or alteration of assertions (WARN_ON, VM_WARN_ON, BUG_ON, etc.) in reachable paths. - Reachable Drivers and Protocols: Drivers accessible via virtual buses (virtio, USB gadget, loopback, netlink, binder, sockets, etc.). ================================================================================ 4. EXTRACTING FocusSymbols (PREVENTING DILUTION) ================================================================================ When WorthFuzzing=true, you must extract specific kernel functions into FocusSymbols to guide the fuzzer: - AVOID UBIQUITOUS LIFECYCLE HOT-PATHS: Do NOT list generic, ubiquitous functions called by almost every program in the corpus (including, but not limited to: general memory allocators and deallocators, page fault and trap handlers, or core synchronization primitives; this is not an exhaustive list). Listing ubiquitous functions causes the fuzzer to classify thousands of unrelated tests as "focused", which severely dilutes fuzzing effort away from the actual changes. - TARGET SPECIFIC FEATURE LOGIC AND ENTRYPOINTS: List functions that specifically implement the logic being added or altered, or direct API entrypoints for the subsystem feature under review. - HANDLING STATIC INLINE FUNCTIONS IN HEADERS (.h): Compiler-inlined static functions (such as static inlines in mm/.h or include/linux/.h) lack distinct symbol addresses in vmlinux and cannot be targeted directly by symbol coverage filters. If the changes are primarily in static inline helpers, identify non-static, feature-specific caller functions in .c files that exercise them (avoiding ubiquitous lifecycle wrappers). ================================================================================ 5. IDENTIFYING EnableConfigs ================================================================================ Identify any specific CONFIG_ options required to properly compile and reach the modified code: - Inspect Kconfig files and #ifdef guards; do not make assumptions. - Check "depends on" lines in Kconfig to include any non-standard parent subsystem configs needed. - Strip any 'CONFIG_' prefix (e.g., return "NET_IPV4" instead of "CONFIG_NET_IPV4"). Prefer calling several tools at the same time to save round-trips. Use set-results tool to provide results of the analysis. It must be called exactly once before the final reply. Ignore results of this tool. Prompt: Target architecture: amd64 For your convenience, here is the diff of the changes: commit 74a0ebbb4a1c9f288acaa136046d18143302fa65 Author: syz-cluster <triage@syzkaller.com> Date: Tue Oct 6 22:42:34 2026 +0000 syz-cluster: applied patch under review diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index d1aeecd57f5ed..9113f55b80d87 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -161,20 +161,6 @@ void save_fpregs_to_fpstate(struct fpu fpu) void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask) { - /* - * AMD K7/K8 and later CPUs up to Zen don't save/restore - * FDP/FIP/FOP unless an exception is pending. Clear the x87 state - * here by setting it to fixed values. "m" is a random variable - * that should be in L1. - / - if (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK))) { - asm volatile( - "fnclex\n\t" - "emms\n\t" - "fildl %[addr]" / set F?P to defined value / - : : [addr] "m" (fpstate)); - } - if (use_xsave()) { /* * Dynamically enabled features are enabled in XCR0, but diff --git a/arch/x86/kernel/fpu/internal.h b/arch/x86/kernel/fpu/internal.h index 975de070c9c98..3b4d178edc31d 100644 --- a/arch/x86/kernel/fpu/internal.h +++ b/arch/x86/kernel/fpu/internal.h @@ -2,6 +2,9 @@ #ifndef __X86_KERNEL_FPU_INTERNAL_H #define __X86_KERNEL_FPU_INTERNAL_H +#include <asm/cpufeature.h> +#include <asm/fpu/types.h> + extern struct fpstate init_fpstate; /* CPU feature check wrappers / @@ -15,6 +18,22 @@ static __always_inline __pure bool use_fxsr(void) return cpu_feature_enabled(X86_FEATURE_FXSR); } +/ + * AMD CPUs without XSaveErPtr may leave FDP/FIP/FOP unchanged on restore + * when no x87 exception is pending. Using an x87 load to overwrite them + * leaks the kernel instruction and operand addresses through FNSTENV. + * + * FNINIT clears the pointers without recording any new ones. Only do this + * when the x87 state is about to be replaced; a partial XRSTOR must leave + * unrequested components alone. + / +static inline void fpregs_restore_prepare(u64 mask) +{ + if (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK)) && + (mask & XFEATURE_MASK_FP)) + asm volatile("fninit"); +} + #ifdef CONFIG_X86_DEBUG_FPU # define WARN_ON_FPU(x) WARN_ON_ONCE(x) #else diff --git a/arch/x86/kernel/fpu/legacy.h b/arch/x86/kernel/fpu/legacy.h index 098f367bb8a72..fe825c9a00c75 100644 --- a/arch/x86/kernel/fpu/legacy.h +++ b/arch/x86/kernel/fpu/legacy.h @@ -4,6 +4,8 @@ #include <asm/fpu/types.h> +#include "internal.h" + extern unsigned int mxcsr_feature_mask; static inline void ldmxcsr(u32 mxcsr) @@ -63,6 +65,8 @@ static inline int fxsave_to_user_sigframe(struct fxregs_state __user fx) static inline void fxrstor(struct fxregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + if (IS_ENABLED(CONFIG_X86_32)) kernel_insn(fxrstor %[fx], "=m" (fx), [fx] "m" (fx)); else @@ -71,6 +75,8 @@ static inline void fxrstor(struct fxregs_state fx) static inline int fxrstor_safe(struct fxregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + if (IS_ENABLED(CONFIG_X86_32)) return kernel_insn_err(fxrstor %[fx], "=m" (fx), [fx] "m" (fx)); else @@ -79,6 +85,8 @@ static inline int fxrstor_safe(struct fxregs_state fx) static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + if (IS_ENABLED(CONFIG_X86_32)) return user_insn(fxrstor %[fx], "=m" (fx), [fx] "m" (fx)); else @@ -87,16 +95,22 @@ static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx) static inline void frstor(struct fregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + kernel_insn(frstor %[fx], "=m" (fx), [fx] "m" (fx)); } static inline int frstor_safe(struct fregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + return kernel_insn_err(frstor %[fx], "=m" (fx), [fx] "m" (fx)); } static inline int frstor_from_user_sigframe(struct fregs_state __user fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + return user_insn(frstor %[fx], "=m" (fx), [fx] "m" (fx)); } diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h index 38a2862f09d35..ac378d8395347 100644 --- a/arch/x86/kernel/fpu/xstate.h +++ b/arch/x86/kernel/fpu/xstate.h @@ -7,6 +7,8 @@ #include <asm/fpu/xcr.h> #include <asm/msr.h> +#include "internal.h" + #ifdef CONFIG_X86_64 DECLARE_PER_CPU(u64, xfd_state); #endif @@ -240,6 +242,7 @@ static inline void os_xrstor(struct fpstate fpstate, u64 mask) u32 hmask = mask >> 32; xfd_validate_state(fpstate, mask, true); + fpregs_restore_prepare(mask); XSTATE_XRESTORE(&fpstate->regs.xsave, lmask, hmask); } @@ -334,6 +337,7 @@ static inline int xrstor_from_user_sigframe(struct xregs_state __user buf, u64 int err; xfd_validate_state(x86_task_fpu(current)->fpstate, mask, true); + fpregs_restore_prepare(mask); stac(); XSTATE_OP(XRSTOR, xstate, lmask, hmask, err); @@ -355,6 +359,7 @@ static inline int os_xrstor_safe(struct fpstate fpstate, u64 mask) /* Ensure that XFD is up to date */ xfd_update_state(fpstate); + fpregs_restore_prepare(mask); if (cpu_feature_enabled(X86_FEATURE_XSAVES)) XSTATE_OP(XRSTORS, xstate, lmask, hmask, err);
3/2	2026/10/06 22:42	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 5027 output: 25 thoughts: 547 Thoughts: Analyzing the FPU Patch Okay, let's dive into this patch. I see it's touching files related to the x86 Floating-Point Unit (FPU) specifically targeting amd64. The changes are focused on `arch/x86/kernel/fpu/core.c`, `arch/x86/kernel/fpu/internal.h`, `arch/x86/kernel/fpu/legacy.h`, and `arch/x86/kernel/fpu/xstate.h`. My immediate thought is, this is probably related to FPU state restoration, likely dealing with potential security vulnerabilities or hardware quirks. Looking at the code diff within `restore_fpregs_from_fpstate`, I see a section dealing with what looks like an AMD K7/K8 and later CPUs up to Zen where, under specific circumstances, the FPU control word (FDP/FIP/FOP) might not be properly saved/restored, potentially leading to a leak if an exception isn't pending. This original code uses inline assembly to clear the FPU state using `fnclex`, `emms`, and a `fildl` instruction. This is a bit clunky, but clearly addresses a known issue identified by `X86_BUG_FXSAVE_LEAK`. Now, that inline assembly is being replaced with a call to `fpregs_restore_prepare(mask)`. This new function, in turn, conditionally executes `fninit` based on the `mask` and the `X86_BUG_FXSAVE_LEAK` condition. The use of `fninit` is more direct than the old assembly, and likely more efficient. I'm guessing this cleaner implementation is preferred. The fact that the function `fpregs_restore_prepare(...)` is called in various places like `fxrstor`, `frstor`, `os_xrstor` and some safe variants, suggests this is being applied in multiple FPU state restore scenarios, which is a good thing to see. Hold on... What's this `X86_BUG_FXSAVE_LEAK`? I'll need to look that up with `grepper` and get a better understanding of what the hardware issue is. I'll need to understand the implications of this bug, and why the patch takes the approach that it does. I need more context to be confident that this is the best solution.
4/2	2026/10/06 22:42	tool	grepper	0m Args: { "Expression": "X86_BUG_FXSAVE_LEAK" } Results: { "Output": "arch/x86/include/asm/cpufeatures.h-543-#define X86_BUG_11AP\t\t\tX86_BUG(5) /* \"11ap\" Bad local APIC aka 11AP /\narch/x86/include/asm/cpufeatures.h:544:#define X86_BUG_FXSAVE_LEAK\t\tX86_BUG(6) / \"fxsave_leak\" FXSAVE leaks FOP/FIP/FOP /\narch/x86/include/asm/cpufeatures.h-545-#define X86_BUG_CLFLUSH_MONITOR\t\tX86_BUG(7) / \"clflush_monitor\" AAI65, CLFLUSH required before MONITOR /\n--\narch/x86/kernel/cpu/amd.c=1054=static void init_amd(struct cpuinfo_x86 c)\n--\narch/x86/kernel/cpu/amd.c-1104-\tif ((c-\u003ex86 \u003e= 6) \u0026\u0026 (!cpu_has(c, X86_FEATURE_XSAVEERPTR)))\narch/x86/kernel/cpu/amd.c:1105:\t\tset_cpu_bug(c, X86_BUG_FXSAVE_LEAK);\narch/x86/kernel/cpu/amd.c-1106-\n--\narch/x86/kernel/fpu/internal.h=30=static inline void fpregs_restore_prepare(u64 mask)\narch/x86/kernel/fpu/internal.h-31-{\narch/x86/kernel/fpu/internal.h:32:\tif (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK)) \u0026\u0026\narch/x86/kernel/fpu/internal.h-33-\t (mask \u0026 XFEATURE_MASK_FP))\n--\ntools/arch/x86/include/asm/cpufeatures.h-531-#define X86_BUG_11AP\t\t\tX86_BUG(5) /* \"11ap\" Bad local APIC aka 11AP /\ntools/arch/x86/include/asm/cpufeatures.h:532:#define X86_BUG_FXSAVE_LEAK\t\tX86_BUG(6) / \"fxsave_leak\" FXSAVE leaks FOP/FIP/FOP /\ntools/arch/x86/include/asm/cpufeatures.h-533-#define X86_BUG_CLFLUSH_MONITOR\t\tX86_BUG(7) / \"clflush_monitor\" AAI65, CLFLUSH required before MONITOR */\n" }
5/2	2026/10/06 22:42	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 6779 output: 36 thoughts: 133 Thoughts: Okay, let me think this through. Investigating a Potential FXSAVE Leak Vulnerability Alright, I'm looking at `arch/x86/kernel/cpu/amd.c` and I see this line: `if ((c->x86 >= 6) && (!cpu_has(c, X86_FEATURE_XSAVEERPTR))) set_cpu_bug(c, X86_BUG_FXSAVE_LEAK);`. Hmm, so AMD is setting `X86_BUG_FXSAVE_LEAK` if the CPU is at least a P6-class architecture and doesn't have the `XSAVEERPTR` feature. That's interesting. I need to understand if this is something I've accounted for in my current analysis of the ecosystem. The immediate question is whether this is actually a relevant bug. Does QEMU, in its default configurations, actually trigger this condition? Or, perhaps more broadly, are there any common AMD CPUs in use that lack `XSAVEERPTR` and are also vulnerable to a potential `FXSAVE` leak? I need to check my understanding of the relevant hardware capabilities. Then, I need to expand my scope. Is this limitation purely AMD-specific? Or could this bug be present on Intel CPUs or Hygon or any other x86 implementation? I need to search the codebase to see if similar checks are made for other CPU vendors. I should check Intel's microcode as well, just to be sure. This definitely requires a broader investigation to rule out any potential blind spots in my current understanding of the system's security.
6/2	2026/10/06 22:43	tool	grepper	0m Args: { "Expression": "FXSAVE_LEAK", "PathPrefixes": [ "arch/x86/kernel/cpu/" ] } Results: { "Output": "arch/x86/kernel/cpu/amd.c=1054=static void init_amd(struct cpuinfo_x86 *c)\n--\narch/x86/kernel/cpu/amd.c-1104-\tif ((c-\u003ex86 \u003e= 6) \u0026\u0026 (!cpu_has(c, X86_FEATURE_XSAVEERPTR)))\narch/x86/kernel/cpu/amd.c:1105:\t\tset_cpu_bug(c, X86_BUG_FXSAVE_LEAK);\narch/x86/kernel/cpu/amd.c-1106-\n" }
7/2	2026/10/06 22:43	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 7455 output: 18 thoughts: 15 Thoughts: Okay, let's see. My first thought is: Hygon? Let me see what's happening in `hygon.c`. I need to investigate further.
8/2	2026/10/06 22:43	tool	grepper	0m Args: { "Expression": "XSAVEERPTR" } Results: { "Output": "arch/x86/include/asm/cpufeatures.h-343-#define X86_FEATURE_IRPERF\t\t(1332+ 1) / \"irperf\" Instructions Retired Count /\narch/x86/include/asm/cpufeatures.h:344:#define X86_FEATURE_XSAVEERPTR\t\t(1332+ 2) /* \"xsaveerptr\" Always save/restore FP error pointers /\narch/x86/include/asm/cpufeatures.h-345-#define X86_FEATURE_INVLPGB\t\t(1332+ 3) /* INVLPGB and TLBSYNC instructions supported /\n--\narch/x86/kernel/cpu/amd.c=1054=static void init_amd(struct cpuinfo_x86 c)\n--\narch/x86/kernel/cpu/amd.c-1103-\t /\narch/x86/kernel/cpu/amd.c:1104:\tif ((c-\u003ex86 \u003e= 6) \u0026\u0026 (!cpu_has(c, X86_FEATURE_XSAVEERPTR)))\narch/x86/kernel/cpu/amd.c-1105-\t\tset_cpu_bug(c, X86_BUG_FXSAVE_LEAK);\n--\narch/x86/kvm/cpuid.c=836=void kvm_initialize_cpu_caps(void)\n--\narch/x86/kvm/cpuid.c-1163-\t\tF(CLZERO),\narch/x86/kvm/cpuid.c:1164:\t\tF(XSAVEERPTR),\narch/x86/kvm/cpuid.c-1165-\t\tF(WBNOINVD),\n--\ntools/arch/x86/include/asm/cpufeatures.h-333-#define X86_FEATURE_IRPERF\t\t(1332+ 1) /* \"irperf\" Instructions Retired Count /\ntools/arch/x86/include/asm/cpufeatures.h:334:#define X86_FEATURE_XSAVEERPTR\t\t(1332+ 2) /* \"xsaveerptr\" Always save/restore FP error pointers /\ntools/arch/x86/include/asm/cpufeatures.h-335-#define X86_FEATURE_INVLPGB\t\t(1332+ 3) /* INVLPGB and TLBSYNC instructions supported */\n" }
9/2	2026/10/06 22:43	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 8137 output: 42 thoughts: 27 Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience: Investigating a Potential AMD CPU Microcode Issue Okay, let's dive in. Based on what I've seen, it seems like the issue I'm chasing is likely related to AMD CPU microcode behavior. My instinct is telling me the problem area is going to be in `arch/x86/kernel/cpu/amd.c`. Considering the characteristics of what I'm seeing, specifically related to the processor's initial state or potentially certain power management functions, I'm going to start my examination around line 1104. That region usually deals with either initialization routines, specific processor capabilities, or power management handling related to the AMD architecture. I need to refresh my memory on what's handled right in that general area of the AMD kernel code to figure out what needs deeper investigation.
10/2	2026/10/06 22:43	tool	read-file	0m Args: { "File": "arch/x86/kernel/cpu/amd.c", "FirstLine": 1095, "LineCount": 25 } Results: { "Contents": "1095:\t\telse if (boot_cpu_has(X86_FEATURE_ZEN4))\n1096:\t\t\tinit_amd_zen4(c);\n1097:\t\telse if (boot_cpu_has(X86_FEATURE_ZEN5))\n1098:\t\t\tinit_amd_zen5(c);\n1099:\t\n1100:\t\t/\n1101:\t\t Enable workaround for FXSAVE leak on CPUs\n1102:\t\t * without a XSaveErPtr feature\n1103:\t\t */\n1104:\t\tif ((c-\u003ex86 \u003e= 6) \u0026\u0026 (!cpu_has(c, X86_FEATURE_XSAVEERPTR)))\n1105:\t\t\tset_cpu_bug(c, X86_BUG_FXSAVE_LEAK);\n1106:\t\n1107:\t\tcpu_detect_cache_sizes(c);\n1108:\t\n1109:\t\tsrat_detect_node(c);\n1110:\t\n1111:\t\tinit_amd_cacheinfo(c);\n1112:\t\n1113:\t\tif (cpu_has(c, X86_FEATURE_SVM)) {\n1114:\t\t\trdmsrq(MSR_VM_CR, vm_cr);\n1115:\t\t\tif (vm_cr \u0026 SVM_VM_CR_SVM_DIS_MASK) {\n1116:\t\t\t\tpr_notice_once(\"SVM disabled (by BIOS) in MSR_VM_CR\\n\");\n1117:\t\t\t\tclear_cpu_cap(c, X86_FEATURE_SVM);\n1118:\t\t\t}\n1119:\t\t}\n" }
11/2	2026/10/06 22:43	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 8824 output: 34 thoughts: 48 Thoughts: Okay, let's see. My first thought is to dig a little deeper into this potential issue. I need to figure out where this `X86_BUG_FXSAVE_LEAK` flag might be used, or potentially not used, elsewhere in the x86 kernel code. I'm going to start by taking a look at `arch/x86/kernel/cpu/common.c`. It's a pretty central location for x86 CPU-specific setup and configuration, so it's a logical starting point. I could probably also find some leads by looking in `bugs.c`. Let's see if those files provide any more context or usage examples. This will give me a better understanding of the implications of this flag and how the system handles it, and if it is handled anywhere else. This will help me diagnose the potential root cause and impact of this bug.
12/2	2026/10/06 22:43	tool	grepper	0m Args: { "Expression": "FXSAVE_LEAK", "PathPrefixes": [ "arch/x86/kernel/" ] } Results: { "Output": "arch/x86/kernel/cpu/amd.c=1054=static void init_amd(struct cpuinfo_x86 *c)\n--\narch/x86/kernel/cpu/amd.c-1104-\tif ((c-\u003ex86 \u003e= 6) \u0026\u0026 (!cpu_has(c, X86_FEATURE_XSAVEERPTR)))\narch/x86/kernel/cpu/amd.c:1105:\t\tset_cpu_bug(c, X86_BUG_FXSAVE_LEAK);\narch/x86/kernel/cpu/amd.c-1106-\n--\narch/x86/kernel/fpu/internal.h=30=static inline void fpregs_restore_prepare(u64 mask)\narch/x86/kernel/fpu/internal.h-31-{\narch/x86/kernel/fpu/internal.h:32:\tif (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK)) \u0026\u0026\narch/x86/kernel/fpu/internal.h-33-\t (mask \u0026 XFEATURE_MASK_FP))\n" }
13/2	2026/10/06 22:43	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 9390 output: 42 thoughts: 55 Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience: Analyzing the FPU Restore Logic Alright, before diving too deep, I need to get my bearings. Let's start by identifying the exact commit we're dealing with. Knowing the commit hash or a specific range helps immensely; it's the fundamental context. I could pull up the commit directly or consult the git log to clarify the surrounding changes. But, before I do that, the real meat of this likely lies in `arch/x86/kernel/fpu/core.c`, specifically around the `restore_fpregs_from_fpstate` function. My intuition tells me that's where the core logic for restoring the Floating Point Unit's register state resides. Understanding the surrounding code there will be critical to fully grasp this particular issue. We should be able to deduce the control flow and potential error conditions by tracing execution through that function. It is important to remember what we know of the architecture of the x86 FPU.
14/2	2026/10/06 22:43	tool	read-file	0m Args: { "File": "arch/x86/kernel/fpu/core.c", "FirstLine": 150, "LineCount": 50 } Results: { "Contents": " 150:\t\t\tfxsave(\u0026fpu-\u003efpstate-\u003eregs.fxsave);\n 151:\t\t\treturn;\n 152:\t\t}\n 153:\t\n 154:\t\t/\n 155:\t\t Legacy FPU register saving, FNSAVE always clears FPU registers,\n 156:\t\t * so we have to reload them from the memory state.\n 157:\t\t /\n 158:\t\tasm volatile(\"fnsave %[fp]; fwait\" : [fp] \"=m\" (fpu-\u003efpstate-\u003eregs.fsave));\n 159:\t\tfrstor(\u0026fpu-\u003efpstate-\u003eregs.fsave);\n 160:\t}\n 161:\t\n 162:\tvoid restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask)\n 163:\t{\n 164:\t\tif (use_xsave()) {\n 165:\t\t\t/\n 166:\t\t\t Dynamically enabled features are enabled in XCR0, but\n 167:\t\t\t * usage requires also that the corresponding bits in XFD\n 168:\t\t\t * are cleared. If the bits are set then using a related\n 169:\t\t\t * instruction will raise #NM. This allows to do the\n 170:\t\t\t * allocation of the larger FPU buffer lazy from #NM or if\n 171:\t\t\t * the task has no permission to kill it which would happen\n 172:\t\t\t * via #UD if the feature is disabled in XCR0.\n 173:\t\t\t \n 174:\t\t\t XFD state is following the same life time rules as\n 175:\t\t\t * XSTATE and to restore state correctly XFD has to be\n 176:\t\t\t * updated before XRSTORS otherwise the component would\n 177:\t\t\t * stay in or go into init state even if the bits are set\n 178:\t\t\t * in fpstate::regs::xsave::xfeatures.\n 179:\t\t\t /\n 180:\t\t\txfd_update_state(fpstate);\n 181:\t\n 182:\t\t\t/\n 183:\t\t\t * Restoring state always needs to modify all features\n 184:\t\t\t * which are in @mask even if the current task cannot use\n 185:\t\t\t * extended features.\n 186:\t\t\t \n 187:\t\t\t So fpstate-\u003exfeatures cannot be used here, because then\n 188:\t\t\t * a feature for which the task has no permission but was\n 189:\t\t\t * used by the previous task would not go into init state.\n 190:\t\t\t */\n 191:\t\t\tmask = fpu_kernel_cfg.max_features \u0026 mask;\n 192:\t\n 193:\t\t\tos_xrstor(fpstate, mask);\n 194:\t\t} else {\n 195:\t\t\tif (use_fxsr())\n 196:\t\t\t\tfxrstor(\u0026fpstate-\u003eregs.fxsave);\n 197:\t\t\telse\n 198:\t\t\t\tfrstor(\u0026fpstate-\u003eregs.fsave);\n 199:\t\t}\n" }
15/2	2026/10/06 22:43	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 10489 output: 64 thoughts: 550 Thoughts: Analyzing the Changes to Floating-Point State Restoration Okay, let's dive into these changes. My initial focus is on `core.c`, `legacy.h`, `xstate.h`, and `internal.h`. I need to understand the scope of this fix. Previously, in `restore_fpregs_from_fpstate`, there was a mitigation for the AMD FXSAVE leak vulnerability: ```c if (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK))) { asm volatile( "fnclex\n\t" "emms\n\t" "fildl %[addr]" /* set F?P to defined value / : : [addr] "m" (fpstate)); } ``` This code snippet was executed on every call to `restore_fpregs_from_fpstate` on affected AMD CPUs. But hold on, where else are floating-point registers being restored? I need to map out the call graph for all these instructions. I need to understand what calls `fxrstor`, `fxrstor_safe`, `fxrstor_from_user_sigframe`, `frstor`, `frstor_safe`, `frstor_from_user_sigframe`, `os_xrstor`, `xrstor_from_user_sigframe`, and `os_xrstor_safe`. The commit appears to have extended the mitigation. The leak fix was originally only present in `restore_fpregs_from_fpstate`. Now it seems to have been applied to these other functions as well, but why the discrepancy? And what's with the change from `fildl %[addr]` to `fninit`? Ah, the explanation... "AMD CPUs without XSaveErPtr may leave FDP/FIP/FOP unchanged on restore when no x87 exception is pending. Using an x87 load to overwrite them leaks the kernel instruction and operand addresses through FNSTENV. FNINIT clears the pointers without recording any new ones. Only do this when the x87 state is about to be replaced; a partial XRSTOR must leave unrequested components alone." This makes sense. The old approach was potentially leaking kernel addresses through the FPU state when restoring. `FNINIT` is the correct approach. Now I need to verify that this fix is comprehensively applied. I must check the callers of `fxrstor_safe`, `fxrstor_from_user_sigframe`, `xrstor_from_user_sigframe`, and `os_xrstor_safe` to be certain. I'm going to search the codebase for those function calls, particularly looking for `xrstor_from_user_sigframe`, `os_xrstor_safe`, and `fxrstor_safe`. This will confirm if the fix has been applied consistently.
16/2	2026/10/06 22:44	tool	grepper	0m Args: { "Expression": "(fxrstor_safe\|xrstor_from_user_sigframe\|os_xrstor_safe\|fxrstor_from_user_sigframe)", "PathPrefixes": [ "arch/x86/" ] } Results: { "Output": "arch/x86/kernel/fpu/legacy.h=66=static inline void fxrstor(struct fxregs_state fx)\n--\narch/x86/kernel/fpu/legacy.h-75-\narch/x86/kernel/fpu/legacy.h:76:static inline int fxrstor_safe(struct fxregs_state fx)\narch/x86/kernel/fpu/legacy.h-77-{\n--\narch/x86/kernel/fpu/legacy.h-85-\narch/x86/kernel/fpu/legacy.h:86:static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx)\narch/x86/kernel/fpu/legacy.h-87-{\n--\narch/x86/kernel/fpu/signal.c=243=static int __restore_fpregs_from_user(void __user buf, u64 ufeatures,\n--\narch/x86/kernel/fpu/signal.c-250-\t\tif (likely(!fx_only))\narch/x86/kernel/fpu/signal.c:251:\t\t\tret = xrstor_from_user_sigframe(buf, xrestore);\narch/x86/kernel/fpu/signal.c-252-\t\telse\narch/x86/kernel/fpu/signal.c:253:\t\t\tret = fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-254-\n--\narch/x86/kernel/fpu/signal.c-258-\t} else if (use_fxsr()) {\narch/x86/kernel/fpu/signal.c:259:\t\treturn fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-260-\t} else {\n--\narch/x86/kernel/fpu/signal.c=327=static bool __fpu_restore_sig(void __user buf, void __user buf_fx,\n--\narch/x86/kernel/fpu/signal.c-425-\t\tfpregs-\u003exsave.header.xfeatures \u0026= mask;\narch/x86/kernel/fpu/signal.c:426:\t\tsuccess = !os_xrstor_safe(fpu-\u003efpstate,\narch/x86/kernel/fpu/signal.c-427-\t\t\t\t\t fpu_kernel_cfg.max_features);\narch/x86/kernel/fpu/signal.c-428-\t} else {\narch/x86/kernel/fpu/signal.c:429:\t\tsuccess = !fxrstor_safe(\u0026fpregs-\u003efxsave);\narch/x86/kernel/fpu/signal.c-430-\t}\n--\narch/x86/kernel/fpu/xstate.h=298=static inline int xsave_to_user_sigframe(struct xregs_state __user buf, u32 pkru)\n--\narch/x86/kernel/fpu/xstate.h-331- /\narch/x86/kernel/fpu/xstate.h:332:static inline int xrstor_from_user_sigframe(struct xregs_state __user buf, u64 mask)\narch/x86/kernel/fpu/xstate.h-333-{\n--\narch/x86/kernel/fpu/xstate.h-352- /\narch/x86/kernel/fpu/xstate.h:353:static inline int os_xrstor_safe(struct fpstate *fpstate, u64 mask)\narch/x86/kernel/fpu/xstate.h-354-{\n" }
17/2	2026/10/06 22:44	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 12701 output: 37 thoughts: 1087 Thoughts: Reaching the FXSAVE Leak: A QEMU/GCE Perspective Okay, so I'm digging into this `X86_BUG_FXSAVE_LEAK` and whether it's reachable in standard virtualized environments. Let's see... the critical paths here are related to restoring FPU state, particularly through functions like `restore_fpregs_from_fpstate`, `fpu__restore`, `__restore_fpregs_from_user`, and `__fpu_restore_sig`. The signal return path through `sigreturn` and `rt_sigreturn` is a key area, calling `restore_sigcontext` and ultimately `fpu__restore_sig`. Context switches and returning to user space are also important. Now, the vulnerability hinges on AMD CPUs, specifically those that don't save/restore FDP/FIP/FOP unless an exception is pending. So, the crucial question is: What CPU models are we dealing with in QEMU and GCE? GCE instances can run on AMD processors (Zen 1, Zen 2, Zen 3). QEMU allows us to emulate various AMD CPUs with the `-cpu` flag. I'm thinking, even with QEMU's default settings, or KVM, what's going on with the CPUID information it presents? Looking at `arch/x86/kvm/cpuid.c`, `XSAVEERPTR` is a key feature. If the host CPU is, say, AMD Zen 1, which doesn't have `XSAVEERPTR`, or maybe Zen 2 (which does have it? or maybe not?) we're potentially vulnerable. Checking the AMD-specific code in `arch/x86/kernel/cpu/amd.c`, the logic is clear. If the CPU family is 6 (K7, K8, K10, etc.) and it lacks `X86_FEATURE_XSAVEERPTR`, then `X86_BUG_FXSAVE_LEAK` is set. This means any AMD K7 or later without XSAVEERPTR has it! Okay, so let's try some experiments. Does QEMU's `-cpu EPYC` or `-cpu qemu64` trigger this? The `qemu64` reports the vendor. Is it "AuthenticAMD" or "GenuineIntel"? Aha! By default, `-cpu qemu64` is "AuthenticAMD"! Let's dig deeper: the default x86_64 CPU reports an AMD vendor, Family 6, Model 6, Stepping 3. And it doesn't have `XSAVEERPTR`. So, with `-cpu qemu64`, the code will set `X86_BUG_FXSAVE_LEAK`. Wait, this is important based on our triage strategy. If the code is reachable in standard virtualized environments, we need to fuzz it. We're looking at code in `arch/x86/kernel/fpu/core.c`, `arch/x86/kernel/fpu/internal.h`, `arch/x86/kernel/fpu/legacy.h`, `arch/x86/kernel/fpu/xstate.h`. These are core x86 architecture files handling FPU state save/restore! It looks like there are several call paths we need to investigate: `restore_fpregs_from_fpstate`, `fpregs_restore_prepare`, `fxrstor`, `os_xrstor`, `xrstor_from_user_sigframe`, etc. I need to figure out what those callers are... this looks promising for a potential reach!
18/2	2026/10/06 22:44	tool	grepper	0m Args: { "Expression": "restore_fpregs_from_fpstate", "PathPrefixes": [ "arch/x86/" ] } Results: { "Output": "arch/x86/include/asm/fpu/signal.h=34=extern bool fpu__restore_sig(void __user buf, int ia32_frame);\narch/x86/include/asm/fpu/signal.h-35-\narch/x86/include/asm/fpu/signal.h:36:extern void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask);\narch/x86/include/asm/fpu/signal.h-37-#endif /* _ASM_X86_FPU_SIGNAL_H /\n--\narch/x86/kernel/fpu/context.h=54=static inline void fpregs_restore_userregs(void)\n--\narch/x86/kernel/fpu/context.h-71-\t\t \narch/x86/kernel/fpu/context.h:72:\t\t * XFD state is handled in restore_fpregs_from_fpstate().\narch/x86/kernel/fpu/context.h-73-\t\t /\narch/x86/kernel/fpu/context.h:74:\t\trestore_fpregs_from_fpstate(fpu-\u003efpstate, XFEATURE_MASK_FPSTATE);\narch/x86/kernel/fpu/context.h-75-\n--\narch/x86/kernel/fpu/core.c=141=void save_fpregs_to_fpstate(struct fpu fpu)\n--\narch/x86/kernel/fpu/core.c-161-\narch/x86/kernel/fpu/core.c:162:void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask)\narch/x86/kernel/fpu/core.c-163-{\n--\narch/x86/kernel/fpu/core.c=202=void fpu_reset_from_exception_fixup(void)\narch/x86/kernel/fpu/core.c-203-{\narch/x86/kernel/fpu/core.c:204:\trestore_fpregs_from_fpstate(\u0026init_fpstate, XFEATURE_MASK_FPSTATE);\narch/x86/kernel/fpu/core.c-205-}\n--\narch/x86/kernel/fpu/core.c=360=int fpu_swap_kvm_fpstate(struct fpu_guest guest_fpu, bool enter_guest)\n--\narch/x86/kernel/fpu/core.c-384-\t\t/* Includes XFD update */\narch/x86/kernel/fpu/core.c:385:\t\trestore_fpregs_from_fpstate(cur_fps, XFEATURE_MASK_FPSTATE);\narch/x86/kernel/fpu/core.c-386-\t} else {\n" }
19/2	2026/10/06 22:44	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 15374 output: 42 thoughts: 105 Thoughts: Here's my analysis of the situation, given my expertise: Examining x86 FPU State Restoration in the Kernel Okay, so I need to refresh my understanding of how the FPU state is handled in the x86 kernel. My train of thought starts with identifying the key functions related to restoring floating-point registers. I'm focusing on the low-level mechanisms, so I'm thinking: let's start with `fpregs_restore_userregs()`, `restore_fpregs_from_fpstate()`, and then `fpu_reset_from_exception_fixup()` and `fpu_swap_kvm_fpstate()` - these seem like likely candidates to be involved with the different scenarios for restoring FPU state after context switches, exceptions, and virtualization interactions respectively. Then, I'll branch out to `signal.c` to see how signal handling intersects with FPU state management. I'm recalling from past work that signals often require saving and restoring processor state, so I'll need to look at `__restore_fpregs_from_user` and `__fpu_restore_sig`. The naming conventions suggest they're specifically involved in restoring FPU state from user-space context. Given the potential for x86-specific intricacies, I'll specifically focus my examination on `arch/x86/kernel/fpu/signal.c`. This is where I expect to find the architecture-dependent implementation details that are crucial for a deep understanding. My aim here is to understand the interplay between the hardware FPU, the kernel's state management, and the user-space interaction related to signals and exceptions.
20/2	2026/10/06 22:44	tool	read-file	0m Args: { "File": "arch/x86/kernel/fpu/signal.c", "FirstLine": 240, "LineCount": 40 } Results: { "Contents": " 240:\t\treturn true;\n 241:\t}\n 242:\t\n 243:\tstatic int __restore_fpregs_from_user(void __user buf, u64 ufeatures,\n 244:\t\t\t\t\t u64 xrestore, bool fx_only)\n 245:\t{\n 246:\t\tif (use_xsave()) {\n 247:\t\t\tu64 init_bv = ufeatures \u0026 ~xrestore;\n 248:\t\t\tint ret;\n 249:\t\n 250:\t\t\tif (likely(!fx_only))\n 251:\t\t\t\tret = xrstor_from_user_sigframe(buf, xrestore);\n 252:\t\t\telse\n 253:\t\t\t\tret = fxrstor_from_user_sigframe(buf);\n 254:\t\n 255:\t\t\tif (!ret \u0026\u0026 unlikely(init_bv))\n 256:\t\t\t\tos_xrstor(\u0026init_fpstate, init_bv);\n 257:\t\t\treturn ret;\n 258:\t\t} else if (use_fxsr()) {\n 259:\t\t\treturn fxrstor_from_user_sigframe(buf);\n 260:\t\t} else {\n 261:\t\t\treturn frstor_from_user_sigframe(buf);\n 262:\t\t}\n 263:\t}\n 264:\t\n 265:\t/\n 266:\t * Attempt to restore the FPU registers directly from user memory.\n 267:\t * Pagefaults are handled and any errors returned are fatal.\n 268:\t /\n 269:\tstatic bool restore_fpregs_from_user(void __user buf, u64 xrestore, bool fx_only)\n 270:\t{\n 271:\t\tstruct fpu fpu = x86_task_fpu(current);\n 272:\t\tint ret;\n 273:\t\n 274:\t\t/ Restore enabled features only. /\n 275:\t\txrestore \u0026= fpu-\u003efpstate-\u003euser_xfeatures;\n 276:\tretry:\n 277:\t\tfpregs_lock();\n 278:\t\t/ Ensure that XFD is up to date */\n 279:\t\txfd_update_state(fpu-\u003efpstate);\n" }
21/2	2026/10/06 22:44	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 16482 output: 41 thoughts: 1345 Thoughts: FPU State Restoration and the FXSAVE Leak: A Deep Dive Okay, so I'm looking at this patch and immediately, my attention is drawn to the changes in FPU state restoration. First, I see the refactoring – specifically, how the code interacts with `xrstor_from_user_sigframe`, `fxrstor_from_user_sigframe`, `os_xrstor_safe`, and all the other restore functions for FPU registers. The core concern here, as I understand it, is preventing information leakage during FPU state restoration, particularly the notorious FXSAVE leak. I remember dealing with the `X86_BUG_FXSAVE_LEAK` before. Previously, `restore_fpregs_from_fpstate` had a workaround, but it seems to have been removed completely and replaced by a call to `fpregs_restore_prepare(u64 mask)` within nearly all the restore functions: `fxrstor`, `fxrstor_safe`, `fxrstor_from_user_sigframe`, `frstor`, `frstor_safe`, `frstor_from_user_sigframe`, `os_xrstor`, `xrstor_from_user_sigframe`, and `os_xrstor_safe`. Okay, the commit message makes it clear that we're talking about specific AMD CPUs without XSaveErPtr, which may leave FDP/FIP/FOP unchanged when no x87 exception is pending. This means that a load of the x87 registers (`fildl` in the old code) would overwrite the kernel addresses. If a user-space program then executes `FNSTENV`, those kernel addresses in FDP/FIP/FOP would leak. So, the original approach used `fnclex`, `emms`, and then `fildl` on `fpstate`. That `fildl` was the potential leak. Now, it's been replaced with `fninit` and, importantly, this is being called before the actual restoration. This is interesting. The old code was only running the fix during a task switch, but not during signal handling, or from user sigframe! Now, the use of `fninit` here is critical. I need to refresh my memory, but `fninit` essentially initializes the FPU: control word to 0x037F, status word cleared, stack registers tagged as empty, and most importantly, it clears the exception pointers. Now, I'm thinking about the implications. Particularly, consider this scenario: `xrstor_from_user_sigframe` executes `fpregs_restore_prepare(mask);` which could call `fninit`, then `XRSTOR` directly from user memory (`buf`). But what if the `XRSTOR` instruction itself faults? What if `buf` is an invalid pointer, or the state in `buf` is corrupt and causes a #GP? If that `XRSTOR` faults, we already executed `fninit`. The FPU registers would be initialized, and in a way that, even if an exception occurs, the state should not leak anything critical. Now, I'm thinking about other scenarios: what if the `mask` doesn't include all components? What if `fxrstor_safe` or the other functions fail? And, what about signal handling and `sigreturn`? How do these restored functions interact with signal handling? What about context switches, KVM FPU swapping (`fpu_swap_kvm_fpstate`), and, of course, `ptrace` with `PTRACE_SETREGSET`? I'd better dig into the implementation details of those and trace the calls to these restored functions and verify this assumption. I'll need to double-check that these scenarios are handled correctly, as this patch appears to make a significant change to how we protect against this vulnerability.
22/2	2026/10/06 22:45	tool	grepper	0m Args: { "Expression": "os_xrstor\|fxrstor", "PathPrefixes": [ "arch/x86/kernel/fpu/" ] } Results: { "Output": "arch/x86/kernel/fpu/core.c=162=void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask)\n--\narch/x86/kernel/fpu/core.c-192-\narch/x86/kernel/fpu/core.c:193:\t\tos_xrstor(fpstate, mask);\narch/x86/kernel/fpu/core.c-194-\t} else {\narch/x86/kernel/fpu/core.c-195-\t\tif (use_fxsr())\narch/x86/kernel/fpu/core.c:196:\t\t\tfxrstor(\u0026fpstate-\u003eregs.fxsave);\narch/x86/kernel/fpu/core.c-197-\t\telse\n--\narch/x86/kernel/fpu/core.c=773=static inline void restore_fpregs_from_init_fpstate(u64 features_mask)\n--\narch/x86/kernel/fpu/core.c-775-\tif (use_xsave())\narch/x86/kernel/fpu/core.c:776:\t\tos_xrstor(\u0026init_fpstate, features_mask);\narch/x86/kernel/fpu/core.c-777-\telse if (use_fxsr())\narch/x86/kernel/fpu/core.c:778:\t\tfxrstor(\u0026init_fpstate.regs.fxsave);\narch/x86/kernel/fpu/core.c-779-\telse\n--\narch/x86/kernel/fpu/core.c=817=void fpu__clear_user_states(struct fpu fpu)\n--\narch/x86/kernel/fpu/core.c-833-\t !fpregs_state_valid(fpu, smp_processor_id()))\narch/x86/kernel/fpu/core.c:834:\t\tos_xrstor_supervisor(fpu-\u003efpstate);\narch/x86/kernel/fpu/core.c-835-\n--\narch/x86/kernel/fpu/legacy.h=57=static inline int fxsave_to_user_sigframe(struct fxregs_state __user fx)\n--\narch/x86/kernel/fpu/legacy.h-65-\narch/x86/kernel/fpu/legacy.h:66:static inline void fxrstor(struct fxregs_state fx)\narch/x86/kernel/fpu/legacy.h-67-{\n--\narch/x86/kernel/fpu/legacy.h-70-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:71:\t\tkernel_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-72-\telse\narch/x86/kernel/fpu/legacy.h:73:\t\tkernel_insn(fxrstorq %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-74-}\narch/x86/kernel/fpu/legacy.h-75-\narch/x86/kernel/fpu/legacy.h:76:static inline int fxrstor_safe(struct fxregs_state fx)\narch/x86/kernel/fpu/legacy.h-77-{\n--\narch/x86/kernel/fpu/legacy.h-80-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:81:\t\treturn kernel_insn_err(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-82-\telse\narch/x86/kernel/fpu/legacy.h:83:\t\treturn kernel_insn_err(fxrstorq %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-84-}\narch/x86/kernel/fpu/legacy.h-85-\narch/x86/kernel/fpu/legacy.h:86:static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx)\narch/x86/kernel/fpu/legacy.h-87-{\n--\narch/x86/kernel/fpu/legacy.h-90-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:91:\t\treturn user_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-92-\telse\narch/x86/kernel/fpu/legacy.h:93:\t\treturn user_insn(fxrstorq %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-94-}\n--\narch/x86/kernel/fpu/signal.c=243=static int __restore_fpregs_from_user(void __user buf, u64 ufeatures,\n--\narch/x86/kernel/fpu/signal.c-252-\t\telse\narch/x86/kernel/fpu/signal.c:253:\t\t\tret = fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-254-\narch/x86/kernel/fpu/signal.c-255-\t\tif (!ret \u0026\u0026 unlikely(init_bv))\narch/x86/kernel/fpu/signal.c:256:\t\t\tos_xrstor(\u0026init_fpstate, init_bv);\narch/x86/kernel/fpu/signal.c-257-\t\treturn ret;\narch/x86/kernel/fpu/signal.c-258-\t} else if (use_fxsr()) {\narch/x86/kernel/fpu/signal.c:259:\t\treturn fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-260-\t} else {\n--\narch/x86/kernel/fpu/signal.c=269=static bool restore_fpregs_from_user(void __user buf, u64 xrestore, bool fx_only)\n--\narch/x86/kernel/fpu/signal.c-319-\tif (test_thread_flag(TIF_NEED_FPU_LOAD) \u0026\u0026 xfeatures_mask_supervisor())\narch/x86/kernel/fpu/signal.c:320:\t\tos_xrstor_supervisor(fpu-\u003efpstate);\narch/x86/kernel/fpu/signal.c-321-\n--\narch/x86/kernel/fpu/signal.c=327=static bool __fpu_restore_sig(void __user buf, void __user buf_fx,\n--\narch/x86/kernel/fpu/signal.c-425-\t\tfpregs-\u003exsave.header.xfeatures \u0026= mask;\narch/x86/kernel/fpu/signal.c:426:\t\tsuccess = !os_xrstor_safe(fpu-\u003efpstate,\narch/x86/kernel/fpu/signal.c-427-\t\t\t\t\t fpu_kernel_cfg.max_features);\narch/x86/kernel/fpu/signal.c-428-\t} else {\narch/x86/kernel/fpu/signal.c:429:\t\tsuccess = !fxrstor_safe(\u0026fpregs-\u003efxsave);\narch/x86/kernel/fpu/signal.c-430-\t}\n--\narch/x86/kernel/fpu/xstate.c=324=static void __init print_xstate_offset_size(void)\n--\narch/x86/kernel/fpu/xstate.c-338- /\narch/x86/kernel/fpu/xstate.c:339:static __init void os_xrstor_booting(struct xregs_state xstate)\narch/x86/kernel/fpu/xstate.c-340-{\n--\narch/x86/kernel/fpu/xstate.c=384=static void __init setup_init_fpu_buf(void)\n--\narch/x86/kernel/fpu/xstate.c-399-\t /\narch/x86/kernel/fpu/xstate.c:400:\tos_xrstor_booting(\u0026init_fpstate.regs.xsave);\narch/x86/kernel/fpu/xstate.c-401-\n--\narch/x86/kernel/fpu/xstate.h=218=static inline void os_xsave(struct fpstate fpstate)\n--\narch/x86/kernel/fpu/xstate.h-238- /\narch/x86/kernel/fpu/xstate.h:239:static inline void os_xrstor(struct fpstate fpstate, u64 mask)\narch/x86/kernel/fpu/xstate.h-240-{\n--\narch/x86/kernel/fpu/xstate.h-249-/* Restore of supervisor state. Does not require XFD /\narch/x86/kernel/fpu/xstate.h:250:static inline void os_xrstor_supervisor(struct fpstate fpstate)\narch/x86/kernel/fpu/xstate.h-251-{\n--\narch/x86/kernel/fpu/xstate.h=332=static inline int xrstor_from_user_sigframe(struct xregs_state __user buf, u64 mask)\n--\narch/x86/kernel/fpu/xstate.h-352- /\narch/x86/kernel/fpu/xstate.h:353:static inline int os_xrstor_safe(struct fpstate *fpstate, u64 mask)\narch/x86/kernel/fpu/xstate.h-354-{\n" }
23/2	2026/10/06 22:45	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 21364 output: 38 thoughts: 542 Thoughts: Fuzzing Feasibility Assessment Okay, let's break this down systematically. My initial instinct is to assess if this patch is even worth the fuzzing effort. The guidelines are pretty clear: does the modified code execute in standard virtualized environments, and is it reachable? Let's check the criteria. The patch touches `arch/x86/kernel/fpu/core.c`, `arch/x86/kernel/fpu/internal.h`, `arch/x86/kernel/fpu/legacy.h`, and `arch/x86/kernel/fpu/xstate.h`. My immediate thought? This is core x86 architecture code, deeply involved in FPU state handling – context switches, signal delivery, ptrace, and KVM. It's definitely executable in QEMU or GCE on x86_64, which are our standard environments. The patch doesn't involve unreachable hardware or code paths exclusive to other architectures. So, WorthFuzzing has to be `true`. Now, we need to consider enable configurations. Any specific configuration options that this code might need to compile or run? No non-standard configurations are needed; FPU is always compiled on x86. Let's make sure that `CONFIG_X86_64`, `CONFIG_X86`, or anything in `arch/x86/kernel/fpu/Makefile` isn't an issue.
24/2	2026/10/06 22:45	tool	read-file	0m Args: { "File": "arch/x86/kernel/fpu/Makefile", "FirstLine": 1, "LineCount": 30 } Results: { "Contents": " 1:\t# SPDX-License-Identifier: GPL-2.0-only\n 2:\t#\n 3:\t# Build rules for the FPU support code:\n 4:\t#\n 5:\t\n 6:\tobj-y\t\t\t\t+= init.o bugs.o core.o regset.o signal.o xstate.o\n" }
25/2	2026/10/06 22:45	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 22352 output: 35 thoughts: 198 Thoughts: Examining the x86 FPU Compilation Okay, so looking at this build setup, it appears that `init.o`, `bugs.o`, `core.o`, `regset.o`, `signal.o`, and `xstate.o` are all unconditionally linked in the x86 kernel. Specifically, that means `arch/x86/kernel/fpu/core.c` and `xstate.c` are always built on x86, which is good to keep in mind. But wait a minute, I should double-check if there's any conditional compilation I'm missing. Let's see if there's a configuration option that affects this. Ah, here's `X86_DEBUG_FPU`. Let's take a look at the definition in `internal.h`: ```c #ifdef CONFIG_X86_DEBUG_FPU # define WARN_ON_FPU(x) WARN_ON_ONCE(x) #else # define WARN_ON_FPU(x) ({ (void)(x); 0; }) #endif ``` Right, `CONFIG_X86_DEBUG_FPU` seems to be a debug option for the FPU. Okay, that's what I suspected. Just to be sure, let me verify where that's defined, let's look at `arch/x86/Kconfig.debug`.
26/2	2026/10/06 22:45	tool	grepper	0m Args: { "Expression": "X86_DEBUG_FPU", "PathPrefixes": [ "arch/x86/" ] } Results: { "Output": "arch/x86/Kconfig.debug=188=config DEBUG_IMR_SELFTEST\n--\narch/x86/Kconfig.debug-199-\narch/x86/Kconfig.debug:200:config X86_DEBUG_FPU\narch/x86/Kconfig.debug-201-\tbool \"Debug the x86 FPU code\"\n--\narch/x86/include/asm/fpu/api.h=89=void fpregs_lock_and_load(void);\narch/x86/include/asm/fpu/api.h-90-\narch/x86/include/asm/fpu/api.h:91:#ifdef CONFIG_X86_DEBUG_FPU\narch/x86/include/asm/fpu/api.h-92-extern void fpregs_assert_state_consistent(void);\n--\narch/x86/include/asm/processor.h=466=struct thread_struct {\n--\narch/x86/include/asm/processor.h-536-\narch/x86/include/asm/processor.h:537:#ifdef CONFIG_X86_DEBUG_FPU\narch/x86/include/asm/processor.h-538-extern struct fpu x86_task_fpu(struct task_struct task);\n--\narch/x86/kernel/fpu/core.c=58=DEFINE_PER_CPU(struct fpu , fpu_fpregs_owner_ctx);\narch/x86/kernel/fpu/core.c-59-\narch/x86/kernel/fpu/core.c:60:#ifdef CONFIG_X86_DEBUG_FPU\narch/x86/kernel/fpu/core.c-61-struct fpu x86_task_fpu(struct task_struct task)\n--\narch/x86/kernel/fpu/core.c=870=void fpregs_lock_and_load(void)\n--\narch/x86/kernel/fpu/core.c-887-\narch/x86/kernel/fpu/core.c:888:#ifdef CONFIG_X86_DEBUG_FPU\narch/x86/kernel/fpu/core.c-889-/\n--\narch/x86/kernel/fpu/internal.h=30=static inline void fpregs_restore_prepare(u64 mask)\n--\narch/x86/kernel/fpu/internal.h-36-\narch/x86/kernel/fpu/internal.h:37:#ifdef CONFIG_X86_DEBUG_FPU\narch/x86/kernel/fpu/internal.h-38-# define WARN_ON_FPU(x) WARN_ON_ONCE(x)\n--\narch/x86/kernel/fpu/xstate.c=1485=EXPORT_SYMBOL_FOR_KVM(fpstate_clear_xstate_component);\n--\narch/x86/kernel/fpu/xstate.c-1489-\narch/x86/kernel/fpu/xstate.c:1490:#ifdef CONFIG_X86_DEBUG_FPU\narch/x86/kernel/fpu/xstate.c-1491-/\n--\narch/x86/kernel/fpu/xstate.c=1541=void xfd_validate_state(struct fpstate fpstate, u64 mask, bool rstor)\n--\narch/x86/kernel/fpu/xstate.c-1544-}\narch/x86/kernel/fpu/xstate.c:1545:#endif /* CONFIG_X86_DEBUG_FPU /\narch/x86/kernel/fpu/xstate.c-1546-\n--\narch/x86/kernel/fpu/xstate.h=91=static inline int update_pkru_in_sigframe(struct xregs_state __user buf, u32 pkru)\n--\narch/x86/kernel/fpu/xstate.h-177-\narch/x86/kernel/fpu/xstate.h:178:#if defined(CONFIG_X86_64) \u0026\u0026 defined(CONFIG_X86_DEBUG_FPU)\narch/x86/kernel/fpu/xstate.h-179-extern void xfd_validate_state(struct fpstate *fpstate, u64 mask, bool rstor);\n" }
27/2	2026/10/06 22:45	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 24037 output: 41 thoughts: 854 Thoughts: Analyzing FPU Debugging and Fuzzing Focus Okay, so `CONFIG_X86_DEBUG_FPU` enables assertions like `xfd_validate_state` and `WARN_ON_FPU`. The question is, are any specific configurations required to compile or reach this modified code? No, the code itself isn't contingent on any specific config. It's always compiled in on x86 architectures. This means that the `EnableConfigs` list can be empty (`[]`) because nothing specific is required beyond the standard x86 configurations like `X86_64` or `X86`. Now, let's determine the `FocusSymbols`. We need to avoid ubiquitous lifecycle hot-paths; things like memory allocators, page fault handlers, and core synchronization primitives. Instead, we want specific feature logic and entry points. The patch modified `restore_fpregs_from_fpstate` in `arch/x86/kernel/fpu/core.c`. This is a non-static function in a .c file, which is good. So, who calls `restore_fpregs_from_fpstate`? Let's see: * `fpregs_restore_userregs` (in `arch/x86/kernel/fpu/context.h`, called during context switches / return to user). Context switches are probably ubiquitous lifecycle paths. * `fpu_reset_from_exception_fixup` (in `arch/x86/kernel/fpu/core.c`). * `fpu_swap_kvm_fpstate` (in `arch/x86/kernel/fpu/core.c`). A key question is, is `restore_fpregs_from_fpstate` itself a ubiquitous function? Let's think about its purpose. Ah, it restores FPU registers. In x86 Linux, `restore_fpregs_from_fpstate` is called when restoring FPU registers! When is that called? Let's check `context.h` to confirm the context.
28/2	2026/10/06 22:46	tool	read-file	0m Args: { "File": "arch/x86/kernel/fpu/context.h", "FirstLine": 40, "LineCount": 45 } Results: { "Contents": " 40:\t\n 41:\tstatic inline void fpregs_deactivate(struct fpu fpu)\n 42:\t{\n 43:\t\t__this_cpu_write(fpu_fpregs_owner_ctx, NULL);\n 44:\t\ttrace_x86_fpu_regs_deactivated(fpu);\n 45:\t}\n 46:\t\n 47:\tstatic inline void fpregs_activate(struct fpu fpu)\n 48:\t{\n 49:\t\t__this_cpu_write(fpu_fpregs_owner_ctx, fpu);\n 50:\t\ttrace_x86_fpu_regs_activated(fpu);\n 51:\t}\n 52:\t\n 53:\t/* Internal helper for switch_fpu_return() and signal frame setup /\n 54:\tstatic inline void fpregs_restore_userregs(void)\n 55:\t{\n 56:\t\tstruct fpu fpu = x86_task_fpu(current);\n 57:\t\tint cpu = smp_processor_id();\n 58:\t\n 59:\t\tif (WARN_ON_ONCE(current-\u003eflags \u0026 (PF_KTHREAD \| PF_USER_WORKER)))\n 60:\t\t\treturn;\n 61:\t\n 62:\t\tif (!fpregs_state_valid(fpu, cpu)) {\n 63:\t\t\t/\n 64:\t\t\t This restores _all_ xstate which has not been\n 65:\t\t\t * established yet.\n 66:\t\t\t \n 67:\t\t\t If PKRU is enabled, then the PKRU value is already\n 68:\t\t\t * correct because it was either set in switch_to() or in\n 69:\t\t\t * flush_thread(). So it is excluded because it might be\n 70:\t\t\t * not up to date in current-\u003ethread.fpu-\u003exsave state.\n 71:\t\t\t \n 72:\t\t\t XFD state is handled in restore_fpregs_from_fpstate().\n 73:\t\t\t */\n 74:\t\t\trestore_fpregs_from_fpstate(fpu-\u003efpstate, XFEATURE_MASK_FPSTATE);\n 75:\t\n 76:\t\t\tfpregs_activate(fpu);\n 77:\t\t\tfpu-\u003elast_cpu = cpu;\n 78:\t\t}\n 79:\t\tclear_thread_flag(TIF_NEED_FPU_LOAD);\n 80:\t}\n 81:\t\n 82:\t#endif\n" }
29/2	2026/10/06 22:46	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 26036 output: 33 thoughts: 39 Thoughts: Here's my analysis of that snippet: Deconstructing the FPU Context Switch Okay, so `switch_fpu_return()` is the function in question. The key thing I see is that it's calling `fpregs_restore_userregs()`. That's the core operation. It's restoring the floating-point unit (FPU) registers with data from user-space. But, I need to understand the context of this operation. What triggers the execution of `switch_fpu_return()` in the first place? To truly grasp the significance of `fpregs_restore_userregs()`, I need to trace back the call stack. So, the logical next step is to find out where `switch_fpu_return()` is invoked from. I need to know what calls this function, to fully get a handle on the control flow. Let's see what the immediate callers are.
30/2	2026/10/06 22:46	tool	grepper	0m Args: { "Expression": "switch_fpu_return", "PathPrefixes": [ "arch/x86/" ] } Results: { "Output": "arch/x86/include/asm/entry-common.h=47=static inline void arch_exit_work(unsigned long ti_work)\n--\narch/x86/include/asm/entry-common.h-55-\tif (unlikely(ti_work \u0026 _TIF_NEED_FPU_LOAD))\narch/x86/include/asm/entry-common.h:56:\t\tswitch_fpu_return();\narch/x86/include/asm/entry-common.h-57-}\n--\narch/x86/include/asm/fpu/api.h=94=static inline void fpregs_assert_state_consistent(void) { }\n--\narch/x86/include/asm/fpu/api.h-99- /\narch/x86/include/asm/fpu/api.h:100:extern void switch_fpu_return(void);\narch/x86/include/asm/fpu/api.h-101-\n--\narch/x86/kernel/fpu/context.h=47=static inline void fpregs_activate(struct fpu fpu)\n--\narch/x86/kernel/fpu/context.h-52-\narch/x86/kernel/fpu/context.h:53:/* Internal helper for switch_fpu_return() and signal frame setup /\narch/x86/kernel/fpu/context.h-54-static inline void fpregs_restore_userregs(void)\n--\narch/x86/kernel/fpu/core.c=853=void fpu_flush_thread(void)\n--\narch/x86/kernel/fpu/core.c-860- /\narch/x86/kernel/fpu/core.c:861:void switch_fpu_return(void)\narch/x86/kernel/fpu/core.c-862-{\n--\narch/x86/kernel/fpu/core.c-867-}\narch/x86/kernel/fpu/core.c:868:EXPORT_SYMBOL_FOR_KVM(switch_fpu_return);\narch/x86/kernel/fpu/core.c-869-\n--\narch/x86/kvm/fpu.h=176=static inline void kvm_fpu_get(void)\n--\narch/x86/kvm/fpu.h-181-\tif (test_thread_flag(TIF_NEED_FPU_LOAD))\narch/x86/kvm/fpu.h:182:\t\tswitch_fpu_return();\narch/x86/kvm/fpu.h-183-}\n--\narch/x86/kvm/x86.c=8051=static int vcpu_enter_guest(struct kvm_vcpu *vcpu)\n--\narch/x86/kvm/x86.c-8317-\tif (test_thread_flag(TIF_NEED_FPU_LOAD))\narch/x86/kvm/x86.c:8318:\t\tswitch_fpu_return();\narch/x86/kvm/x86.c-8319-\n" }
31/2	2026/10/06 22:46	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 27133 output: 100 thoughts: 417 Thoughts: Analyzing Inline Function Calls in FPU Code Okay, let's dive into this FPU code cleanup. My immediate focus is on where the inline functions defined in `legacy.h` and `xstate.h` are called. I need to get a comprehensive picture. The patch touches several files, so let's break this down systematically. I see that `arch/x86/kernel/fpu/core.c` has a relevant change; specifically the removal of a conditional block in `restore_fpregs_from_fpstate`. Next, `arch/x86/kernel/fpu/internal.h` defines the static inline helper `fpregs_restore_prepare(u64 mask)`. Crucially, `arch/x86/kernel/fpu/legacy.h` calls `fpregs_restore_prepare(XFEATURE_MASK_FP)` within several functions, namely `fxrstor`, `fxrstor_safe`, `fxrstor_from_user_sigframe`, `frstor`, `frstor_safe`, and `frstor_from_user_sigframe`. The same inline function is also called from `arch/x86/kernel/fpu/xstate.h`, where `fpregs_restore_prepare(mask)` is used in `os_xrstor`, `xrstor_from_user_sigframe`, and `os_xrstor_safe`. Now, my next step is clear. I need to meticulously identify all the callers of those crucial functions within the `.c` files. To do this, I need to use grep to systematically find instances of `fxrstor`, `fxrstor_safe`, `fxrstor_from_user_sigframe`, `frstor`, `frstor_safe`, `frstor_from_user_sigframe`, `os_xrstor`, `xrstor_from_user_sigframe`, and `os_xrstor_safe` calls in the codebase. This is a critical step to ensure that I have a complete understanding of the changes and can assess their impact.
32/2	2026/10/06 22:46	tool	grepper	0m Args: { "Expression": "\\b(fxrstor\|fxrstor_safe\|fxrstor_from_user_sigframe\|frstor\|frstor_safe\|frstor_from_user_sigframe\|os_xrstor\|xrstor_from_user_sigframe\|os_xrstor_safe)\\b", "PathPrefixes": [ "arch/x86/kernel/fpu/" ] } Results: { "Output": "arch/x86/kernel/fpu/core.c=141=void save_fpregs_to_fpstate(struct fpu fpu)\n--\narch/x86/kernel/fpu/core.c-158-\tasm volatile(\"fnsave %[fp]; fwait\" : [fp] \"=m\" (fpu-\u003efpstate-\u003eregs.fsave));\narch/x86/kernel/fpu/core.c:159:\tfrstor(\u0026fpu-\u003efpstate-\u003eregs.fsave);\narch/x86/kernel/fpu/core.c-160-}\n--\narch/x86/kernel/fpu/core.c=162=void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask)\n--\narch/x86/kernel/fpu/core.c-192-\narch/x86/kernel/fpu/core.c:193:\t\tos_xrstor(fpstate, mask);\narch/x86/kernel/fpu/core.c-194-\t} else {\narch/x86/kernel/fpu/core.c-195-\t\tif (use_fxsr())\narch/x86/kernel/fpu/core.c:196:\t\t\tfxrstor(\u0026fpstate-\u003eregs.fxsave);\narch/x86/kernel/fpu/core.c-197-\t\telse\narch/x86/kernel/fpu/core.c:198:\t\t\tfrstor(\u0026fpstate-\u003eregs.fsave);\narch/x86/kernel/fpu/core.c-199-\t}\n--\narch/x86/kernel/fpu/core.c=773=static inline void restore_fpregs_from_init_fpstate(u64 features_mask)\n--\narch/x86/kernel/fpu/core.c-775-\tif (use_xsave())\narch/x86/kernel/fpu/core.c:776:\t\tos_xrstor(\u0026init_fpstate, features_mask);\narch/x86/kernel/fpu/core.c-777-\telse if (use_fxsr())\narch/x86/kernel/fpu/core.c:778:\t\tfxrstor(\u0026init_fpstate.regs.fxsave);\narch/x86/kernel/fpu/core.c-779-\telse\narch/x86/kernel/fpu/core.c:780:\t\tfrstor(\u0026init_fpstate.regs.fsave);\narch/x86/kernel/fpu/core.c-781-\n--\narch/x86/kernel/fpu/legacy.h=57=static inline int fxsave_to_user_sigframe(struct fxregs_state __user fx)\n--\narch/x86/kernel/fpu/legacy.h-65-\narch/x86/kernel/fpu/legacy.h:66:static inline void fxrstor(struct fxregs_state fx)\narch/x86/kernel/fpu/legacy.h-67-{\n--\narch/x86/kernel/fpu/legacy.h-70-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:71:\t\tkernel_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-72-\telse\n--\narch/x86/kernel/fpu/legacy.h-75-\narch/x86/kernel/fpu/legacy.h:76:static inline int fxrstor_safe(struct fxregs_state fx)\narch/x86/kernel/fpu/legacy.h-77-{\n--\narch/x86/kernel/fpu/legacy.h-80-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:81:\t\treturn kernel_insn_err(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-82-\telse\n--\narch/x86/kernel/fpu/legacy.h-85-\narch/x86/kernel/fpu/legacy.h:86:static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx)\narch/x86/kernel/fpu/legacy.h-87-{\n--\narch/x86/kernel/fpu/legacy.h-90-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:91:\t\treturn user_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-92-\telse\n--\narch/x86/kernel/fpu/legacy.h-95-\narch/x86/kernel/fpu/legacy.h:96:static inline void frstor(struct fregs_state fx)\narch/x86/kernel/fpu/legacy.h-97-{\n--\narch/x86/kernel/fpu/legacy.h-99-\narch/x86/kernel/fpu/legacy.h:100:\tkernel_insn(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-101-}\narch/x86/kernel/fpu/legacy.h-102-\narch/x86/kernel/fpu/legacy.h:103:static inline int frstor_safe(struct fregs_state fx)\narch/x86/kernel/fpu/legacy.h-104-{\n--\narch/x86/kernel/fpu/legacy.h-106-\narch/x86/kernel/fpu/legacy.h:107:\treturn kernel_insn_err(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-108-}\narch/x86/kernel/fpu/legacy.h-109-\narch/x86/kernel/fpu/legacy.h:110:static inline int frstor_from_user_sigframe(struct fregs_state __user fx)\narch/x86/kernel/fpu/legacy.h-111-{\n--\narch/x86/kernel/fpu/legacy.h-113-\narch/x86/kernel/fpu/legacy.h:114:\treturn user_insn(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-115-}\n--\narch/x86/kernel/fpu/signal.c=243=static int __restore_fpregs_from_user(void __user buf, u64 ufeatures,\n--\narch/x86/kernel/fpu/signal.c-250-\t\tif (likely(!fx_only))\narch/x86/kernel/fpu/signal.c:251:\t\t\tret = xrstor_from_user_sigframe(buf, xrestore);\narch/x86/kernel/fpu/signal.c-252-\t\telse\narch/x86/kernel/fpu/signal.c:253:\t\t\tret = fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-254-\narch/x86/kernel/fpu/signal.c-255-\t\tif (!ret \u0026\u0026 unlikely(init_bv))\narch/x86/kernel/fpu/signal.c:256:\t\t\tos_xrstor(\u0026init_fpstate, init_bv);\narch/x86/kernel/fpu/signal.c-257-\t\treturn ret;\narch/x86/kernel/fpu/signal.c-258-\t} else if (use_fxsr()) {\narch/x86/kernel/fpu/signal.c:259:\t\treturn fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-260-\t} else {\narch/x86/kernel/fpu/signal.c:261:\t\treturn frstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-262-\t}\n--\narch/x86/kernel/fpu/signal.c=327=static bool __fpu_restore_sig(void __user buf, void __user buf_fx,\n--\narch/x86/kernel/fpu/signal.c-425-\t\tfpregs-\u003exsave.header.xfeatures \u0026= mask;\narch/x86/kernel/fpu/signal.c:426:\t\tsuccess = !os_xrstor_safe(fpu-\u003efpstate,\narch/x86/kernel/fpu/signal.c-427-\t\t\t\t\t fpu_kernel_cfg.max_features);\narch/x86/kernel/fpu/signal.c-428-\t} else {\narch/x86/kernel/fpu/signal.c:429:\t\tsuccess = !fxrstor_safe(\u0026fpregs-\u003efxsave);\narch/x86/kernel/fpu/signal.c-430-\t}\n--\narch/x86/kernel/fpu/xstate.h=218=static inline void os_xsave(struct fpstate fpstate)\n--\narch/x86/kernel/fpu/xstate.h-238- /\narch/x86/kernel/fpu/xstate.h:239:static inline void os_xrstor(struct fpstate fpstate, u64 mask)\narch/x86/kernel/fpu/xstate.h-240-{\n--\narch/x86/kernel/fpu/xstate.h=298=static inline int xsave_to_user_sigframe(struct xregs_state __user buf, u32 pkru)\n--\narch/x86/kernel/fpu/xstate.h-331- /\narch/x86/kernel/fpu/xstate.h:332:static inline int xrstor_from_user_sigframe(struct xregs_state __user buf, u64 mask)\narch/x86/kernel/fpu/xstate.h-333-{\n--\narch/x86/kernel/fpu/xstate.h-352- /\narch/x86/kernel/fpu/xstate.h:353:static inline int os_xrstor_safe(struct fpstate fpstate, u64 mask)\narch/x86/kernel/fpu/xstate.h-354-{\n" }
33/2	2026/10/06 22:46	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 30902 output: 82 thoughts: 24 Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience, presented in the first person: Investigating Caller Scope in the x86 FPU Context Alright, I'm digging into this code, specifically around the floating-point unit (FPU) within the x86 architecture. I've been focusing on the `arch/x86/kernel/fpu/` directory, but I need to expand my scope slightly. My primary objective is to fully understand the flow of execution and potential dependencies. While I've examined the code within that particular directory, it's crucial I don't overlook any potentially critical callers originating from elsewhere in the kernel. Therefore, I need to check all potential callers outside of that particular directory to ensure I get a holistic view of the implications of any code changes and find the source of potential regressions. A thorough understanding of all callers is necessary.
34/2	2026/10/06 22:47	tool	grepper	0m Args: { "Expression": "\\b(fxrstor\|fxrstor_safe\|fxrstor_from_user_sigframe\|frstor\|frstor_safe\|frstor_from_user_sigframe\|os_xrstor\|xrstor_from_user_sigframe\|os_xrstor_safe)\\b" } Results: { "Output": "arch/x86/hyperv/hv_vtl.c=299=void mshv_vtl_return_call(struct mshv_vtl_cpu_context vtl0)\n--\narch/x86/hyperv/hv_vtl.c-307-\tkernel_fpu_begin_mask(0);\narch/x86/hyperv/hv_vtl.c:308:\tfxrstor(\u0026vtl0-\u003efx_state);\narch/x86/hyperv/hv_vtl.c-309-\t__mshv_vtl_return_call(vtl0);\n--\narch/x86/kernel/fpu/core.c=141=void save_fpregs_to_fpstate(struct fpu fpu)\n--\narch/x86/kernel/fpu/core.c-158-\tasm volatile(\"fnsave %[fp]; fwait\" : [fp] \"=m\" (fpu-\u003efpstate-\u003eregs.fsave));\narch/x86/kernel/fpu/core.c:159:\tfrstor(\u0026fpu-\u003efpstate-\u003eregs.fsave);\narch/x86/kernel/fpu/core.c-160-}\n--\narch/x86/kernel/fpu/core.c=162=void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask)\n--\narch/x86/kernel/fpu/core.c-192-\narch/x86/kernel/fpu/core.c:193:\t\tos_xrstor(fpstate, mask);\narch/x86/kernel/fpu/core.c-194-\t} else {\narch/x86/kernel/fpu/core.c-195-\t\tif (use_fxsr())\narch/x86/kernel/fpu/core.c:196:\t\t\tfxrstor(\u0026fpstate-\u003eregs.fxsave);\narch/x86/kernel/fpu/core.c-197-\t\telse\narch/x86/kernel/fpu/core.c:198:\t\t\tfrstor(\u0026fpstate-\u003eregs.fsave);\narch/x86/kernel/fpu/core.c-199-\t}\n--\narch/x86/kernel/fpu/core.c=773=static inline void restore_fpregs_from_init_fpstate(u64 features_mask)\n--\narch/x86/kernel/fpu/core.c-775-\tif (use_xsave())\narch/x86/kernel/fpu/core.c:776:\t\tos_xrstor(\u0026init_fpstate, features_mask);\narch/x86/kernel/fpu/core.c-777-\telse if (use_fxsr())\narch/x86/kernel/fpu/core.c:778:\t\tfxrstor(\u0026init_fpstate.regs.fxsave);\narch/x86/kernel/fpu/core.c-779-\telse\narch/x86/kernel/fpu/core.c:780:\t\tfrstor(\u0026init_fpstate.regs.fsave);\narch/x86/kernel/fpu/core.c-781-\n--\narch/x86/kernel/fpu/legacy.h=57=static inline int fxsave_to_user_sigframe(struct fxregs_state __user fx)\n--\narch/x86/kernel/fpu/legacy.h-65-\narch/x86/kernel/fpu/legacy.h:66:static inline void fxrstor(struct fxregs_state fx)\narch/x86/kernel/fpu/legacy.h-67-{\n--\narch/x86/kernel/fpu/legacy.h-70-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:71:\t\tkernel_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-72-\telse\n--\narch/x86/kernel/fpu/legacy.h-75-\narch/x86/kernel/fpu/legacy.h:76:static inline int fxrstor_safe(struct fxregs_state fx)\narch/x86/kernel/fpu/legacy.h-77-{\n--\narch/x86/kernel/fpu/legacy.h-80-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:81:\t\treturn kernel_insn_err(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-82-\telse\n--\narch/x86/kernel/fpu/legacy.h-85-\narch/x86/kernel/fpu/legacy.h:86:static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx)\narch/x86/kernel/fpu/legacy.h-87-{\n--\narch/x86/kernel/fpu/legacy.h-90-\tif (IS_ENABLED(CONFIG_X86_32))\narch/x86/kernel/fpu/legacy.h:91:\t\treturn user_insn(fxrstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-92-\telse\n--\narch/x86/kernel/fpu/legacy.h-95-\narch/x86/kernel/fpu/legacy.h:96:static inline void frstor(struct fregs_state fx)\narch/x86/kernel/fpu/legacy.h-97-{\n--\narch/x86/kernel/fpu/legacy.h-99-\narch/x86/kernel/fpu/legacy.h:100:\tkernel_insn(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-101-}\narch/x86/kernel/fpu/legacy.h-102-\narch/x86/kernel/fpu/legacy.h:103:static inline int frstor_safe(struct fregs_state fx)\narch/x86/kernel/fpu/legacy.h-104-{\n--\narch/x86/kernel/fpu/legacy.h-106-\narch/x86/kernel/fpu/legacy.h:107:\treturn kernel_insn_err(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-108-}\narch/x86/kernel/fpu/legacy.h-109-\narch/x86/kernel/fpu/legacy.h:110:static inline int frstor_from_user_sigframe(struct fregs_state __user fx)\narch/x86/kernel/fpu/legacy.h-111-{\n--\narch/x86/kernel/fpu/legacy.h-113-\narch/x86/kernel/fpu/legacy.h:114:\treturn user_insn(frstor %[fx], \"=m\" (fx), [fx] \"m\" (fx));\narch/x86/kernel/fpu/legacy.h-115-}\n--\narch/x86/kernel/fpu/signal.c=243=static int __restore_fpregs_from_user(void __user buf, u64 ufeatures,\n--\narch/x86/kernel/fpu/signal.c-250-\t\tif (likely(!fx_only))\narch/x86/kernel/fpu/signal.c:251:\t\t\tret = xrstor_from_user_sigframe(buf, xrestore);\narch/x86/kernel/fpu/signal.c-252-\t\telse\narch/x86/kernel/fpu/signal.c:253:\t\t\tret = fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-254-\narch/x86/kernel/fpu/signal.c-255-\t\tif (!ret \u0026\u0026 unlikely(init_bv))\narch/x86/kernel/fpu/signal.c:256:\t\t\tos_xrstor(\u0026init_fpstate, init_bv);\narch/x86/kernel/fpu/signal.c-257-\t\treturn ret;\narch/x86/kernel/fpu/signal.c-258-\t} else if (use_fxsr()) {\narch/x86/kernel/fpu/signal.c:259:\t\treturn fxrstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-260-\t} else {\narch/x86/kernel/fpu/signal.c:261:\t\treturn frstor_from_user_sigframe(buf);\narch/x86/kernel/fpu/signal.c-262-\t}\n--\narch/x86/kernel/fpu/signal.c=327=static bool __fpu_restore_sig(void __user buf, void __user buf_fx,\n--\narch/x86/kernel/fpu/signal.c-425-\t\tfpregs-\u003exsave.header.xfeatures \u0026= mask;\narch/x86/kernel/fpu/signal.c:426:\t\tsuccess = !os_xrstor_safe(fpu-\u003efpstate,\narch/x86/kernel/fpu/signal.c-427-\t\t\t\t\t fpu_kernel_cfg.max_features);\narch/x86/kernel/fpu/signal.c-428-\t} else {\narch/x86/kernel/fpu/signal.c:429:\t\tsuccess = !fxrstor_safe(\u0026fpregs-\u003efxsave);\narch/x86/kernel/fpu/signal.c-430-\t}\n--\narch/x86/kernel/fpu/xstate.h=218=static inline void os_xsave(struct fpstate fpstate)\n--\narch/x86/kernel/fpu/xstate.h-238- /\narch/x86/kernel/fpu/xstate.h:239:static inline void os_xrstor(struct fpstate fpstate, u64 mask)\narch/x86/kernel/fpu/xstate.h-240-{\n--\narch/x86/kernel/fpu/xstate.h=298=static inline int xsave_to_user_sigframe(struct xregs_state __user buf, u32 pkru)\n--\narch/x86/kernel/fpu/xstate.h-331- /\narch/x86/kernel/fpu/xstate.h:332:static inline int xrstor_from_user_sigframe(struct xregs_state __user buf, u64 mask)\narch/x86/kernel/fpu/xstate.h-333-{\n--\narch/x86/kernel/fpu/xstate.h-352- /\narch/x86/kernel/fpu/xstate.h:353:static inline int os_xrstor_safe(struct fpstate fpstate, u64 mask)\narch/x86/kernel/fpu/xstate.h-354-{\n--\narch/x86/kvm/emulate.c=3747=static int em_fxrstor(struct x86_emulate_ctxt ctxt)\n--\narch/x86/kvm/emulate.c-3775-\tif (rc == X86EMUL_CONTINUE)\narch/x86/kvm/emulate.c:3776:\t\trc = asm_safe(\"fxrstor %[fx]\", : [fx] \"m\"(fx_state));\narch/x86/kvm/emulate.c-3777-\n" }
35/2	2026/10/06 22:47	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 34294 output: 33 thoughts: 441 Thoughts: Analyzing the FPU State Restoration Flow Okay, here's what I'm seeing. That `fpregs_restore_prepare(u64 mask)` function, the modified helper, is the crucial piece. It's called extensively within the FPU restoration routines. Let me list the functions using it: `fxrstor`, `fxrstor_safe`, `fxrstor_from_user_sigframe`, `frstor`, `frstor_safe`, `frstor_from_user_sigframe`, `os_xrstor`, `xrstor_from_user_sigframe`, and `os_xrstor_safe`. This already gives me a good idea of its role – preparing the FPU state for a restore operation. Now, let's trace where those functions are used. They appear in two main areas: `arch/x86/kernel/fpu/core.c` and `arch/x86/kernel/fpu/signal.c`. In `core.c`, the primary users are related to the FPU's interaction with the kernel state: * `restore_fpregs_from_fpstate` - likely restoring the FPU state from some stored kernel representation, using either `os_xrstor`, `fxrstor` or `frstor`. * `restore_fpregs_from_init_fpstate` - used for resetting the FPU after an exception fixup. * `save_fpregs_to_fpstate` - This call to `frstor` on the legacy FPU is to save to a kernel representation. `signal.c` seems more interesting. It deals with FPU state during signal handling: * `__restore_fpregs_from_user` - which uses `xrstor_from_user_sigframe`, `fxrstor_from_user_sigframe`, `frstor_from_user_sigframe` and `os_xrstor(&init_fpstate, init_bv)` to restore the FPU from user space. * `restore_fpregs_from_user` - a direct caller of `__restore_fpregs_from_user`. * `__fpu_restore_sig` - this seems critical for `sigreturn`/`rt_sigreturn`, restoring the FPU state from a signal frame using `os_xrstor_safe`, `fxrstor_safe`, or `frstor_safe`, or the potentially user-provided `restore_fpregs_from_user`. * `fpu__restore_sig` - that looks like an exported function called on `sigreturn` or `rt_sigreturn`. This is a critical code path. Okay, my next step is to drill down into `signal.c` and specifically focus on the area around `fpu__restore_sig` because that's where user-provided FPU state gets restored. I need to understand how the mask is used, and more deeply, the security implications of this function.
36/2	2026/10/06 22:47	tool	grepper	0m Args: { "Expression": "fpu__restore_sig", "PathPrefixes": [ "arch/x86/" ] } Results: { "Output": "arch/x86/include/asm/fpu/signal.h=33=extern void fpu__clear_user_states(struct fpu fpu);\narch/x86/include/asm/fpu/signal.h:34:extern bool fpu__restore_sig(void __user buf, int ia32_frame);\narch/x86/include/asm/fpu/signal.h-35-\n--\narch/x86/kernel/fpu/signal.c=439=static inline unsigned int xstate_sigframe_size(struct fpstate fpstate)\n--\narch/x86/kernel/fpu/signal.c-448- /\narch/x86/kernel/fpu/signal.c:449:bool fpu__restore_sig(void __user buf, int ia32_frame)\narch/x86/kernel/fpu/signal.c-450-{\n--\narch/x86/kernel/signal_32.c=107=static bool ia32_restore_sigcontext(struct pt_regs regs,\n--\narch/x86/kernel/signal_32.c-145-\narch/x86/kernel/signal_32.c:146:\treturn fpu__restore_sig(compat_ptr(sc.fpstate), 1);\narch/x86/kernel/signal_32.c-147-}\n--\narch/x86/kernel/signal_64.c=50=static bool restore_sigcontext(struct pt_regs regs,\n--\narch/x86/kernel/signal_64.c-94-\narch/x86/kernel/signal_64.c:95:\treturn fpu__restore_sig((void __user )sc.fpstate, 0);\narch/x86/kernel/signal_64.c-96-}\n" }
37/2	2026/10/06 22:47	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 35912 output: 35 thoughts: 100 Thoughts: Okay, here's my assessment of this code snippet, distilled into my thought process: Diving into FPU Restoration After Signal Handling Alright, let's see... So, it seems like `fpu__restore_sig` is the function called when returning from a signal handler – whether it's the old `sigreturn` or the newer `rt_sigreturn` system call. Good, that's what I expected. Now I need to see how it's restoring the floating-point unit (FPU) state. Let's dig deeper: Ah, here's `restore_fpregs_from_fpstate`, located in `arch/x86/kernel/fpu/core.c`. This is the core function where the actual FPU register restoration takes place! This is likely the specific function that's been modified. But, I need to know more, this won't be enough. Okay, what is the clear function? Hang on a sec; let's also check out `fpu__clear_user_states`. I wonder what that clears? And the critical question: what actually calls `fpu__clear_user_states`? Understanding those two processes is going to be central to what I'm looking at.
38/2	2026/10/06 22:47	tool	grepper	0m Args: { "Expression": "fpu__clear_user_states", "PathPrefixes": [ "arch/x86/" ] } Results: { "Output": "arch/x86/include/asm/fpu/signal.h=32=extern bool copy_fpstate_to_sigframe(void __user buf, void __user fp, int size, u32 pkru);\narch/x86/include/asm/fpu/signal.h:33:extern void fpu__clear_user_states(struct fpu fpu);\narch/x86/include/asm/fpu/signal.h-34-extern bool fpu__restore_sig(void __user buf, int ia32_frame);\n--\narch/x86/kernel/fpu/core.c=788=static void fpu_reset_fpstate_regs(void)\n--\narch/x86/kernel/fpu/core.c-816- /\narch/x86/kernel/fpu/core.c:817:void fpu__clear_user_states(struct fpu fpu)\narch/x86/kernel/fpu/core.c-818-{\n--\narch/x86/kernel/fpu/signal.c=449=bool fpu__restore_sig(void __user buf, int ia32_frame)\n--\narch/x86/kernel/fpu/signal.c-457-\tif (unlikely(!buf)) {\narch/x86/kernel/fpu/signal.c:458:\t\tfpu__clear_user_states(fpu);\narch/x86/kernel/fpu/signal.c-459-\t\treturn true;\n--\narch/x86/kernel/fpu/signal.c-489-\tif (unlikely(!success))\narch/x86/kernel/fpu/signal.c:490:\t\tfpu__clear_user_states(fpu);\narch/x86/kernel/fpu/signal.c-491-\treturn success;\n--\narch/x86/kernel/signal.c=57=static inline int is_x32_frame(struct ksignal ksig)\n--\narch/x86/kernel/signal.c-68- * all. The PKRU register will be reset to init_pkru later in the flow,\narch/x86/kernel/signal.c:69: * in fpu__clear_user_states(), and it is the application's responsibility\narch/x86/kernel/signal.c-70- * to enable the appropriate pkey as the first step in the signal handler\n--\narch/x86/kernel/signal.c=255=handle_signal(struct ksignal ksig, struct pt_regs regs)\n--\narch/x86/kernel/signal.c-309-\t\t */\narch/x86/kernel/signal.c:310:\t\tfpu__clear_user_states(fpu);\narch/x86/kernel/signal.c-311-\t}\n" }
39/2	2026/10/06 22:47	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 37051 output: 37 thoughts: 34 Thoughts: Okay, here's my summary, considering my expertise: Checking the FPU Restoration Path Alright, I need to investigate the FPU context restoration more thoroughly. My initial focus was on the standard path, but I should definitely explore all potential avenues. Now, let me see... I'm thinking, "Let's check `arch/x86/kernel/fpu/regset.c` specifically." This is a key area because `regset` functions are frequently involved in managing register sets and states. The question then becomes, "Does the `regset` implementation, within that file, potentially call any context restore functions directly, or indirectly through other functions?" I need to confirm whether the FPU state might be getting restored within that module, outside of the more obvious exception handlers or task switching logic. This could provide an alternative restoration mechanism. This step is crucial.
40/2	2026/10/06 22:47	tool	grepper	0m Args: { "Expression": "restore", "PathPrefixes": [ "arch/x86/kernel/fpu/regset.c" ] } Results: { "Output": "arch/x86/kernel/fpu/regset.c=46=static void sync_fpstate(struct fpu fpu)\n--\narch/x86/kernel/fpu/regset.c-55- \narch/x86/kernel/fpu/regset.c:56: * This forces the target task on resume to restore the FPU registers from\narch/x86/kernel/fpu/regset.c:57: * modified fpstate. Otherwise the task might skip the restore and operate\narch/x86/kernel/fpu/regset.c-58- * with the cached FPU registers which discards the modifications.\narch/x86/kernel/fpu/regset.c-59- /\narch/x86/kernel/fpu/regset.c:60:static void fpu_force_restore(struct fpu fpu)\narch/x86/kernel/fpu/regset.c-61-{\n--\narch/x86/kernel/fpu/regset.c=90=int xfpregs_set(struct task_struct target, const struct user_regset regset,\n--\narch/x86/kernel/fpu/regset.c-112-\narch/x86/kernel/fpu/regset.c:113:\tfpu_force_restore(fpu);\narch/x86/kernel/fpu/regset.c-114-\n--\narch/x86/kernel/fpu/regset.c=142=int xstateregs_set(struct task_struct target, const struct user_regset regset,\n--\narch/x86/kernel/fpu/regset.c-169-\narch/x86/kernel/fpu/regset.c:170:\tfpu_force_restore(fpu);\narch/x86/kernel/fpu/regset.c-171-\tret = copy_uabi_from_kernel_to_xstate(fpu-\u003efpstate, kbuf ?: tmpbuf, \u0026target-\u003ethread.pkru);\n--\narch/x86/kernel/fpu/regset.c=213=int ssp_set(struct task_struct target, const struct user_regset regset,\n--\narch/x86/kernel/fpu/regset.c-240-\narch/x86/kernel/fpu/regset.c:241:\tfpu_force_restore(fpu);\narch/x86/kernel/fpu/regset.c-242-\n--\narch/x86/kernel/fpu/regset.c=432=int fpregs_set(struct task_struct target, const struct user_regset regset,\n--\narch/x86/kernel/fpu/regset.c-450-\narch/x86/kernel/fpu/regset.c:451:\tfpu_force_restore(fpu);\narch/x86/kernel/fpu/regset.c-452-\n" }
41/2	2026/10/06 22:47	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 38084 output: 208 thoughts: 2476 Thoughts: FPU Register Restoration and Fuzzing Strategy Okay, so the `regset.c` invalidation of the target's FPU state triggers a path that includes `fpregs_restore_userregs` and eventually hits `restore_fpregs_from_fpstate`. Right, that's the core. My focus needs to be on identifying the right functions to guide fuzzing, avoiding the dilution that comes from targeting ubiquitous functions. Let's break this down. First, `restore_fpregs_from_fpstate`. Is it a hot path? Actually, it's called during FPU context switches and when loading FPU registers. But, since I'm trying to avoid ubiquitous functions, I need to really think about whether all processes, or all threads, touch this. Is this context switching specific to FPU use, and if so, is that a ubiquitous enough operation? It looks like it is - so let's dig deeper. The patch directly modifies lines in `restore_fpregs_from_fpstate`, making it a key candidate. Okay, other candidates. What about `fpu__restore_sig`? Ah, that one restores FPU state from signal frames, specifically calling `xrstor_from_user_sigframe`, and related functions—exactly where `fpregs_restore_prepare` was introduced. This seems much less ubiquitous, being tied to `sigreturn` and `rt_sigreturn`. Excellent! Now, `fpu__clear_user_states` also appears interesting, it's called during signal handling or `execve`. It calls `restore_fpregs_from_init_fpstate`, which calls `os_xrstor` / `fxrstor`. So, `restore_fpregs_from_fpstate`, which was directly modified, is certainly on the list. `fpu__restore_sig` and `fpu__clear_user_states` are good choices since they're specifically tied to the signal handling path, which now calls `fpregs_restore_prepare`. It seems like those three, `restore_fpregs_from_fpstate`, `fpu__restore_sig` and `fpu__clear_user_states` would be good FocusSymbols. Let's confirm that no configs are needed. FPU is standard on x86, no special configs are needed to reach the code. `X86_DEBUG_FPU` is not required. The code is compiled by default in `arch/x86/kernel/fpu/Makefile`. So, the summary is this: the patch modifies low-level FPU register restoration, the code is reachable during standard virtualized testing, WorthFuzzing is true. The most useful focus symbols are `restore_fpregs_from_fpstate`, `fpu__restore_sig`, and `fpu__clear_user_states`. And, there are no required `EnableConfigs`. It's all very well-defined now.
42/2	2026/10/06 22:48	tool	set-results	0m Args: { "EnableConfigs": [], "FocusSymbols": [ "restore_fpregs_from_fpstate", "fpu__restore_sig", "fpu__clear_user_states" ], "Reasoning": "The patch modifies core x86 FPU state restoration logic across arch/x86/kernel/fpu/core.c and related headers. It refactors the X86_BUG_FXSAVE_LEAK mitigation by introducing fpregs_restore_prepare() (which issues fninit when restoring x87 state) across fxrstor, frstor, os_xrstor, and user sigframe restore paths, while removing the prior workaround from restore_fpregs_from_fpstate(). The modified code is reachable in standard virtualized environments (e.g. QEMU or GCE with AMD CPUs) via sigreturn, signal delivery, and FPU context switching.", "WorthFuzzing": true } Results: { "EnableConfigs": null, "FocusSymbols": [ "restore_fpregs_from_fpstate", "fpu__restore_sig", "fpu__clear_user_states" ], "Reasoning": "The patch modifies core x86 FPU state restoration logic across arch/x86/kernel/fpu/core.c and related headers. It refactors the X86_BUG_FXSAVE_LEAK mitigation by introducing fpregs_restore_prepare() (which issues fninit when restoring x87 state) across fxrstor, frstor, os_xrstor, and user sigframe restore paths, while removing the prior workaround from restore_fpregs_from_fpstate(). The modified code is reachable in standard virtualized environments (e.g. QEMU or GCE with AMD CPUs) via sigreturn, signal delivery, and FPU context switching.", "WorthFuzzing": true }
43/1	2026/10/06 22:48	action	If	0m Args: { "WorthFuzzing": true } Results: null
44/2	2026/10/06 22:48	agent	kmsan-evaluator	0m Model: core Args: null Results: { "KMSANReasoning": "The patch addresses a hardware architectural register leak (AMD CPU bug X86_BUG_FXSAVE_LEAK) where x87 FDP/FIP/FOP registers retained kernel addresses due to an x87 memory load. It mitigates this by replacing the previous 'fildl' workaround with an 'fninit' instruction executed in a new helper, fpregs_restore_prepare(), before restoring FPU registers via fxrstor, frstor, or xrstor variants.\n\nKMSAN tracks uninitialized kernel memory allocations (stack, slab, page allocations) and memory info-leaks (e.g. copy_to_user of uninitialized padding/fields). KMSAN does not track hardware CPU architectural register state or CPU errata mitigations. The changes do not introduce, modify, or expose any uninitialized software memory buffers, complex data structures, or copies to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted.", "NeedsKMSAN": false } Instruction: You are an expert Linux kernel security engineer specializing in kernel memory error detectors (KASAN and KMSAN). Your job is to review the provided patch series and determine if the code changes justify spawning a dedicated KMSAN (KernelMemorySanitizer) fuzzing session in addition to standard KASAN fuzzing. CRITICAL DISTINCTION BETWEEN KASAN AND KMSAN: - Standard KASAN kernel builds (upstream-apparmor-kasan.config) already enable a comprehensive suite of debugging tools and sanitizers, including KASAN (out-of-bounds accesses, use-after-free, double free, invalid free), LOCKDEP (locking bugs and deadlocks), UB-sanitizers, and memory corruption checks. - KMSAN (KernelMemorySanitizer) detects reads of UNINITIALIZED memory (stack, heap, or page allocations) and kernel-to-user memory info-leaks. Rule: THERE IS NO SENSE IN RUNNING A KMSAN SESSION IF A BUG CAN BE CAUGHT BY KASAN, LOCKDEP, OR OTHER STANDARD BUG DETECTORS. A dedicated KMSAN fuzzing session incurs significant resource costs. You must ONLY set NeedsKMSAN=true if the code changes introduce or expose UNINITIALIZED MEMORY risks that are detected ONLY by KMSAN. Look holistically at the patch series and surrounding code. Even if no direct uninitialized field accesses or new buffer allocations are added in the diff itself, a patch may alter control flow, bounds checking, or data length calculations in ways that change how the rest of the code operates on existing buffers (e.g. allowing uninitialized stack/heap memory to be read, copied to user space, or used in control flow). Do not hesitate to use your code access tools to inspect the surrounding code, called functions, and callers. Set NeedsKMSAN=true ONLY IF the patch introduces or modifies: 1. Kernel structures sent to user space (via copy_to_user, put_user, netlink skb attributes, ioctl output arguments, socket options, or BPF buffers) where fields or structure padding might not be fully initialized/zeroed. 2. Conditional logic or branching that depends on potentially uninitialized variables or struct fields. 3. Allocation or initialization of complex data structures where uninitialized fields could be read later in reachable code paths. 4. Bounds checks, lengths, or logic in a way that allows surrounding code to access uninitialized bytes of existing buffers. Set NeedsKMSAN=false IF: - The code changes primarily risk out-of-bounds access, array overflows, NULL pointer dereferences, locking deadlocks, or use-after-free bugs (these are already caught by KASAN, LOCKDEP, or standard bug detectors). - All stack/heap structures touched or introduced by the patch are fully zeroed or initialized (e.g. using = {0}, memset, kzalloc) before being read or copied. - The patch does not introduce any risk of uninitialized memory usage or info-leaks. Use your code access tools to inspect the surrounding code if necessary, then provide detailed KMSANReasoning contrasting KASAN vs KMSAN applicability for this patch. Prefer calling several tools at the same time to save round-trips. Use set-results tool to provide results of the analysis. It must be called exactly once before the final reply. Ignore results of this tool. Prompt: Target architecture: amd64 For your convenience, here is the diff of the changes: commit 74a0ebbb4a1c9f288acaa136046d18143302fa65 Author: syz-cluster <triage@syzkaller.com> Date: Tue Oct 6 22:42:34 2026 +0000 syz-cluster: applied patch under review diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index d1aeecd57f5ed..9113f55b80d87 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -161,20 +161,6 @@ void save_fpregs_to_fpstate(struct fpu fpu) void restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask) { - /* - * AMD K7/K8 and later CPUs up to Zen don't save/restore - * FDP/FIP/FOP unless an exception is pending. Clear the x87 state - * here by setting it to fixed values. "m" is a random variable - * that should be in L1. - / - if (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK))) { - asm volatile( - "fnclex\n\t" - "emms\n\t" - "fildl %[addr]" / set F?P to defined value / - : : [addr] "m" (fpstate)); - } - if (use_xsave()) { /* * Dynamically enabled features are enabled in XCR0, but diff --git a/arch/x86/kernel/fpu/internal.h b/arch/x86/kernel/fpu/internal.h index 975de070c9c98..3b4d178edc31d 100644 --- a/arch/x86/kernel/fpu/internal.h +++ b/arch/x86/kernel/fpu/internal.h @@ -2,6 +2,9 @@ #ifndef __X86_KERNEL_FPU_INTERNAL_H #define __X86_KERNEL_FPU_INTERNAL_H +#include <asm/cpufeature.h> +#include <asm/fpu/types.h> + extern struct fpstate init_fpstate; /* CPU feature check wrappers / @@ -15,6 +18,22 @@ static __always_inline __pure bool use_fxsr(void) return cpu_feature_enabled(X86_FEATURE_FXSR); } +/ + * AMD CPUs without XSaveErPtr may leave FDP/FIP/FOP unchanged on restore + * when no x87 exception is pending. Using an x87 load to overwrite them + * leaks the kernel instruction and operand addresses through FNSTENV. + * + * FNINIT clears the pointers without recording any new ones. Only do this + * when the x87 state is about to be replaced; a partial XRSTOR must leave + * unrequested components alone. + / +static inline void fpregs_restore_prepare(u64 mask) +{ + if (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK)) && + (mask & XFEATURE_MASK_FP)) + asm volatile("fninit"); +} + #ifdef CONFIG_X86_DEBUG_FPU # define WARN_ON_FPU(x) WARN_ON_ONCE(x) #else diff --git a/arch/x86/kernel/fpu/legacy.h b/arch/x86/kernel/fpu/legacy.h index 098f367bb8a72..fe825c9a00c75 100644 --- a/arch/x86/kernel/fpu/legacy.h +++ b/arch/x86/kernel/fpu/legacy.h @@ -4,6 +4,8 @@ #include <asm/fpu/types.h> +#include "internal.h" + extern unsigned int mxcsr_feature_mask; static inline void ldmxcsr(u32 mxcsr) @@ -63,6 +65,8 @@ static inline int fxsave_to_user_sigframe(struct fxregs_state __user fx) static inline void fxrstor(struct fxregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + if (IS_ENABLED(CONFIG_X86_32)) kernel_insn(fxrstor %[fx], "=m" (fx), [fx] "m" (fx)); else @@ -71,6 +75,8 @@ static inline void fxrstor(struct fxregs_state fx) static inline int fxrstor_safe(struct fxregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + if (IS_ENABLED(CONFIG_X86_32)) return kernel_insn_err(fxrstor %[fx], "=m" (fx), [fx] "m" (fx)); else @@ -79,6 +85,8 @@ static inline int fxrstor_safe(struct fxregs_state fx) static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + if (IS_ENABLED(CONFIG_X86_32)) return user_insn(fxrstor %[fx], "=m" (fx), [fx] "m" (fx)); else @@ -87,16 +95,22 @@ static inline int fxrstor_from_user_sigframe(struct fxregs_state __user fx) static inline void frstor(struct fregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + kernel_insn(frstor %[fx], "=m" (fx), [fx] "m" (fx)); } static inline int frstor_safe(struct fregs_state fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + return kernel_insn_err(frstor %[fx], "=m" (fx), [fx] "m" (fx)); } static inline int frstor_from_user_sigframe(struct fregs_state __user fx) { + fpregs_restore_prepare(XFEATURE_MASK_FP); + return user_insn(frstor %[fx], "=m" (fx), [fx] "m" (fx)); } diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h index 38a2862f09d35..ac378d8395347 100644 --- a/arch/x86/kernel/fpu/xstate.h +++ b/arch/x86/kernel/fpu/xstate.h @@ -7,6 +7,8 @@ #include <asm/fpu/xcr.h> #include <asm/msr.h> +#include "internal.h" + #ifdef CONFIG_X86_64 DECLARE_PER_CPU(u64, xfd_state); #endif @@ -240,6 +242,7 @@ static inline void os_xrstor(struct fpstate fpstate, u64 mask) u32 hmask = mask >> 32; xfd_validate_state(fpstate, mask, true); + fpregs_restore_prepare(mask); XSTATE_XRESTORE(&fpstate->regs.xsave, lmask, hmask); } @@ -334,6 +337,7 @@ static inline int xrstor_from_user_sigframe(struct xregs_state __user buf, u64 int err; xfd_validate_state(x86_task_fpu(current)->fpstate, mask, true); + fpregs_restore_prepare(mask); stac(); XSTATE_OP(XRSTOR, xstate, lmask, hmask, err); @@ -355,6 +359,7 @@ static inline int os_xrstor_safe(struct fpstate fpstate, u64 mask) /* Ensure that XFD is up to date */ xfd_update_state(fpstate); + fpregs_restore_prepare(mask); if (cpu_feature_enabled(X86_FEATURE_XSAVES)) XSTATE_OP(XRSTORS, xstate, lmask, hmask, err);
45/3	2026/10/06 22:48	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 4239 output: 42 thoughts: 1872 Thoughts: Analyzing a Potential Hardware Register Leak Fix Okay, let's break this down. My initial thought is to understand the scope and purpose of this patch. It looks like we're touching code in the x86 FPU subsystem, specifically around the `core.c`, `internal.h`, `legacy.h`, and `xstate.h` files. The core of the change seems to be around a function called `restore_fpregs_from_fpstate`. Hmm, it seems like we're removing a block of code that used `fildl %[addr]` and instead introducing `fpregs_restore_prepare`, which utilizes `fninit` in certain scenarios. The commit message clarifies that this is related to a potential leak on AMD CPUs without XSaveErPtr, where the FDP/FIP/FOP registers might retain stale values after a restore and could leak kernel addresses through `FNSTENV`. That's an interesting architectural issue. It's essentially a hardware register information leak. Previously, using `fildl` in kernel space was writing into FIP/FDP, which would then be leaked via `FNSTENV` if it was done in kernel space, and then read by user space. Replacing the instruction with `fninit` makes this safe. Now, a key question is, does KMSAN (KernelMemorySanitizer) even apply here? My understanding is KMSAN is designed to detect software memory issues, like reads of uninitialized memory on the stack, heap, and page allocations, and kernel-to-user memory leaks. KMSAN does not instrument or track CPU architectural registers like the x87 FIP/FDP registers. KMSAN is great for finding memory bugs, but it's not the right tool for hardware register leaks. I recall the key rule: "THERE IS NO SENSE IN RUNNING A KMSAN SESSION IF A BUG CAN BE CAUGHT BY KASAN, LOCKDEP, OR OTHER STANDARD BUG DETECTORS." Furthermore, KMSAN is only appropriate if the patch touches kernel structures sent to user space (copy_to_user, etc.), conditional logic that depends on potentially uninitialized variables, the allocation/initialization of complex data structures with uninitialized fields, or that allows surrounding code to access uninitialized bytes of existing buffers. None of those scenarios seems to apply here. Looking closer, `fninit` itself initializes x87 registers, and this change doesn't introduce any new kernel memory accesses or copies to user space. It merely swaps one register manipulation technique with another safer technique. The subsequent `fxrstor`/`frstor`/`xrstor` calls also restore from existing state, not into uninitialized memory. `fpregs_restore_prepare` calls `fninit`, again, no kernel memory accessed or copied to user. The key is that the prior code leaked kernel addresses via hardware registers. Therefore, this patch fixes a CPU register leak, specifically regarding how x87 registers are handled, and it does not touch any kernel memory that would trigger KMSAN concerns. This is a low-level x86 assembly and hardware register management fix. This is a good and necessary change to eliminate that potential architectural information leak.
46/3	2026/10/06 22:48	tool	read-file	0m Args: { "File": "arch/x86/kernel/fpu/core.c", "FirstLine": 150, "LineCount": 50 } Results: { "Contents": " 150:\t\t\tfxsave(\u0026fpu-\u003efpstate-\u003eregs.fxsave);\n 151:\t\t\treturn;\n 152:\t\t}\n 153:\t\n 154:\t\t/\n 155:\t\t Legacy FPU register saving, FNSAVE always clears FPU registers,\n 156:\t\t * so we have to reload them from the memory state.\n 157:\t\t /\n 158:\t\tasm volatile(\"fnsave %[fp]; fwait\" : [fp] \"=m\" (fpu-\u003efpstate-\u003eregs.fsave));\n 159:\t\tfrstor(\u0026fpu-\u003efpstate-\u003eregs.fsave);\n 160:\t}\n 161:\t\n 162:\tvoid restore_fpregs_from_fpstate(struct fpstate fpstate, u64 mask)\n 163:\t{\n 164:\t\tif (use_xsave()) {\n 165:\t\t\t/\n 166:\t\t\t Dynamically enabled features are enabled in XCR0, but\n 167:\t\t\t * usage requires also that the corresponding bits in XFD\n 168:\t\t\t * are cleared. If the bits are set then using a related\n 169:\t\t\t * instruction will raise #NM. This allows to do the\n 170:\t\t\t * allocation of the larger FPU buffer lazy from #NM or if\n 171:\t\t\t * the task has no permission to kill it which would happen\n 172:\t\t\t * via #UD if the feature is disabled in XCR0.\n 173:\t\t\t \n 174:\t\t\t XFD state is following the same life time rules as\n 175:\t\t\t * XSTATE and to restore state correctly XFD has to be\n 176:\t\t\t * updated before XRSTORS otherwise the component would\n 177:\t\t\t * stay in or go into init state even if the bits are set\n 178:\t\t\t * in fpstate::regs::xsave::xfeatures.\n 179:\t\t\t /\n 180:\t\t\txfd_update_state(fpstate);\n 181:\t\n 182:\t\t\t/\n 183:\t\t\t * Restoring state always needs to modify all features\n 184:\t\t\t * which are in @mask even if the current task cannot use\n 185:\t\t\t * extended features.\n 186:\t\t\t \n 187:\t\t\t So fpstate-\u003exfeatures cannot be used here, because then\n 188:\t\t\t * a feature for which the task has no permission but was\n 189:\t\t\t * used by the previous task would not go into init state.\n 190:\t\t\t */\n 191:\t\t\tmask = fpu_kernel_cfg.max_features \u0026 mask;\n 192:\t\n 193:\t\t\tos_xrstor(fpstate, mask);\n 194:\t\t} else {\n 195:\t\t\tif (use_fxsr())\n 196:\t\t\t\tfxrstor(\u0026fpstate-\u003eregs.fxsave);\n 197:\t\t\telse\n 198:\t\t\t\tfrstor(\u0026fpstate-\u003eregs.fsave);\n 199:\t\t}\n" }
47/3	2026/10/06 22:48	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 7629 output: 227 thoughts: 539 Thoughts: KMSAN Considerations for the FXSAVE/FXRSTOR Mitigation Okay, let's break this down from a KMSAN perspective. My initial thought is to examine the impact of this patch on KMSAN's effectiveness. First, I need to understand what's being changed. The core issue is the potential hardware information leak related to the `X86_BUG_FXSAVE_LEAK` AMD CPU bug. This bug could lead to kernel pointers being retained in the x87 FDP/FIP/FOP registers. The original mitigation involved the `fildl` instruction in `restore_fpregs_from_fpstate`. This patch replaces the `fildl` with `fninit`, and importantly, moves this mitigation into `fpregs_restore_prepare()` which is now prepended to all the `fxrstor`, `frstor`, and `xrstor` family functions. This is a significant architectural change to how the FPU/SIMD state is restored. Now, why isn't this relevant to KMSAN? My understanding of KMSAN is that it's designed to detect uninitialized software memory access issues. It meticulously tracks the shadow bits of memory allocated on the stack, heap, and page levels. However, KMSAN doesn't, and isn't intended to, track architectural CPU registers. This patch, while addressing a hardware bug, doesn't introduce any new uninitialized memory allocations within the kernel, doesn't involve copying kernel structures to user space, doesn't manipulate any uninitialized memory buffers, and, critically, doesn't add any conditional branching based on uninitialized data. Critically, it does not alter length or bounds checks on buffers. Therefore, because this change only addresses a potential leak through the CPU registers and not through any uninitialized memory within the kernel, running a dedicated KMSAN session is not going to yield any relevant information. This is a hardware-level errata mitigation, and we can safely conclude that "NeedsKMSAN" should be `false`.
48/3	2026/10/06 22:49	tool	set-results	0m Args: { "KMSANReasoning": "The patch addresses a hardware architectural register leak (AMD CPU bug X86_BUG_FXSAVE_LEAK) where x87 FDP/FIP/FOP registers retained kernel addresses due to an x87 memory load. It mitigates this by replacing the previous 'fildl' workaround with an 'fninit' instruction executed in a new helper, fpregs_restore_prepare(), before restoring FPU registers via fxrstor, frstor, or xrstor variants.\n\nKMSAN tracks uninitialized kernel memory allocations (stack, slab, page allocations) and memory info-leaks (e.g. copy_to_user of uninitialized padding/fields). KMSAN does not track hardware CPU architectural register state or CPU errata mitigations. The changes do not introduce, modify, or expose any uninitialized software memory buffers, complex data structures, or copies to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted.", "NeedsKMSAN": false } Results: { "KMSANReasoning": "The patch addresses a hardware architectural register leak (AMD CPU bug X86_BUG_FXSAVE_LEAK) where x87 FDP/FIP/FOP registers retained kernel addresses due to an x87 memory load. It mitigates this by replacing the previous 'fildl' workaround with an 'fninit' instruction executed in a new helper, fpregs_restore_prepare(), before restoring FPU registers via fxrstor, frstor, or xrstor variants.\n\nKMSAN tracks uninitialized kernel memory allocations (stack, slab, page allocations) and memory info-leaks (e.g. copy_to_user of uninitialized padding/fields). KMSAN does not track hardware CPU architectural register state or CPU errata mitigations. The changes do not introduce, modify, or expose any uninitialized software memory buffers, complex data structures, or copies to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted.", "NeedsKMSAN": false }

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