Masked user access avoids the address/size verification by access_ok(). Allthough its main purpose is to skip the speculation in the verification of user address and size hence avoid the need of spec mitigation, it also has the advantage of reducing the amount of instructions required so it even benefits to platforms that don't need speculation mitigation, especially when the size of the copy is not know at build time. So implement masked user access on powerpc. The only requirement is to have memory gap that faults between the top user space and the real start of kernel area. On 64 bits platforms the address space is divided that way: 0xffffffffffffffff +------------------+ | | | kernel space | | | 0xc000000000000000 +------------------+ <== PAGE_OFFSET |//////////////////| |//////////////////| 0x8000000000000000 |//////////////////| |//////////////////| |//////////////////| 0x0010000000000000 +------------------+ <== TASK_SIZE_MAX | | | user space | | | 0x0000000000000000 +------------------+ Kernel is always above 0x8000000000000000 and user always below, with a gap in-between. It leads to a 4 instructions sequence: 80: 7c 69 1b 78 mr r9,r3 84: 7c 63 fe 76 sradi r3,r3,63 88: 7d 29 18 78 andc r9,r9,r3 8c: 79 23 00 4c rldimi r3,r9,0,1 This sequence leaves r3 unmodified when it is below 0x8000000000000000 and clamps it to 0x8000000000000000 if it is above. On 32 bits it is more tricky. In theory user space can go up to 0xbfffffff while kernel will usually start at 0xc0000000. So a gap needs to be added in-between. Allthough in theory a single 4k page would suffice, it is easier and more efficient to enforce a 128k gap below kernel, as it simplifies the masking. e500 has the isel instruction which allows selecting one value or the other without branch and that instruction is not speculative, so use it. Allthough GCC usually generates code using that instruction, it is safer to use inline assembly to be sure. The result is: 14: 3d 20 bf fe lis r9,-16386 18: 7c 03 48 40 cmplw r3,r9 1c: 7c 69 18 5e iselgt r3,r9,r3 On other ones, when kernel space is over 0x80000000 and user space is below, the logic in mask_user_address_simple() leads to a 3 instruction sequence: 14: 7c 69 fe 70 srawi r9,r3,31 18: 7c 63 48 78 andc r3,r3,r9 1c: 51 23 00 00 rlwimi r3,r9,0,0,0 This is the default on powerpc 8xx. When the limit between user space and kernel space is not 0x80000000, mask_user_address_32() is used and a 6 instructions sequence is generated: 24: 54 69 7c 7e srwi r9,r3,17 28: 21 29 57 ff subfic r9,r9,22527 2c: 7d 29 fe 70 srawi r9,r9,31 30: 75 2a b0 00 andis. r10,r9,45056 34: 7c 63 48 78 andc r3,r3,r9 38: 7c 63 53 78 or r3,r3,r10 The constraint is that TASK_SIZE be aligned to 128K in order to get the most optimal number of instructions. When CONFIG_PPC_BARRIER_NOSPEC is not defined, fallback on the test-based masking as it is quicker than the 6 instructions sequence but not quicker than the 3 instructions sequences above. As an exemple, allthough barrier_nospec() voids on the 8xx, this change has the following impact on strncpy_from_user(): the length of the function is reduced from 488 to 340 bytes: Start of the function with the patch: 00000000 : 0: 7c ab 2b 79 mr. r11,r5 4: 40 81 01 48 ble 14c 8: 7c 89 fe 70 srawi r9,r4,31 c: 7c 84 48 78 andc r4,r4,r9 10: 51 24 00 00 rlwimi r4,r9,0,0,0 14: 94 21 ff f0 stwu r1,-16(r1) 18: 3d 20 dc 00 lis r9,-9216 1c: 7d 3a c3 a6 mtspr 794,r9 20: 2f 8b 00 03 cmpwi cr7,r11,3 24: 40 9d 00 b8 ble cr7,dc ... Start of the function without the patch: 00000000 : 0: 7c a0 2b 79 mr. r0,r5 4: 40 81 01 10 ble 114 8: 2f 84 00 00 cmpwi cr7,r4,0 c: 41 9c 01 30 blt cr7,13c 10: 3d 20 80 00 lis r9,-32768 14: 7d 24 48 50 subf r9,r4,r9 18: 7f 80 48 40 cmplw cr7,r0,r9 1c: 7c 05 03 78 mr r5,r0 20: 41 9d 01 00 bgt cr7,120 24: 3d 20 80 00 lis r9,-32768 28: 7d 25 48 50 subf r9,r5,r9 2c: 7f 84 48 40 cmplw cr7,r4,r9 30: 38 e0 ff f2 li r7,-14 34: 41 9d 00 e4 bgt cr7,118 38: 94 21 ff e0 stwu r1,-32(r1) 3c: 3d 20 dc 00 lis r9,-9216 40: 7d 3a c3 a6 mtspr 794,r9 44: 2b 85 00 03 cmplwi cr7,r5,3 48: 40 9d 01 6c ble cr7,1b4 ... 118: 7c e3 3b 78 mr r3,r7 11c: 4e 80 00 20 blr 120: 7d 25 4b 78 mr r5,r9 124: 3d 20 80 00 lis r9,-32768 128: 7d 25 48 50 subf r9,r5,r9 12c: 7f 84 48 40 cmplw cr7,r4,r9 130: 38 e0 ff f2 li r7,-14 134: 41 bd ff e4 bgt cr7,118 138: 4b ff ff 00 b 38 13c: 38 e0 ff f2 li r7,-14 140: 4b ff ff d8 b 118 ... Signed-off-by: Christophe Leroy --- v2: Added 'likely()' to the test in mask_user_address_fallback() --- arch/powerpc/include/asm/task_size_32.h | 6 +- arch/powerpc/include/asm/uaccess.h | 78 +++++++++++++++++++++++++ 2 files changed, 81 insertions(+), 3 deletions(-) diff --git a/arch/powerpc/include/asm/task_size_32.h b/arch/powerpc/include/asm/task_size_32.h index 42a64bbd1964..725ddbf06217 100644 --- a/arch/powerpc/include/asm/task_size_32.h +++ b/arch/powerpc/include/asm/task_size_32.h @@ -13,7 +13,7 @@ #define MODULES_SIZE (CONFIG_MODULES_SIZE * SZ_1M) #define MODULES_VADDR (MODULES_END - MODULES_SIZE) #define MODULES_BASE (MODULES_VADDR & ~(UL(SZ_4M) - 1)) -#define USER_TOP MODULES_BASE +#define USER_TOP (MODULES_BASE - SZ_4M) #endif #ifdef CONFIG_PPC_BOOK3S_32 @@ -21,11 +21,11 @@ #define MODULES_SIZE (CONFIG_MODULES_SIZE * SZ_1M) #define MODULES_VADDR (MODULES_END - MODULES_SIZE) #define MODULES_BASE (MODULES_VADDR & ~(UL(SZ_256M) - 1)) -#define USER_TOP MODULES_BASE +#define USER_TOP (MODULES_BASE - SZ_4M) #endif #ifndef USER_TOP -#define USER_TOP ASM_CONST(CONFIG_PAGE_OFFSET) +#define USER_TOP ((ASM_CONST(CONFIG_PAGE_OFFSET) - SZ_128K) & ~(UL(SZ_128K) - 1)) #endif #if CONFIG_TASK_SIZE < USER_TOP diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h index 49254f7d9069..0b8e8ed37a14 100644 --- a/arch/powerpc/include/asm/uaccess.h +++ b/arch/powerpc/include/asm/uaccess.h @@ -2,6 +2,8 @@ #ifndef _ARCH_POWERPC_UACCESS_H #define _ARCH_POWERPC_UACCESS_H +#include + #include #include #include @@ -435,6 +437,82 @@ static __must_check __always_inline bool __user_access_begin(const void __user * #define user_access_save prevent_user_access_return #define user_access_restore restore_user_access +/* + * Masking the user address is an alternative to a conditional + * user_access_begin that can avoid the fencing. This only works + * for dense accesses starting at the address. + */ +static inline void __user *mask_user_address_simple(const void __user *ptr) +{ + unsigned long addr = (unsigned long)ptr; + unsigned long mask = (unsigned long)((long)addr >> (BITS_PER_LONG - 1)); + + addr = ((addr & ~mask) & (~0UL >> 1)) | (mask & (1UL << (BITS_PER_LONG - 1))); + + return (void __user *)addr; +} + +static inline void __user *mask_user_address_isel(const void __user *ptr) +{ + unsigned long addr; + + asm("cmplw %1, %2; iselgt %0, %2, %1" : "=r"(addr) : "r"(ptr), "r"(TASK_SIZE) : "cr0"); + + return (void __user *)addr; +} + +/* TASK_SIZE is a multiple of 128K for shifting by 17 to the right */ +static inline void __user *mask_user_address_32(const void __user *ptr) +{ + unsigned long addr = (unsigned long)ptr; + unsigned long mask = (unsigned long)((long)((TASK_SIZE >> 17) - 1 - (addr >> 17)) >> 31); + + addr = (addr & ~mask) | (TASK_SIZE & mask); + + return (void __user *)addr; +} + +static inline void __user *mask_user_address_fallback(const void __user *ptr) +{ + unsigned long addr = (unsigned long)ptr; + + return (void __user *)(likely(addr < TASK_SIZE) ? addr : TASK_SIZE); +} + +static inline void __user *mask_user_address(const void __user *ptr) +{ +#ifdef MODULES_VADDR + const unsigned long border = MODULES_VADDR; +#else + const unsigned long border = PAGE_OFFSET; +#endif + + if (IS_ENABLED(CONFIG_PPC64)) + return mask_user_address_simple(ptr); + if (IS_ENABLED(CONFIG_E500)) + return mask_user_address_isel(ptr); + if (TASK_SIZE <= UL(SZ_2G) && border >= UL(SZ_2G)) + return mask_user_address_simple(ptr); + if (IS_ENABLED(CONFIG_PPC_BARRIER_NOSPEC)) + return mask_user_address_32(ptr); + return mask_user_address_fallback(ptr); +} + +static __always_inline void __user *__masked_user_access_begin(const void __user *p, + unsigned long dir) +{ + void __user *ptr = mask_user_address(p); + + might_fault(); + allow_user_access(ptr, dir); + + return ptr; +} + +#define masked_user_access_begin(p) __masked_user_access_begin(p, KUAP_READ_WRITE) +#define masked_user_read_access_begin(p) __masked_user_access_begin(p, KUAP_READ) +#define masked_user_write_access_begin(p) __masked_user_access_begin(p, KUAP_WRITE) + #define unsafe_get_user(x, p, e) do { \ __long_type(*(p)) __gu_val; \ __typeof__(*(p)) __user *__gu_addr = (p); \ -- 2.49.0