After do_check() returns, the verifier runs several instruction rewrite
passes. Some of them patch or remove one instruction at a time. Each
operation moves the remaining instruction and auxiliary-data arrays and
adjusts all branch offsets, making the overall work quadratic in the
program length.

A privileged loader can submit 131072 unconditional jumps by zero followed
by a valid return. Verification finishes quickly, but bpf_opt_remove_nops()
then spends a long time removing each jump separately. Since this
post-verification work neither checks for signals nor reschedules, a pending
SIGKILL cannot terminate the task until the rewrite finishes.

Make bpf_patch_insn_data() and verifier_remove_insns() common cancellation
and rescheduling points. These helpers run from BPF_PROG_LOAD process
context, and bpf_patch_insn_data() can already sleep while reallocating
auxiliary data. Callers already handle NULL or propagate an error, so a
fatal signal can abort without leaving a partially accepted program visible.

This does not reduce the quadratic cost of the rewrite passes, but it makes
the work preemptible and allows a killed loader to be torn down promptly.

Fixes: 52875a04f4b2 ("bpf: verifier: remove dead code")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/fixups.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index 52d3cec33672..9401fffcedfd 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -8,6 +8,7 @@
 #include <linux/bsearch.h>
 #include <linux/sort.h>
 #include <linux/perf_event.h>
+#include <linux/sched/signal.h>
 #include <net/xdp.h>
 #include "disasm.h"
 
@@ -306,12 +307,28 @@ static void adjust_poke_descs(struct bpf_prog *prog, u32 off, u32 len)
 	}
 }
 
+/*
+ * Some post-verification instruction rewriting passes require an
+ * O(prog->len) operation per instruction. Keep their shared primitives
+ * killable and preemptible.
+ */
+static bool bpf_rewrite_must_abort(void)
+{
+	if (fatal_signal_pending(current))
+		return true;
+	cond_resched();
+	return false;
+}
+
 struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 off,
 				     const struct bpf_insn *patch, u32 len)
 {
 	struct bpf_prog *new_prog;
 	struct bpf_insn_aux_data *new_data = NULL;
 
+	if (bpf_rewrite_must_abort())
+		return NULL;
+
 	if (len > 1) {
 		new_data = vrealloc(env->insn_aux_data,
 				    array_size(env->prog->len + len - 1,
@@ -523,6 +540,9 @@ static int verifier_remove_insns(struct bpf_verifier_env *env, u32 off, u32 cnt)
 	unsigned int orig_prog_len = env->prog->len;
 	int err;
 
+	if (bpf_rewrite_must_abort())
+		return -EINTR;
+
 	if (bpf_prog_is_offloaded(env->prog->aux))
 		bpf_prog_offload_remove_insns(env, off, cnt);
 
@@ -2666,4 +2686,3 @@ int bpf_remove_fastcall_spills_fills(struct bpf_verifier_env *env)
 
 	return 0;
 }
-
-- 
2.53.0