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bpf: Fix aux usage after do_check_insn()

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We must terminate the speculative analysis if the just-analyzed insn had
nospec_result set. Using cur_aux() here is wrong because insn_idx might
have been incremented by do_check_insn(). Therefore, introduce and use
insn_aux variable.

Also change cur_aux(env)->nospec in case do_check_insn() ever manages to
increment insn_idx but still fail.

Change the warning to check the insn class (which prevents it from
triggering for ldimm64, for which nospec_result would not be
problematic) and use verifier_bug_if().

In line with Eduard's suggestion, do not introduce prev_aux() because
that requires one to understand that after do_check_insn() call what was
current became previous. This would at-least require a comment.

Fixes: d6f1c85f22 ("bpf: Fall back to nospec for Spectre v1")
Reported-by: Paul Chaignon <paul.chaignon@gmail.com>
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Reported-by: syzbot+dc27c5fb8388e38d2d37@syzkaller.appspotmail.com
Link: https://lore.kernel.org/bpf/685b3c1b.050a0220.2303ee.0010.GAE@google.com/
Link: https://lore.kernel.org/bpf/4266fd5de04092aa4971cbef14f1b4b96961f432.camel@gmail.com/
Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250705190908.1756862-2-luis.gerhorst@fau.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Luis Gerhorst 2025-07-05 21:09:07 +02:00 committed by Alexei Starovoitov
parent 0f626c98fd
commit dadb59104c
1 changed files with 14 additions and 5 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

19
kernel/bpf/verifier.c
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@ -19953,6 +19953,7 @@ static int do_check(struct bpf_verifier_env *env)
for (;;) {
struct bpf_insn *insn;
struct bpf_insn_aux_data *insn_aux;
int err;
/* reset current history entry on each new instruction */
@ -19966,6 +19967,7 @@ static int do_check(struct bpf_verifier_env *env)
}
insn = &insns[env->insn_idx];
insn_aux = &env->insn_aux_data[env->insn_idx];
if (++env->insn_processed > BPF_COMPLEXITY_LIMIT_INSNS) {
verbose(env,
@ -20042,7 +20044,7 @@ static int do_check(struct bpf_verifier_env *env)
/* Reduce verification complexity by stopping speculative path
* verification when a nospec is encountered.
*/
if (state->speculative && cur_aux(env)->nospec)
if (state->speculative && insn_aux->nospec)
goto process_bpf_exit;
err = do_check_insn(env, &do_print_state);
@ -20050,11 +20052,11 @@ static int do_check(struct bpf_verifier_env *env)
/* Prevent this speculative path from ever reaching the
* insn that would have been unsafe to execute.
*/
cur_aux(env)->nospec = true;
insn_aux->nospec = true;
/* If it was an ADD/SUB insn, potentially remove any
* markings for alu sanitization.
*/
cur_aux(env)->alu_state = 0;
insn_aux->alu_state = 0;
goto process_bpf_exit;
} else if (err < 0) {
return err;
@ -20063,7 +20065,7 @@ static int do_check(struct bpf_verifier_env *env)
}
WARN_ON_ONCE(err);
if (state->speculative && cur_aux(env)->nospec_result) {
if (state->speculative && insn_aux->nospec_result) {
/* If we are on a path that performed a jump-op, this
* may skip a nospec patched-in after the jump. This can
* currently never happen because nospec_result is only
@ -20072,8 +20074,15 @@ static int do_check(struct bpf_verifier_env *env)
* never skip the following insn. Still, add a warning
* to document this in case nospec_result is used
* elsewhere in the future.
*
* All non-branch instructions have a single
* fall-through edge. For these, nospec_result should
* already work.
*/
WARN_ON_ONCE(env->insn_idx != prev_insn_idx + 1);
if (verifier_bug_if(BPF_CLASS(insn->code) == BPF_JMP ||
BPF_CLASS(insn->code) == BPF_JMP32, env,
"speculation barrier after jump instruction may not have the desired effect"))
return -EFAULT;
process_bpf_exit:
mark_verifier_state_scratched(env);
err = update_branch_counts(env, env->cur_state);