[PATCH bpf-next v1] bpf: Fix func_info_aux desync after dead code elimination

[PATCH bpf-next v1] bpf: Fix func_info_aux desync after dead code elimination

[Kumar Kartikeya Dwivedi]

The verifier keeps per-subprogram metadata in three parallel arrays:
subprog_info, func_info, and func_info_aux. Dead code elimination can
remove whole subprograms, and adjust_subprog_starts_after_remove()
shifts subprog_info and func_info to close the gap, but leaves
func_info_aux in place. From that point on, func_info_aux[i] no longer
describes subprogram i.

Shift func_info_aux together with func_info so the three arrays stay
aligned after subprogram removal.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://lore.kernel.org/bpf/20260808064523.DE3E71F000E9@smtp.kernel.org
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/fixups.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index c4bd70befbb5..37ea85e974f1 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -401,13 +401,17 @@ static int adjust_subprog_starts_after_remove(struct bpf_verifier_env *env,
 			sizeof(*env->subprog_info) * move);
 		env->subprog_cnt -= j - i;

-		/* remove func_info */
+		/* remove func_info and its aux */
 		if (aux->func_info) {
 			move = aux->func_info_cnt - j;

 			memmove(aux->func_info + i,
 				aux->func_info + j,
 				sizeof(*aux->func_info) * move);
+			if (aux->func_info_aux)
+				memmove(aux->func_info_aux + i,
+					aux->func_info_aux + j,
+					sizeof(*aux->func_info_aux) * move);
 			aux->func_info_cnt -= j - i;
 			/* func_info->insn_off is set after all code rewrites,
 			 * in adjust_btf_func() - no need to adjust
--
2.53.0

Re: [PATCH bpf-next v1] bpf: Fix func_info_aux desync after dead code elimination

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Failure to update `env->exception_callback_subprog` during dead code elimination leads to an out-of-bounds array read and potential crash during JIT.
--

--- Patch [1]: [PATCH bpf-next v1] bpf: Fix func_info_aux desync after dead code elimination ---
commit f1d27d196a7979de10f6e09614e6262cf8069cae
Author: Kumar Kartikeya Dwivedi <memxor@gmail.com>

bpf: Fix func_info_aux desync after dead code elimination

The commit shifts func_info_aux together with func_info so the three parallel
arrays stay aligned after subprogram removal during dead code elimination.

Link: https://lore.kernel.org/bpf/20260808064523.DE3E71F000E9@smtp.kernel.org

> diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
> index c4bd70befbb5c..37ea85e974f14 100644
> --- a/kernel/bpf/fixups.c
> +++ b/kernel/bpf/fixups.c
> @@ -401,13 +401,17 @@ static int adjust_subprog_starts_after_remove(struct bpf_verifier_env *env,
>  			sizeof(*env->subprog_info) * move);
>  		env->subprog_cnt -= j - i;

[Severity: High]
This isn't a bug introduced by this patch, but should we also be adjusting
env->exception_callback_subprog here?

When dead code elimination removes an unused static function that comes before
an exception callback, env->subprog_cnt is decremented and subprograms are
shifted down. However, env->exception_callback_subprog is left unadjusted.

Later, in jit_subprogs(), the func array is allocated with the new, smaller
env->subprog_cnt. Accessing it using the outdated
env->exception_callback_subprog index could result in an out-of-bounds array
read and a potential kernel panic:

kernel/bpf/fixups.c:jit_subprogs() {
    ...
    prog->aux->bpf_exception_cb = (void *)func[env->exception_callback_subprog]->bpf_func;
    ...
}

Does env->exception_callback_subprog need to be updated to reflect the shift
when subprograms are removed?

>  
> -		/* remove func_info */
> +		/* remove func_info and its aux */
>  		if (aux->func_info) {
>  			move = aux->func_info_cnt - j;
>  
>  			memmove(aux->func_info + i,
>  				aux->func_info + j,
>  				sizeof(*aux->func_info) * move);
> +			if (aux->func_info_aux)
> +				memmove(aux->func_info_aux + i,
> +					aux->func_info_aux + j,
> +					sizeof(*aux->func_info_aux) * move);
>  			aux->func_info_cnt -= j - i;
>  			/* func_info->insn_off is set after all code rewrites,
>  			 * in adjust_btf_func() - no need to adjust

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260812231506.3558128-1-memxor@gmail.com?part=1

Re: [PATCH bpf-next v1] bpf: Fix func_info_aux desync after dead code elimination

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1185 bytes --]

>     bpf: Fix func_info_aux desync after dead code elimination
>
>     The verifier keeps per-subprogram metadata in three parallel arrays:
>     subprog_info, func_info, and func_info_aux. Dead code elimination can
>     remove whole subprograms, and adjust_subprog_starts_after_remove()
>     shifts subprog_info and func_info to close the gap, but leaves
>     func_info_aux in place. From that point on, func_info_aux[i] no longer
>     describes subprogram i.
>
>     Shift func_info_aux together with func_info so the three arrays stay
>     aligned after subprogram removal.
>
>     Reported-by: Sashiko <sashiko-bot@kernel.org>
>     Link: https://lore.kernel.org/bpf/20260808064523.DE3E71F000E9@smtp.kernel.org
>     Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

This looks like a bug fix for func_info_aux array alignment. Should this
include:

  Fixes: 51c39bb1d5d1 ("bpf: Introduce function-by-function verification")


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/31651414371