Re: [PATCH bpf] bpf: Check the helper function is valid in get_helper_proto

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Aug 12, 2025 at 3:12 PM Jiri Olsa <jolsa@kernel.org> wrote:
>
> From: Jiri Olsa <olsajiri@gmail.com>
>
> syzbot reported an verifier bug [1] where the helper func pointer
> could be NULL due to disabled config option.
>
> As Alexei suggested we could check on that in get_helper_proto
> directly. Excluding tail_call helper from the check, because it
> is NULL by design and valid in all configs.
>
> [1] https://lore.kernel.org/bpf/68904050.050a0220.7f033.0001.GAE@google.com/
> Reported-by: syzbot+a9ed3d9132939852d0df@syzkaller.appspotmail.com
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/bpf/verifier.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index c4f69a9e9af6..5e38489656e2 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11344,6 +11344,13 @@ static bool can_elide_value_nullness(enum bpf_map_type type)
>         }
>  }
>
> +static bool is_valid_proto(const struct bpf_func_proto *fn)
> +{
> +       if (fn == &bpf_tail_call_proto)
> +               return true;

ugh... what if we set bpf_tail_call_proto's .func to (void *)0xDEADBAD
or some such and avoid this special casing?

> +       return fn && fn->func;
> +}
> +
>  static int get_helper_proto(struct bpf_verifier_env *env, int func_id,
>                             const struct bpf_func_proto **ptr)
>  {
> @@ -11354,7 +11361,7 @@ static int get_helper_proto(struct bpf_verifier_env *env, int func_id,
>                 return -EINVAL;
>
>         *ptr = env->ops->get_func_proto(func_id, env->prog);
> -       return *ptr ? 0 : -EINVAL;

so we explicitly do not want WARN/BUG/verifier_bug() if
!is_valid_proto(), is that right?

> +       return is_valid_proto(*ptr) ? 0 : -EINVAL;
>  }
>
>  static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> --
> 2.50.1
>