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

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Aug 12, 2025 at 03:32:40PM -0700, Andrii Nakryiko wrote:
> 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?

right, that's an option, will change

> 
> > +       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?

yes, I don't think it's verifier bug if option is missing, with this change
we will fail earlier in check_helper_call->get_helper_proto

jirka

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