Re: [PATCHv2 bpf] bpf: Check the helper function is valid in get_helper_proto

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Aug 13, 2025 at 03:44:43PM -0700, Andrii Nakryiko wrote:
> On Wed, Aug 13, 2025 at 6:38 AM 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>
> > ---
> > v2 changes:
> > - set bpf_tail_call_proto.func to -1 so we can skip the extra check [Andrii]
> >
> >  kernel/bpf/core.c     | 5 ++++-
> >  kernel/bpf/verifier.c | 2 +-
> >  2 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 5d1650af899d..0f6e9a3d9960 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -3024,7 +3024,10 @@ EXPORT_SYMBOL_GPL(bpf_event_output);
> >
> >  /* Always built-in helper functions. */
> >  const struct bpf_func_proto bpf_tail_call_proto = {
> > -       .func           = NULL,
> > +       /* func is unused for tail_call, we set it to pass the
> > +        * get_helper_proto check
> > +        */
> > +       .func           = (void *) 1,
> 
> we seem to have BPF_PTR_POISON in include/linux/poison.h, let's use
> that instead of 1?

ah I did not know about this macro, thanks

jirka


> 
> pw-bot: cr
> 
> 
> >         .gpl_only       = false,
> >         .ret_type       = RET_VOID,
> >         .arg1_type      = ARG_PTR_TO_CTX,
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index c4f69a9e9af6..c89e2b1bc644 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -11354,7 +11354,7 @@ 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;
> > +       return *ptr && (*ptr)->func ? 0 : -EINVAL;
> >  }
> >
> >  static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > --
> > 2.50.1
> >