On Wed, Aug 13, 2025 at 6:38 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > From: Jiri Olsa <olsajiri@xxxxxxxxx> > > 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@xxxxxxxxxx/ > Reported-by: syzbot+a9ed3d9132939852d0df@xxxxxxxxxxxxxxxxxxxxxxxxx > Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx> > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx> > --- > 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? 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 >
