Re: [PATCH] bpf: Fix off-by-one in tail call count limiting

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

On Thu, Jul 29, 2021 at 3:29 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Jul 29, 2021 at 2:38 PM Johan Almbladh
> <johan.almbladh@anyfinetworks.com> wrote:
> >
> > On Wed, Jul 28, 2021 at 9:13 PM Yonghong Song <yhs@fb.com> wrote:
> > > I also checked arm/arm64 jit. I saw the following comments:
> > >
> > >          /* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
> > >           *      goto out;
> > >           * tail_call_cnt++;
> > >           */
> > >
> > > Maybe we have this MAX_TAIL_CALL_CNT + 1 issue
> > > for arm/arm64 jit?
> >
> > That wouldn't be unreasonable. I don't have an arm or arm64 setup
> > available right now, but I can try to test it in qemu.
>
> On a brief check, there seems to be quite a mess in terms of the code
> and comments.
>
> E.g., in arch/x86/net/bpf_jit_comp32.c:
>
>         /*
>          * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
>          *     goto out;
>          */
>
>                             ^^^^ here comment is wrong
>
>         [...]
>
>         /* cmp edx,hi */
>         EMIT3(0x83, add_1reg(0xF8, IA32_EBX), hi);
>         EMIT2(IA32_JNE, 3);
>         /* cmp ecx,lo */
>         EMIT3(0x83, add_1reg(0xF8, IA32_ECX), lo);
>
>         /* ja out */
>         EMIT2(IA32_JAE, jmp_label(jmp_label1, 2));
>
>         ^^^ JAE is >=, right? But the comment says JA.
>
>
> As for arch/x86/net/bpf_jit_comp.c, both comment and the code seem to
> do > MAX_TAIL_CALL_CNT, but you are saying JIT is correct. What am I
> missing?
>
> Can you please check all the places where MAX_TAIL_CALL_CNT is used
> throughout the code? Let's clean this up in one go.
>
> Also, given it's so easy to do this off-by-one error, can you please
> add a negative test validating that 33 tail calls are not allowed? I
> assume we have a positive test that allows exactly MAX_TAIL_CALL_CNT,
> but please double-check that as well.

Ok, I see that you've added this in your bpf tests patch set. Please
consider, additionally, implementing a similar test as part of
selftests/bpf (specifically in test_progs). We run test_progs
continuously in CI for every incoming patch/patchset, so it has much
higher chances of capturing any regressions.

I'm also thinking that this MAX_TAIL_CALL_CNT change should probably
go into the bpf-next tree. First, this off-by-one behavior was around
for a while and it doesn't cause serious issues, even if abused. But
on the other hand, it will make your tail call tests fail, when
applied into bpf-next without your change. So I think we should apply
both into bpf-next.

On a related topic, please don't forget to include the target kernel
tree for your patches: [PATCH bpf] or [PATCH bpf-next].


>
> I also wonder if it would make sense to convert these
> internal-but-sort-of-advertised constants like MAX_TAIL_CALL_CNT and
> BPF_COMPLEXITY_LIMIT_INSNS into enums so that they can be "discovered"
> from BTF. This should be discussed/attempted outside of this fix,
> though. Just bringing it up here.