netdev - Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.

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Message-ID: <5f87ca47436f3_b7602088f@john-XPS-13-9370.notmuch>
Date:   Wed, 14 Oct 2020 21:04:23 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        john fastabend <john.fastabend@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.

Andrii Nakryiko wrote:
> On Wed, Oct 14, 2020 at 10:59 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > From: Alexei Starovoitov <ast@...nel.org>
> >
> > The 64-bit JEQ/JNE handling in reg_set_min_max() was clearing reg->id in either
> > true or false branch. In the case 'if (reg->id)' check was done on the other
> > branch the counter part register would have reg->id == 0 when called into
> > find_equal_scalars(). In such case the helper would incorrectly identify other
> > registers with id == 0 as equivalent and propagate the state incorrectly.

One thought. It seems we should never have reg->id=0 in find_equal_scalars()
would it be worthwhile to add an additional check here? Something like,

  if (known_reg->id == 0)
	return

Or even a WARN_ON_ONCE() there? Not sold either way, but maybe worth thinking
about.

> > Fix it by preserving ID across reg_set_min_max().
> > In other words any kind of comparison operator on the scalar register
> > should preserve its ID to recognize:
> > r1 = r2
> > if (r1 == 20) {
> >   #1 here both r1 and r2 == 20
> > } else if (r2 < 20) {
> >   #2 here both r1 and r2 < 20
> > }
> >
> > The patch is addressing #1 case. The #2 was working correctly already.
> >
> > Fixes: 75748837b7e5 ("bpf: Propagate scalar ranges through register assignments.")
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> > ---
> 
> Number of underscores is a bit subtle a difference, but this fixes the bug, so:
> 
> Acked-by: Andrii Nakryiko <andrii@...nel.org>
> 

Nice catch,

Acked-by: John Fastabend <john.fastabend@...il.com>

> 
> >  kernel/bpf/verifier.c                         | 38 ++++++++++++-------
> >  .../testing/selftests/bpf/verifier/regalloc.c | 26 +++++++++++++
> >  2 files changed, 51 insertions(+), 13 deletions(-)
> >
> 
> [...]