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

From: John Fastabend <john.fastabend@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	John Fastabend <john.fastabend@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.
Date: Wed, 14 Oct 2020 21:27:17 -0700	[thread overview]
Message-ID: <5f87cfa5b1a77_b7602087e@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20201015041952.n3crk6kvtbgev6rw@ast-mbp.dhcp.thefacebook.com>

Alexei Starovoitov wrote:
> On Wed, Oct 14, 2020 at 09:04:23PM -0700, John Fastabend wrote:
> > Andrii Nakryiko wrote:
> > > On Wed, Oct 14, 2020 at 10:59 AM Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > From: Alexei Starovoitov <ast@kernel.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.
> 
> That cannot happen anymore due to
> if (dst_reg->id && !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id))
> check in the caller.
> I prefer not to repeat the same check twice. Also I really don't like defensive programming.
> if (known_reg->id == 0)
>        return;
> is exactly that.
> If we had that already, as Andrii argued in the original thread, we would have
> never noticed this issue. <, >, <= ops would have worked, but == would be
> sort-of working. It would mark one branch instead of both, and sometimes
> neither of the branches. I'd rather have bugs like this one hurting and caught
> quickly instead of warm feeling of being safe and sailing into unknown.

Agree. Although a WARN_ON_ONCE would have also been caught.