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

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Message-ID: <20201015043327.stqhrupw2adhd5hl@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 14 Oct 2020 21:33:27 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        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.

On Wed, Oct 14, 2020 at 09:27:17PM -0700, John Fastabend wrote:
> 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@...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.
> > 
> > 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.

Right. Such WARN_ON_ONCE would definitely have been nice either in the caller
or in the callee. If I could have thought that id could be zero somehow here.
In retrospect it makes sense that there is possibility that IDs of regs in
this_branch and other_branch may diverge.
Hence I'm adding the warn to check for this specific divergence.