linux-kernel - Re: [PATCH] bpf: 32-bit RSH verification must truncate input before the ALU op

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Date:   Fri, 5 Oct 2018 19:48:55 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: 32-bit RSH verification must truncate input before
 the ALU op

On Fri, Oct 5, 2018 at 7:45 PM Edward Cree <ecree@...arflare.com> wrote:
> On 05/10/18 17:17, Jann Horn wrote:
> > When I wrote commit 468f6eafa6c4 ("bpf: fix 32-bit ALU op verification"), I
> > assumed that, in order to emulate 64-bit arithmetic with 32-bit logic, it
> > is sufficient to just truncate the output to 32 bits; and so I just moved
> > the register size coercion that used to be at the start of the function to
> > the end of the function.
> >
> > That assumption is true for almost every op, but not for 32-bit right
> > shifts, because those can propagate information towards the least
> > significant bit. Fix it by always truncating inputs for 32-bit ops to 32
> > bits.
> >
> > Also get rid of the coerce_reg_to_size() after the ALU op, since that has
> > no effect.
> Might be worth saying something like "because src_reg is passed by value".
> > Fixes: 468f6eafa6c4 ("bpf: fix 32-bit ALU op verification")
> > Acked-by: Daniel Borkmann <daniel@...earbox.net>
> > Signed-off-by: Jann Horn <jannh@...gle.com>
> > ---
> Acked-by: Edward Cree <ecree@...arflare.com>
> >  kernel/bpf/verifier.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index bb07e74b34a2..465952a8e465 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -2896,6 +2896,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> >       u64 umin_val, umax_val;
> >       u64 insn_bitness = (BPF_CLASS(insn->code) == BPF_ALU64) ? 64 : 32;
> Incidentally, I don't see why this needs to be a u64 (rather than say a u8).

Yeah, the size of the integer doesn't really matter there... but it's
being compared against other u64 values further down, so I also don't
see a particular need to change it.