Re: [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers

From: Song Liu <liu.song.a23@gmail.com>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@fb.com>,
	afabre@cloudflare.com, Marek Majkowski <marek@cloudflare.com>,
	bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>
Subject: Re: [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers
Date: Fri, 1 Mar 2019 15:42:08 -0800	[thread overview]
Message-ID: <CAPhsuW6DEUtLMGNgYCOjLOBWQGqiPQ=0XWC4i-QnWRxfBu=CRA@mail.gmail.com> (raw)
In-Reply-To: <e1250fdc-96f7-e020-eaff-7e0b67be9ead@iogearbox.net>

On Fri, Mar 1, 2019 at 3:23 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 03/02/2019 12:18 AM, Song Liu wrote:
> > On Fri, Mar 1, 2019 at 1:06 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >>
> >> Marek reported that he saw an issue with the below snippet in that
> >> timing measurements where off when loaded as unpriv while results
> >> were reasonable when loaded as privileged:
> >>
> >>     [...]
> >>     uint64_t a = bpf_ktime_get_ns();
> >>     uint64_t b = bpf_ktime_get_ns();
> >>     uint64_t delta = b - a;
> >>     if ((int64_t)delta > 0) {
> >>     [...]
> >>
> >> Turns out there is a bug where a corner case is missing in the fix
> >> d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar
> >> type from different paths"), namely fixup_bpf_calls() only checks
> >> whether aux has a non-zero alu_state, but it also needs to test for
> >> the case of BPF_ALU_NON_POINTER since in both occasions we need to
> >> skip the masking rewrite (as there is nothing to mask).
> >>
> >> Fixes: d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar type from different paths")
> >> Reported-by: Marek Majkowski <marek@cloudflare.com>
> >> Reported-by: Arthur Fabre <afabre@cloudflare.com>
> >> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> >> Link: https://lore.kernel.org/netdev/CAJPywTJqP34cK20iLM5YmUMz9KXQOdu1-+BZrGMAGgLuBWz7fg@mail.gmail.com/T/
> >> ---
> >>  [ Test case will be routed via bpf-next to avoid useless merge churn
> >>    due to test_verifier rework in bpf-next. ]
> >>
> >>  kernel/bpf/verifier.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >> index 8f295b790297..5fcce2f4209d 100644
> >> --- a/kernel/bpf/verifier.c
> >> +++ b/kernel/bpf/verifier.c
> >> @@ -6920,7 +6920,8 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
> >>                         u32 off_reg;
> >>
> >>                         aux = &env->insn_aux_data[i + delta];
> >> -                       if (!aux->alu_state)
> >> +                       if (!aux->alu_state ||
> >> +                           aux->alu_state == BPF_ALU_NON_POINTER)
> >
> > alu_state is a bitmap. Shall we check "aux->alu_state &
> > BPF_ALU_NON_POINTER" here?
>
> The state in this case can only ever be BPF_ALU_NON_POINTER, any other
> setting from sanitize_val_alu() would be a violation.

I see. Thanks for the explanation.

Acked-by: Song Liu <songliubraving@fb.com>