[PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

[Dmitry Vyukov]

On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs at fb.com> wrote:
> On 6/9/21 11:20 AM, Kees Cook wrote:
> > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote:
> >> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov
> >> <alexei.starovoitov at gmail.com> wrote:
> >>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs at fb.com> wrote:
> >>>> On 6/5/21 8:01 AM, Kurt Manucredo wrote:
> >>>>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
> >>>>> kernel/bpf/core.c:1414:2.
> >>>>
> >>>> This is not enough. We need more information on why this happens
> >>>> so we can judge whether the patch indeed fixed the issue.
> >>>>
> >>>>>
> >>>>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid
> >>>>> missing them and return with error when detected.
> >>>>>
> >>>>> Reported-and-tested-by: syzbot+bed360704c521841c85d at syzkaller.appspotmail.com
> >>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0 at gmail.com>
> >>>>> ---
> >>>>>
> >>>>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231
> >>>>>
> >>>>> Changelog:
> >>>>> ----------
> >>>>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals.
> >>>>>        Fix commit message.
> >>>>> v3 - Make it clearer what the fix is for.
> >>>>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> >>>>>        check in check_alu_op() in verifier.c.
> >>>>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> >>>>>        check in ___bpf_prog_run().
> >>>>>
> >>>>> thanks
> >>>>>
> >>>>> kind regards
> >>>>>
> >>>>> Kurt
> >>>>>
> >>>>>    kernel/bpf/verifier.c | 30 +++++++++---------------------
> >>>>>    1 file changed, 9 insertions(+), 21 deletions(-)
> >>>>>
> >>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>>>> index 94ba5163d4c5..ed0eecf20de5 100644
> >>>>> --- a/kernel/bpf/verifier.c
> >>>>> +++ b/kernel/bpf/verifier.c
> >>>>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> >>>>>        u32_min_val = src_reg.u32_min_value;
> >>>>>        u32_max_val = src_reg.u32_max_value;
> >>>>>
> >>>>> +     if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) &&
> >>>>> +                     umax_val >= insn_bitness) {
> >>>>> +             /* Shifts greater than 31 or 63 are undefined.
> >>>>> +              * This includes shifts by a negative number.
> >>>>> +              */
> >>>>> +             verbose(env, "invalid shift %lld\n", umax_val);
> >>>>> +             return -EINVAL;
> >>>>> +     }
> >>>>
> >>>> I think your fix is good. I would like to move after
> >>>
> >>> I suspect such change will break valid programs that do shift by register.
> >>>
> >>>> the following code though:
> >>>>
> >>>>           if (!src_known &&
> >>>>               opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
> >>>>                   __mark_reg_unknown(env, dst_reg);
> >>>>                   return 0;
> >>>>           }
> >>>>
> >>>>> +
> >>>>>        if (alu32) {
> >>>>>                src_known = tnum_subreg_is_const(src_reg.var_off);
> >>>>>                if ((src_known &&
> >>>>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> >>>>>                scalar_min_max_xor(dst_reg, &src_reg);
> >>>>>                break;
> >>>>>        case BPF_LSH:
> >>>>> -             if (umax_val >= insn_bitness) {
> >>>>> -                     /* Shifts greater than 31 or 63 are undefined.
> >>>>> -                      * This includes shifts by a negative number.
> >>>>> -                      */
> >>>>> -                     mark_reg_unknown(env, regs, insn->dst_reg);
> >>>>> -                     break;
> >>>>> -             }
> >>>>
> >>>> I think this is what happens. For the above case, we simply
> >>>> marks the dst reg as unknown and didn't fail verification.
> >>>> So later on at runtime, the shift optimization will have wrong
> >>>> shift value (> 31/64). Please correct me if this is not right
> >>>> analysis. As I mentioned in the early please write detailed
> >>>> analysis in commit log.
> >>>
> >>> The large shift is not wrong. It's just undefined.
> >>> syzbot has to ignore such cases.
> >>
> >> Hi Alexei,
> >>
> >> The report is produced by KUBSAN. I thought there was an agreement on
> >> cleaning up KUBSAN reports from the kernel (the subset enabled on
> >> syzbot at least).
> >> What exactly cases should KUBSAN ignore?
> >> +linux-hardening/kasan-dev for KUBSAN false positive
> >
> > Can check_shl_overflow() be used at all? Best to just make things
> > readable and compiler-happy, whatever the implementation. :)
>
> This is not a compile issue. If the shift amount is a constant,
> compiler should have warned and user should fix the warning.
>
> This is because user code has
> something like
>      a << s;
> where s is a unknown variable and
> verifier just marked the result of a << s as unknown value.
> Verifier may not reject the code depending on how a << s result
> is used.
>
> If bpf program writer uses check_shl_overflow() or some kind
> of checking for shift value and won't do shifting if the
> shifting may cause an undefined result, there should not
> be any kubsan warning.

I guess the main question: what should happen if a bpf program writer
does _not_ use compiler nor check_shl_overflow()?