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

[Yonghong Song <yhs@fb.com>]

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@gmail.com> wrote:
>>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@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@syzkaller.appspotmail.com
>>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@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.

>