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

[Yonghong Song <yhs@fb.com>]

On 6/9/21 10:32 PM, Dmitry Vyukov wrote:
> On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@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@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.
> 
> I guess the main question: what should happen if a bpf program writer
> does _not_ use compiler nor check_shl_overflow()?

If kubsan is not enabled, everything should work as expected even with
shl overflow may cause undefined result.

if kubsan is enabled, the reported shift-out-of-bounds warning
should be ignored. You could disasm the insn to ensure that
there indeed exists a potential shl overflow.