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Date: Wed, 9 Jun 2021 23:06:31 -0700
From: Yonghong Song <yhs@...com>
To: Dmitry Vyukov <dvyukov@...gle.com>
CC: Kees Cook <keescook@...omium.org>,
Alexei Starovoitov
<alexei.starovoitov@...il.com>,
Kurt Manucredo <fuzzybritches0@...il.com>,
<syzbot+bed360704c521841c85d@...kaller.appspotmail.com>,
Andrii Nakryiko
<andrii@...nel.org>,
Alexei Starovoitov <ast@...nel.org>, bpf
<bpf@...r.kernel.org>,
Daniel Borkmann <daniel@...earbox.net>,
"David S.
Miller" <davem@...emloft.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John
Fastabend <john.fastabend@...il.com>,
Martin KaFai Lau <kafai@...com>, KP
Singh <kpsingh@...nel.org>,
Jakub Kicinski <kuba@...nel.org>, LKML
<linux-kernel@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>,
Song Liu <songliubraving@...com>,
syzkaller-bugs
<syzkaller-bugs@...glegroups.com>, <nathan@...nel.org>,
Nick Desaulniers
<ndesaulniers@...gle.com>,
Clang-Built-Linux ML
<clang-built-linux@...glegroups.com>,
<linux-kernel-mentees@...ts.linuxfoundation.org>,
Shuah Khan
<skhan@...uxfoundation.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
kasan-dev
<kasan-dev@...glegroups.com>
Subject: Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run
On 6/9/21 10:32 PM, Dmitry Vyukov wrote:
> On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@...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@...il.com> wrote:
>>>>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@...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@...kaller.appspotmail.com
>>>>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@...il.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.
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