linux-kernel - Re: [syzbot] BUG: unable to handle kernel access to user memory in sock

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Message-ID: <bf2e19a3-3e3a-0eb1-ae37-4cc3b1a7af42@codethink.co.uk>
Date:   Mon, 15 Mar 2021 14:41:42 +0000
From:   Ben Dooks <ben.dooks@...ethink.co.uk>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+c23c5421600e9b454849@...kaller.appspotmail.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>, andrii@...nel.org,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>, kpsingh@...nel.org,
        Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Song Liu <songliubraving@...com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Yonghong Song <yhs@...com>
Subject: Re: [syzbot] BUG: unable to handle kernel access to user memory in
 sock_ioctl

On 15/03/2021 11:52, Dmitry Vyukov wrote:
> On Mon, Mar 15, 2021 at 12:30 PM Ben Dooks <ben.dooks@...ethink.co.uk> wrote:
>>
>> On 14/03/2021 11:03, Dmitry Vyukov wrote:
>>> On Sun, Mar 14, 2021 at 11:01 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>>>>> On Wed, Mar 10, 2021 at 7:28 PM syzbot
>>>>> <syzbot+c23c5421600e9b454849@...kaller.appspotmail.com> wrote:
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> syzbot found the following issue on:
>>>>>>
>>>>>> HEAD commit:    0d7588ab riscv: process: Fix no prototype for arch_dup_tas..
>>>>>> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=122c343ad00000
>>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=c23c5421600e9b454849
>>>>>> userspace arch: riscv64
>>>>>>
>>>>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>>>>
>>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>>>> Reported-by: syzbot+c23c5421600e9b454849@...kaller.appspotmail.com
>>>>>
>>>>> +riscv maintainers
>>>>>
>>>>> Another case of put_user crashing.
>>>>
>>>> There are 58 crashes in sock_ioctl already. Somehow there is a very
>>>> significant skew towards crashing with this "user memory without
>>>> uaccess routines" in schedule_tail and sock_ioctl of all places in the
>>>> kernel that use put_user... This looks very strange... Any ideas
>>>> what's special about these 2 locations?
>>>
>>> I could imagine if such a crash happens after a previous stack
>>> overflow and now task data structures are corrupted. But f_getown does
>>> not look like a function that consumes way more than other kernel
>>> syscalls...
>>
>> The last crash I looked at suggested somehow put_user got re-entered
>> with the user protection turned back on. Either there is a path through
>> one of the kernel handlers where this happens or there's something
>> weird going on with qemu.
> 
> Is there any kind of tracking/reporting that would help to localize
> it? I could re-reproduce with that code.

I'm not sure. I will have a go at debugging on qemu today just to make
sure I can reproduce here before I have to go into the office and fix
my Icicle board for real hardware tests.

I think my first plan post reproduction is to stuff some trace points
into the fault handlers to see if we can get a idea of faults being
processed, etc.

Maybe also add a check in the fault handler to see if the fault was
in a fixable region and post an error if that happens / maybe retry
the instruction with the relevant SR_SUM flag set.

Hopefully tomorrow I can get a run on real hardware to confirm.
Would have been better if the Unmatched board I ordered last year
would turn up.




-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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