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Subject: Re: WARNING in request_end
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        syzbot <syzbot+ef054c4d3f64cd7f7cec@syzkaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Tue, 25 Sep 2018 12:49:56 +0300
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On 25.09.2018 12:38, Dmitry Vyukov wrote:
> On Tue, Sep 25, 2018 at 11:18 AM, Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>> On 24.09.2018 17:44, Miklos Szeredi wrote:
>>> On Mon, Sep 24, 2018 at 2:29 PM, syzbot
>>> <syzbot+ef054c4d3f64cd7f7cec@syzkaller.appspotmail.com> wrote:
>>>> Hello,
>>>>
>>>> syzbot found the following crash on:
>>>>
>>>> HEAD commit:    6bf4ca7fbc85 Linux 4.19-rc5
>>>> git tree:       upstream
>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=159149c6400000
>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=22a62640793a83c9
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=ef054c4d3f64cd7f7cec
>>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>>>
>>>> Unfortunately, I don't have any reproducer for this crash yet.
>>>>
>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>>> Reported-by: syzbot+ef054c4d3f64cd7f7cec@syzkaller.appspotmail.com
>>>>
>>>> WARNING: CPU: 0 PID: 9445 at fs/fuse/dev.c:390 request_end+0x82e/0xaa0
>>>
>>> And  there we have the bug likely caused by the set_bit(FR_SENT, ...)
>>> not being inside the fpq->lock-ed region.
>>>
>>> So that needs to be fixed anyway, apparently.
>>
>> I can't confirm, since I haven't found yet the direct way, that set_bit() results
>> in this stack...
>>
>> We have one more (unrelated) possible use-after-free here:
>>
>> cpu0                                       cpu1
>> fuse_dev_do_write()                     fuse_dev_do_write()
>>   req = request_find(fpq, oh.unique)    ...
>>   spin_unlock(&fpq->lock)               ...
>>   ...                                   req = request_find(fpq, oh.unique)
>>   ...                                   spin_unlock(&fpq->lock)
>>   queue_interrupt(&fc->iq, req);        ...
>>   ...                                   ...
>>   ...                                   ...
>> request freed                           ...
>> ...                                     queue_interrupt(&fc->iq, req); <- use after free
>>
>> Something like below is needed:
> 
> There is a bunch of open bugs in fuse on syzbot dashboard, perhaps
> it's one of them:
> 
> https://syzkaller.appspot.com/bug?id=19aabec97cbf73dd0475d6e599113a7861c4b306
> https://syzkaller.appspot.com/bug?id=24aa489e6929205e40ec4aa52cd8f47897f2ad63
> https://syzkaller.appspot.com/bug?id=400d6a977a0dbd8836d7c7ec8481782a674ee855
> https://syzkaller.appspot.com/bug?id=ff9ab4a23afa7553fb79f745a92be87ba4144508
> https://syzkaller.appspot.com/bug?id=d0f258de27b6d7ccecbba09385b3376cc4a12ffe
> https://syzkaller.appspot.com/bug?id=e8077bce636d52d9c40e1ea904699c27b7454354

I can't find fuse_dev_do_write() there, but it's possible this race could appear
in another function. So, Dmitry, I won't add reference to one of tham. Let's
check, which will disappear in the future.

Thanks,
Kirill