linux-kernel - Re: INFO: task hung in fanotify_handle

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Message-ID: <CACT4Y+ZabUTpP6HZQVkVDvMYbktn=gP2C-m=ueKK3NRh7teY5A@mail.gmail.com>
Date:   Mon, 15 Oct 2018 14:29:14 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Jan Kara <jack@...e.cz>
Cc:     syzbot <syzbot+29143581b0ded3213e99@...kaller.appspotmail.com>,
        Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: INFO: task hung in fanotify_handle_event

On Mon, Oct 15, 2018 at 2:15 PM, Jan Kara <jack@...e.cz> wrote:
> Hello,
>
> On Mon 15-10-18 04:32:02, syzbot wrote:
>> syzbot found the following crash on:
>>
>> HEAD commit:    90ad18418c2d Merge git://git.kernel.org/pub/scm/linux/kern..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=17f1776e400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=88e9a8a39dc0be2d
>> dashboard link: https://syzkaller.appspot.com/bug?extid=29143581b0ded3213e99
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=123459d6400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+29143581b0ded3213e99@...kaller.appspotmail.com
>
> Syzbot has apparently generated fanotify watch for FAN_OPEN_PERM event and
> then the process got stuck waiting for userspace to respond to that event -
> which never happened. So everything works as designed here - the process
> placing FAN_OPEN_PERM mark is responsible for replying to the generated
> events as all opens hang waiting for responses. That's why the
> functionality is behind CAP_SYS_ADMIN after all... Could we fix syzbot to
> actually generate replies for these events?

Hi Jan,

Thanks for looking into it!

Is there a reliable way to kill such processes?
Or admins are never supposed to kill any root processes and have not
bugs whatsoever? :)

syzkaller probably capable of generating replies in some cases, but
unfortunately it can't work this way. It's practically not possible to
ensure that it will always generate a proper reply and it will be
actually delivered and the process won't be killed in the middle, or
another thread won't crash or call exit_group concurrently, etc. The
thing either needs to be reliable, work without any but's and be
reliably killable, or it's not suitable for stress testing.
If there is no reliable way to kill it, I think we need to disable
FAN_OPEN_PERM entirely.