linux-kernel - Re: [syzbot] INFO: task hung in sys_io

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Message-ID: <CACT4Y+Y=7aT65CA4n+sy5n75e53rWc+E3_K+-e6jxU=QQQOATg@mail.gmail.com>
Date:   Mon, 9 Aug 2021 11:24:22 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     syzbot <syzbot+d40a01556c761b2cb385@...kaller.appspotmail.com>,
        bcrl@...ck.org, linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        viro@...iv.linux.org.uk
Subject: Re: [syzbot] INFO: task hung in sys_io_destroy

On Fri, 6 Aug 2021 at 22:39, Jeff Moyer <jmoyer@...hat.com> wrote:
>
> syzbot <syzbot+d40a01556c761b2cb385@...kaller.appspotmail.com> writes:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    1d67c8d993ba Merge tag 'soc-fixes-5.14-1' of git://git.ker..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11b40232300000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=f1b998c1afc13578
> > dashboard link: https://syzkaller.appspot.com/bug?extid=d40a01556c761b2cb385
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12453812300000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11225922300000
> >
> > Bisection is inconclusive: the issue happens on the oldest tested release.
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=127cac6a300000
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=117cac6a300000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=167cac6a300000
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+d40a01556c761b2cb385@...kaller.appspotmail.com
> >
> > INFO: task syz-executor299:8807 blocked for more than 143 seconds.
> >       Not tainted 5.14.0-rc1-syzkaller #0
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > task:syz-executor299 state:D stack:29400 pid: 8807 ppid:  8806 flags:0x00000000
> > Call Trace:
> >  context_switch kernel/sched/core.c:4683 [inline]
> >  __schedule+0x93a/0x26f0 kernel/sched/core.c:5940
> >  schedule+0xd3/0x270 kernel/sched/core.c:6019
> >  schedule_timeout+0x1db/0x2a0 kernel/time/timer.c:1854
> >  do_wait_for_common kernel/sched/completion.c:85 [inline]
> >  __wait_for_common kernel/sched/completion.c:106 [inline]
> >  wait_for_common kernel/sched/completion.c:117 [inline]
> >  wait_for_completion+0x176/0x280 kernel/sched/completion.c:138
> >  __do_sys_io_destroy fs/aio.c:1402 [inline]
> >  __se_sys_io_destroy fs/aio.c:1380 [inline]
> >  __x64_sys_io_destroy+0x17e/0x1e0 fs/aio.c:1380
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> The reproducer is creating a thread, issuing a IOCB_CMD_PREAD from a
> pipe in that thread, and then calling io_destroy from another thread.
> Because there is no writer on the other end of the pipe, the read will
> block.  Note that it also is not submitted asynchronously, as that's not
> supported.
>
> io_destroy is "hanging" because it's waiting for the read to finish.  If
> the read thread is killed, cleanup happens as usual.  I'm not sure I
> could classify this as a kernel bug.

Hi Jeff,

Thanks for looking into this. I suspect the reproducer may create a
fork bomb that DoSed the kernel so that it can't make progress for 140
seconds. FTR, I've added it to
https://github.com/google/syzkaller/issues/498#issuecomment-895071514
to take a closer look.