Date:   Mon, 21 Mar 2022 15:41:13 +0100
From:   Jann Horn <jannh@...gle.com>
To:     syzbot <syzbot+011e4ea1da6692cf881c@...kaller.appspotmail.com>,
        David Howells <dhowells@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: [syzbot] possible deadlock in pipe_write

This also looks like a watch_queue bug.

On Mon, Mar 21, 2022 at 3:34 AM syzbot
<syzbot+011e4ea1da6692cf881c@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    56e337f2cf13 Revert "gpio: Revert regression in sysfs-gpio..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13f00f7e700000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d35f9bc6884af6c9
> dashboard link: https://syzkaller.appspot.com/bug?extid=011e4ea1da6692cf881c
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133235c5700000

The syz reproducer is:

#{"threaded":true,"procs":1,"slowdown":1,"sandbox":"","close_fds":false}
pipe(&(0x7f0000000240)={<r0=>0xffffffffffffffff, <r1=>0xffffffffffffffff})
pipe2(&(0x7f00000001c0)={0xffffffffffffffff, <r2=>0xffffffffffffffff}, 0x80)
splice(r0, 0x0, r2, 0x0, 0x1ff, 0x0)
vmsplice(r1, &(0x7f00000006c0)=[{&(0x7f0000000080)="b5", 0x1}], 0x1, 0x0)

That 0x80 is O_NOTIFICATION_PIPE (==O_EXCL).

It looks like the bug is that when you try to splice between a normal
pipe and a notification pipe, get_pipe_info(..., true) fails, so
splice() falls back to treating the notification pipe like a normal
pipe - so we end up in iter_file_splice_write(), which first locks the
input pipe, then calls vfs_iter_write(), which locks the output pipe.

I think this probably (?) can't actually lead to deadlocks, since
you'd need another way to nest locking a normal pipe into locking a
watch_queue pipe, but the lockdep annotations don't make that clear.

> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1248ca89700000
>
> Bisection is inconclusive: the issue happens on the oldest tested release.
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12f235c5700000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=11f235c5700000
> console output: https://syzkaller.appspot.com/x/log.txt?x=16f235c5700000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+011e4ea1da6692cf881c@...kaller.appspotmail.com
>
> ============================================
> WARNING: possible recursive locking detected
> 5.17.0-rc8-syzkaller-00003-g56e337f2cf13 #0 Not tainted
> --------------------------------------------
> syz-executor190/3593 is trying to acquire lock:
> ffff888078020868 (&pipe->mutex/1){+.+.}-{3:3}, at: __pipe_lock fs/pipe.c:103 [inline]
> ffff888078020868 (&pipe->mutex/1){+.+.}-{3:3}, at: pipe_write+0x132/0x1c00 fs/pipe.c:431
>
> but task is already holding lock:
> ffff888078020468 (&pipe->mutex/1){+.+.}-{3:3}, at: pipe_lock_nested fs/pipe.c:82 [inline]
> ffff888078020468 (&pipe->mutex/1){+.+.}-{3:3}, at: pipe_lock fs/pipe.c:90 [inline]
> ffff888078020468 (&pipe->mutex/1){+.+.}-{3:3}, at: pipe_wait_readable+0x39b/0x420 fs/pipe.c:1049
>
> other info that might help us debug this:
>  Possible unsafe locking scenario:
>
>        CPU0
>        ----
>   lock(&pipe->mutex/1);
>   lock(&pipe->mutex/1);
>
>  *** DEADLOCK ***
>
>  May be due to missing lock nesting notation
>
> 1 lock held by syz-executor190/3593:
>  #0: ffff888078020468 (&pipe->mutex/1){+.+.}-{3:3}, at: pipe_lock_nested fs/pipe.c:82 [inline]
>  #0: ffff888078020468 (&pipe->mutex/1){+.+.}-{3:3}, at: pipe_lock fs/pipe.c:90 [inline]
>  #0: ffff888078020468 (&pipe->mutex/1){+.+.}-{3:3}, at: pipe_wait_readable+0x39b/0x420 fs/pipe.c:1049
>
> stack backtrace:
> CPU: 1 PID: 3593 Comm: syz-executor190 Not tainted 5.17.0-rc8-syzkaller-00003-g56e337f2cf13 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
>  print_deadlock_bug kernel/locking/lockdep.c:2956 [inline]
>  check_deadlock kernel/locking/lockdep.c:2999 [inline]
>  validate_chain kernel/locking/lockdep.c:3788 [inline]
>  __lock_acquire.cold+0x213/0x3a9 kernel/locking/lockdep.c:5027
>  lock_acquire kernel/locking/lockdep.c:5639 [inline]
>  lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5604
>  __mutex_lock_common kernel/locking/mutex.c:600 [inline]
>  __mutex_lock+0x12f/0x12f0 kernel/locking/mutex.c:733
>  __pipe_lock fs/pipe.c:103 [inline]
>  pipe_write+0x132/0x1c00 fs/pipe.c:431
>  call_write_iter include/linux/fs.h:2074 [inline]
>  do_iter_readv_writev+0x47a/0x750 fs/read_write.c:725
>  do_iter_write+0x188/0x710 fs/read_write.c:851
>  vfs_iter_write+0x70/0xa0 fs/read_write.c:892
>  iter_file_splice_write+0x723/0xc70 fs/splice.c:689
>  do_splice_from fs/splice.c:767 [inline]
>  do_splice+0xb7e/0x1960 fs/splice.c:1079
>  __do_splice+0x134/0x250 fs/splice.c:1144
>  __do_sys_splice fs/splice.c:1350 [inline]
>  __se_sys_splice fs/splice.c:1332 [inline]
>  __x64_sys_splice+0x198/0x250 fs/splice.c:1332
>  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
> RIP: 0033:0x7fb9ac14bca9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 81 14 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fb9ac0fe308 EFLAGS: 00000246 ORIG_RAX: 0000000000000113
> RAX: fffffff
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> syzbot can test patches for this issue, for details see:
> https://goo.gl/tpsmEJ#testing-patches
>

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