[syzbot] possible deadlock in pipe_write

[syzbot]

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
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+011e4e...@syzkaller.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 syzk...@googlegroups.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

[Jann Horn]

This also looks like a watch_queue bug.

On Mon, Mar 21, 2022 at 3:34 AM syzbot
<syzbot+011e4e...@syzkaller.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.

[David Howells]

Jann Horn <ja...@google.com> wrote:

> 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.

Is this then a bug/feature in iter_file_splice_write() rather than in the
watch queue code, per se?

David

[Jann Horn]

I think at least when you call splice() on two normal pipes from
userspace, it'll never go through this codepath for real pipes,
because pipe-to-pipe splicing is special-cased? And sendfile() bails
out in that case because pipes don't have a .splice_read() handler.

And with notification pipes, we don't take that special path in
splice(), and so we hit the lockdep warning. But I don't know whether
that makes it the fault of notification pipes...

Maybe it would be enough to just move the "if (pipe->watch_queue)"
check in pipe_write() up above the __pipe_lock(pipe)?

[Jann Horn]

[coming back to this thread 1.5 years later...]
I've turned that idea into a fix, let's have syzbot try it out before
I submit the fix patch:

#syz test: https://github.com/thejh/linux.git 56c486e68166

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-and-tested-by: syzbot+011e4e...@syzkaller.appspotmail.com

Tested on:

commit: 56c486e6 fs/pipe: Fix lockdep false-positive in watchq..
git tree: https://github.com/thejh/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=128b880ce80000
kernel config: https://syzkaller.appspot.com/x/.config?x=1e6a76f6c7029ca2
dashboard link: https://syzkaller.appspot.com/bug?extid=011e4ea1da6692cf881c
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.