Re: BUG: soft lockup in snd_virmidi_output_trigger

[Takashi Iwai]

On Mon, 09 Apr 2018 07:58:01 +0200,
syzbot wrote:
> 
> Hello,
> 
> syzbot hit the following crash on upstream commit
> 3fd14cdcc05a682b03743683ce3a726898b20555 (Fri Apr 6 19:15:41 2018 +0000)
> Merge tag 'mtd/for-4.17' of git://git.infradead.org/linux-mtd
> syzbot dashboard link:
> https://syzkaller.appspot.com/bug?extid=619d9f40141d826b097e
> 
> Unfortunately, I don't have any reproducer for this crash yet.
> Raw console output:
> https://syzkaller.appspot.com/x/log.txt?id=4594231414882304
> Kernel config:
> https://syzkaller.appspot.com/x/.config?id=-5813481738265533882
> compiler: gcc (GCC) 8.0.1 20180301 (experimental)
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+619d9f40141d826b097e@xxxxxxxxxxxxxxxxxxxxxxxxx
> It will help syzbot understand when the bug is fixed. See footer for
> details.
> If you forward the report, please keep this part and the footer.
> 
> 8021q: adding VLAN 0 to HW filter on device team0
> IPv6: ADDRCONF(NETDEV_UP): team0: link is not ready
> 8021q: adding VLAN 0 to HW filter on device team0
> IPv6: ADDRCONF(NETDEV_UP): team0: link is not ready
> 8021q: adding VLAN 0 to HW filter on device team0
> watchdog: BUG: soft lockup - CPU#1 stuck for 134s! [syz-executor2:10431]
> Modules linked in:
> irq event stamp: 35856
> hardirqs last  enabled at (35855): [<ffffffff874a1734>]
> __raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:160
> [inline]
> hardirqs last  enabled at (35855): [<ffffffff874a1734>]
> _raw_spin_unlock_irqrestore+0x74/0xc0 kernel/locking/spinlock.c:184
> hardirqs last disabled at (35856): [<ffffffff87600901>]
> interrupt_entry+0xb1/0xf0 arch/x86/entry/entry_64.S:624
> softirqs last  enabled at (162): [<ffffffff87800778>]
> __do_softirq+0x778/0xaf5 kernel/softirq.c:311
> softirqs last disabled at (95): [<ffffffff81467a61>] invoke_softirq
> kernel/softirq.c:365 [inline]
> softirqs last disabled at (95): [<ffffffff81467a61>]
> irq_exit+0x1d1/0x200  kernel/softirq.c:405
> CPU: 1 PID: 10431 Comm: syz-executor2 Not tainted 4.16.0+ #4
> Hardware name: Google Google Compute Engine/Google Compute Engine,
> BIOS  Google 01/01/2011
> RIP: 0010:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783
> [inline]
> RIP: 0010:__raw_spin_unlock_irqrestore
> include/linux/spinlock_api_smp.h:160  [inline]
> RIP: 0010:_raw_spin_unlock_irqrestore+0xa1/0xc0
> kernel/locking/spinlock.c:184
> RSP: 0018:ffff880184db7780 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff13
> RAX: dffffc0000000000 RBX: 0000000000000282 RCX: 0000000000000000
> RDX: 1ffffffff1162e55 RSI: 0000000000000001 RDI: 0000000000000282
> RBP: ffff880184db7790 R08: ffffed0035d21962 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801ae90cb08
> R13: ffff880184db7810 R14: 0000000000000001 R15: ffff8801cb9a5880
> FS:  00007fc943ad8700(0000) GS:ffff8801db100000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fc943ad7db8 CR3: 00000001b070d000 CR4: 00000000001406e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  spin_unlock_irqrestore include/linux/spinlock.h:365 [inline]
>  snd_virmidi_output_trigger+0x522/0x6c0 sound/core/seq/seq_virmidi.c:205
>  snd_rawmidi_output_trigger sound/core/rawmidi.c:150 [inline]
>  snd_rawmidi_kernel_write1+0x519/0x700 sound/core/rawmidi.c:1288
>  snd_rawmidi_write+0x2e2/0xdc0 sound/core/rawmidi.c:1338
>  __vfs_write+0x10b/0x880 fs/read_write.c:485
>  vfs_write+0x1f8/0x560 fs/read_write.c:549
>  ksys_write+0xf9/0x250 fs/read_write.c:598
>  SYSC_write fs/read_write.c:610 [inline]
>  SyS_write+0x24/0x30 fs/read_write.c:607
>  do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
>  entry_SYSCALL_64_after_hwframe+0x42/0xb7
> RIP: 0033:0x455259
> RSP: 002b:00007fc943ad7c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 00007fc943ad86d4 RCX: 0000000000455259
> RDX: 00000000e78e624c RSI: 0000000020000040 RDI: 0000000000000014
> RBP: 000000000072c010 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
> R13: 00000000000006ca R14: 00000000006fd390 R15: 0000000000000002
> Code: c7 a8 72 b1 88 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea
> 03 80  3c 02 00 75 21 48 83 3d 4e 5b 67 01 00 74 0e 48 89 df 57 9d
> <0f> 1f 44 00  00 eb bb 0f 0b 0f 0b e8 6f 29 68 fa eb 97 e8 68 29

This is quite new, and I still don't figure out what went wrong.

At first I thought of the endless loop in snd_virmidi_output_trigger()
(e.g the error handling of snd_midi_event_encode() continues the loop,
and it may go into an endless loop in theory.  But, practically,
snd_midi_event_encode() never returns an error, so it shouldn't 
matter.)  But it doesn't explain why the report points to
spin_unlock_IRQ restore().

If you have a similar report or any relevant information, please let
me know.


thanks,

Takashi