Re: [syzbot] possible deadlock in snd_mixer_oss_ioctl1

[Pavel Skripkin <paskripkin@gmail.com>]

On 10/23/21 22:35, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    cf6c9d12750c Add linux-next specific files for 20211022
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=15d3d4c4b00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e694204255ade3a3
> dashboard link: https://syzkaller.appspot.com/bug?extid=ace149a75a9a0a399ac7
> 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=17c0530cb00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1129c952b00000
> 
> The issue was bisected to:
> 
> commit 411cef6adfb38a5bb6bd9af3941b28198e7fb680
> Author: Takashi Iwai <tiwai@suse.de>
> Date:   Wed Oct 20 16:48:46 2021 +0000
> 
>      ALSA: mixer: oss: Fix racy access to slots
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=167f6454b00000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=157f6454b00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=117f6454b00000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+ace149a75a9a0a399ac7@syzkaller.appspotmail.com
> Fixes: 411cef6adfb3 ("ALSA: mixer: oss: Fix racy access to slots")
> 
> ============================================
> WARNING: possible recursive locking detected
> 5.15.0-rc6-next-20211022-syzkaller #0 Not tainted
> --------------------------------------------
> syz-executor206/6529 is trying to acquire lock:
> ffff888021ff09c8 (&mixer->reg_mutex){+.+.}-{3:3}, at: snd_mixer_oss_set_volume sound/core/oss/mixer_oss.c:316 [inline]
> ffff888021ff09c8 (&mixer->reg_mutex){+.+.}-{3:3}, at: snd_mixer_oss_ioctl1+0x630/0x19a0 sound/core/oss/mixer_oss.c:375
> 
> but task is already holding lock:
> ffff888021ff09c8 (&mixer->reg_mutex){+.+.}-{3:3}, at: snd_mixer_oss_set_volume sound/core/oss/mixer_oss.c:300 [inline]
> ffff888021ff09c8 (&mixer->reg_mutex){+.+.}-{3:3}, at: snd_mixer_oss_ioctl1+0x4b1/0x19a0 sound/core/oss/mixer_oss.c:375
> 
> other info that might help us debug this:
>   Possible unsafe locking scenario:
> 
>         CPU0
>         ----
>    lock(&mixer->reg_mutex);
>    lock(&mixer->reg_mutex);
> 
>   *** DEADLOCK ***
> 
>   May be due to missing lock nesting notation
> 

Looks like typo in previous ALSA patch. [1] should fix it


[1] 
https://lore.kernel.org/alsa-devel/20211024140315.16704-1-paskripkin@gmail.com/T/#u




With regards,
Pavel Skripkin