[syzbot] UBSAN: shift-out-of-bounds in snd_timer_user_ccallback

[Dongliang Mu]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 5ff2756a Merge tag 'nfs-for-5.13-2' of git://git.linux-nfs..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17872d5bd00000
> kernel config: https://syzkaller.appspot.com/x/.config?x=770708ea7cfd4916
> dashboard link: https://syzkaller.appspot.com/bug?extid=d102fa5b35335a7e544e
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d102fa... at syzkaller.appspotmail.com
>
> ================================================================================
> UBSAN: shift-out-of-bounds in sound/core/timer.c:1376:23
> shift exponent 105 is too large for 32-bit type 'int'
> CPU: 1 PID: 10368 Comm: syz-executor.1 Not tainted 5.13.0-rc3-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
> __dump_stack lib/dump_stack.c:79 [inline]
> dump_stack+0x141/0x1d7 lib/dump_stack.c:120
> ubsan_epilogue+0xb/0x5a lib/ubsan.c:148
> __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x181 lib/ubsan.c:327
> snd_timer_user_ccallback.cold+0x19/0x1e sound/core/timer.c:1376
>
> snd_timer_notify1+0x243/0x3b0 sound/core/timer.c:525

The root cause of this bug is in the snd_timer_notify1 [1]. At the end
of this function, it calls "ts->ccallback(ts, event + 100, &tstamp,
resolution)".

Here the variable event is 5. It adds 100 and is passed as 2nd
argument of snd_timer_user_ccallback.

>From the variable naming, the 2nd argument should an event, and in the
range of event enumeration. In fact, 105 (event + 100) is out of this
range. I don't quite understand the meaning of adding 100. Any thought
here?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/sound/core/timer.c?id=5ff2756afde08b266fbb673849899fec694f39f1#n497

--
My best regards to you.

     No System Is Safe!
     Dongliang Mu