linux-kernel - Re: [syzbot] UBSAN: shift-out-of-bounds in snd_timer_user

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Message-ID: <5c3fbdf8-bfa3-a50e-edb9-81fbce84d9cb@perex.cz>
Date:   Wed, 2 Jun 2021 16:19:13 +0200
From:   Jaroslav Kysela <perex@...ex.cz>
To:     Dongliang Mu <mudongliangabcd@...il.com>, allen.lkml@...il.com,
        alsa-devel@...a-project.org, Joe Perches <joe@...ches.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        pierre-louis.bossart@...ux.intel.com, romain.perier@...il.com,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>, tiwai@...e.com
Subject: Re: [syzbot] UBSAN: shift-out-of-bounds in snd_timer_user_ccallback

On 02. 06. 21 15:18, Dongliang Mu wrote:
>> 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...@...kaller.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?

It seems that the original intent was to move the event to the M... events:

     SNDRV_TIMER_EVENT_MSTART = SNDRV_TIMER_EVENT_START + 10,

So the added value should be 10 which should not break the shift range (8
/resume/ + 10 = 18).

					Jaroslav

> 
> [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
> 


-- 
Jaroslav Kysela <perex@...ex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.