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Subject: Re: [PATCH -next] ALSA: timer: Fix use-after-free problem
To:     Takashi Iwai <tiwai@suse.de>
CC:     <perex@perex.cz>, <tiwai@suse.com>, <broonie@kernel.org>,
        <joe@perches.com>, <alsa-devel@alsa-project.org>,
        <linux-kernel@vger.kernel.org>, <rui.xiang@huawei.com>
References: <20211102134107.35126-1-wangwensheng4@huawei.com>
 <s5h5ytaps0n.wl-tiwai@suse.de>
From:   "wangwensheng (C)" <wangwensheng4@huawei.com>
Message-ID: <3b02dd76-d952-e38e-bc0c-c8a121919720@huawei.com>
Date:   Wed, 3 Nov 2021 11:27:58 +0800
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?? 2021/11/2 22:10, Takashi Iwai д??:
> On Tue, 02 Nov 2021 14:41:07 +0100,
> Wang Wensheng wrote:
>>
>> When the timer instance was add into ack_list but was not currently in
>> process, the user could stop it via snd_timer_stop1() without delete it
>> from the ack_list. Then the user could free the timer instance and when
>> it was actually processed UAF occurred.
>>
>> This issue could be reproduced via testcase snd_timer01 in ltp - running
>> several instances of that testcase at the same time.
>>
>> What I actually met was that the ack_list of the timer broken and the
>> kernel went into deadloop with irqoff. That could be detected by
>> hardlockup detector on board or when we run it on qemu, we could use gdb
>> to dump the ack_list when the console has no response.
>>
>> To fix this issue, we introduce a new flag SNDRV_TIMER_IFLG_ACKING to
>> indicate the state where the timer instance is in ack_list but not
>> currently processed and check against the new flag in snd_timer_stop1()
>> and delete it from ack_list if the flag is set.
>>
>> Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
> 
> Thanks for the patch.  Just through a quick glance, I wonder whether
> it'd be easier to do list_del_init(&timeri->ack_list) unconditionally
> before the check of timeri->flags in snd_timer1_stop().  Ditto for
> active_list.  So something like:
> 

Thanks for your suggestions. It looks much more efficient and easier and 
is also effective. I will take this as a second version.

> --- a/sound/core/timer.c
> +++ b/sound/core/timer.c
> @@ -624,13 +624,13 @@ static int snd_timer_stop1(struct snd_timer_instance *timeri, bool stop)
>   	if (!timer)
>   		return -EINVAL;
>   	spin_lock_irqsave(&timer->lock, flags);
> +	list_del_init(&timeri->ack_list);
> +	list_del_init(&timeri->active_list);
>   	if (!(timeri->flags & (SNDRV_TIMER_IFLG_RUNNING |
>   			       SNDRV_TIMER_IFLG_START))) {
>   		result = -EBUSY;
>   		goto unlock;
>   	}
> -	list_del_init(&timeri->ack_list);
> -	list_del_init(&timeri->active_list);
>   	if (timer->card && timer->card->shutdown)
>   		goto unlock;
>   	if (stop) {
> 
> 
> Takashi