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SubjectRe: [PATCH -next] ALSA: timer: Fix use-after-free problem
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在 2021/11/2 22:10, Takashi Iwai 写道:
> 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

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