Re: [PATCH] ALSA: aloop: Fix peer runtime UAF during format-change stop

[Takashi Iwai <tiwai@suse.de>]

On Thu, 23 Apr 2026 05:22:22 +0200,
Cássio Gabriel wrote:
> 
> loopback_check_format() may stop the capture side when playback starts
> with parameters that no longer match a running capture stream. Commit
> 826af7fa62e3 ("ALSA: aloop: Fix racy access at PCM trigger") moved
> the peer lookup under cable->lock, but the actual snd_pcm_stop() still
> runs after dropping that lock.
> 
> A concurrent close can clear the capture entry from cable->streams[] and
> detach or free its runtime while the playback trigger path still holds a
> stale peer substream pointer.
> 
> Keep a per-cable count of in-flight peer stops before dropping
> cable->lock, make free_cable() wait for those stops before detaching the
> runtime, and take the peer stream lock around snd_pcm_stop(). This
> preserves the existing behavior while making the peer runtime lifetime
> explicit.
> 
> Reported-by: syzbot+8fa95c41eafbc9d2ff6f@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=8fa95c41eafbc9d2ff6f
> Fixes: 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback")
> Cc: stable@vger.kernel.org
> Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
> ---
>  sound/drivers/aloop.c | 56 ++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 40 insertions(+), 16 deletions(-)
> 
> diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
> index aa0d2fcb1a18..a997ee262740 100644
> --- a/sound/drivers/aloop.c
> +++ b/sound/drivers/aloop.c
> @@ -99,6 +99,9 @@ struct loopback_ops {
>  struct loopback_cable {
>  	spinlock_t lock;
>  	struct loopback_pcm *streams[2];
> +	/* in-flight peer stops running outside cable->lock */
> +	atomic_t stop_count;
> +	wait_queue_head_t stop_wait;
>  	struct snd_pcm_hardware hw;
>  	/* flags */
>  	unsigned int valid;
> @@ -337,10 +340,10 @@ static bool is_access_interleaved(snd_pcm_access_t access)
>  static int loopback_check_format(struct loopback_cable *cable, int stream)
>  {
>  	struct loopback_pcm *dpcm_play, *dpcm_capt;
> +	struct loopback_pcm *stop_dpcm = NULL;
>  	struct snd_pcm_runtime *runtime, *cruntime;
>  	struct loopback_setup *setup;
>  	struct snd_card *card;
> -	bool stop_capture = false;
>  	int check;
>  
>  	scoped_guard(spinlock_irqsave, &cable->lock) {
> @@ -366,8 +369,11 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
>  				return 0;
>  			if (stream == SNDRV_PCM_STREAM_CAPTURE)
>  				return -EIO;
> -			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING)
> -				stop_capture = true;
> +			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING) {
> +				/* close must not free the peer runtime below */
> +				atomic_inc(&cable->stop_count);
> +				stop_dpcm = dpcm_capt;
> +			}
>  		}
>  
>  		setup = get_setup(dpcm_play);
> @@ -396,8 +402,18 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
>  		}
>  	}
>  
> -	if (stop_capture)
> -		snd_pcm_stop(dpcm_capt->substream, SNDRV_PCM_STATE_DRAINING);
> +	if (stop_dpcm) {
> +		struct snd_pcm_substream *stop_substream = stop_dpcm->substream;
> +		unsigned long flags;
> +
> +		snd_pcm_stream_lock_irqsave_nested(stop_substream, flags);
> +		if (stop_substream->runtime && snd_pcm_running(stop_substream))
> +			snd_pcm_stop(stop_substream, SNDRV_PCM_STATE_DRAINING);
> +		snd_pcm_stream_unlock_irqrestore(stop_substream, flags);
> +
> +		if (atomic_dec_and_test(&cable->stop_count))
> +			wake_up(&cable->stop_wait);
> +	}

Do we need to complicate this handling?  IOW, can it be simply be like
below?

	if (stop_capture) {
		snd_pcm_stop(dpcm_capt->substream, SNDRV_PCM_STATE_DRAINING);
		if (atomic_dec_and_test(&cable->stop_count))
			wake_up(&cable->stop_wait);
	}


thanks,

Takashi