ALSA: aloop: Fix peer runtime UAF during format-change stop

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, and make free_cable() wait for those stops before
detaching the runtime. 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
Suggested-by: Takashi Iwai <tiwai@suse.com>
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260424-alsa-aloop-peer-stop-uaf-v2-1-94e68101db8a@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index aa0d2fcb1a180ca149afd2695cf270b02757220d..a37a1695f51c7d763ac50d5fbdedca4b8f96e3a8 100644 (file)
--- 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;
@@ -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)
+                       else if (cruntime->state == SNDRV_PCM_STATE_RUNNING) {
+                               /* close must not free the peer runtime below */
+                               atomic_inc(&cable->stop_count);
                                stop_capture = true;
+                       }
                }
 
                setup = get_setup(dpcm_play);
@@ -396,8 +402,11 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
                }
        }
 
-       if (stop_capture)
+       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);
+       }
 
        return 0;
 }
@@ -1049,23 +1058,29 @@ static void free_cable(struct snd_pcm_substream *substream)
        struct loopback *loopback = substream->private_data;
        int dev = get_cable_index(substream);
        struct loopback_cable *cable;
+       struct loopback_pcm *dpcm;
+       bool other_alive;
 
        cable = loopback->cables[substream->number][dev];
        if (!cable)
                return;
-       if (cable->streams[!substream->stream]) {
-               /* other stream is still alive */
-               guard(spinlock_irq)(&cable->lock);
-               cable->streams[substream->stream] = NULL;
-       } else {
-               struct loopback_pcm *dpcm = substream->runtime->private_data;
 
-               if (cable->ops && cable->ops->close_cable && dpcm)
-                       cable->ops->close_cable(dpcm);
-               /* free the cable */
-               loopback->cables[substream->number][dev] = NULL;
-               kfree(cable);
+       scoped_guard(spinlock_irq, &cable->lock) {
+               cable->streams[substream->stream] = NULL;
+               other_alive = cable->streams[!substream->stream];
        }
+
+       /* Pair with the stop_count increment in loopback_check_format(). */
+       wait_event(cable->stop_wait, !atomic_read(&cable->stop_count));
+       if (other_alive)
+               return;
+
+       dpcm = substream->runtime->private_data;
+       if (cable->ops && cable->ops->close_cable && dpcm)
+               cable->ops->close_cable(dpcm);
+       /* free the cable */
+       loopback->cables[substream->number][dev] = NULL;
+       kfree(cable);
 }
 
 static int loopback_jiffies_timer_open(struct loopback_pcm *dpcm)
@@ -1260,6 +1275,8 @@ static int loopback_open(struct snd_pcm_substream *substream)
                        goto unlock;
                }
                spin_lock_init(&cable->lock);
+               atomic_set(&cable->stop_count, 0);
+               init_waitqueue_head(&cable->stop_wait);
                cable->hw = loopback_pcm_hardware;
                if (loopback->timer_source)
                        cable->ops = &loopback_snd_timer_ops;