[PATCH] ALSA: pcm: wake linked drain waiters on unlink

snd_pcm_drain() on a linked stream parks an on-stack wait entry on the
drained peer's runtime->sleep, and after schedule_timeout() removes it
only if that peer is still found in the caller's group.  If group
membership changes during the wait and the sleep ends by signal or
timeout (so autoremove_wake_function() does not run), finish_wait() is
skipped and snd_pcm_drain() returns with the entry still queued on that
stream's sleep list; a later wake_up() then walks a freed stack frame.
This is reachable by unlinking either the drained or the draining stream.

Unlike the close path (snd_pcm_drop() -> snd_pcm_post_stop()),
snd_pcm_unlink() never wakes the sleep queues.  Wake every group member
under the group lock before the membership change, so a linked drainer is
released and drops its entry while the streams are still grouped.

The window was opened when snd_pcm_link_rwsem stopped being held across
the wait and the removal became conditional on group membership (see
Fixes). The later switch to finish_wait() kept that conditional removal,
so the signal/timeout case remained.

Fixes: f57f3df03a8e ("ALSA: pcm: More fine-grained PCM link locking")
Cc: [email protected]
Assisted-by: Claude:claude-opus-5
Signed-off-by: Norbert Szetei <[email protected]>
---
 sound/core/pcm_native.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 7dc0060617f1..c9d0c2bb55b2 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -2367,6 +2367,7 @@ static void relink_to_local(struct snd_pcm_substream *substream)

 static int snd_pcm_unlink(struct snd_pcm_substream *substream)
 {
+	struct snd_pcm_substream *s;
 	struct snd_pcm_group *group;
 	bool nonatomic = substream->pcm->nonatomic;
 	bool do_free = false;
@@ -2379,6 +2380,12 @@ static int snd_pcm_unlink(struct snd_pcm_substream *substream)
 	group = substream->group;
 	snd_pcm_group_lock_irq(group, nonatomic);

+	/* release drain waiters before changing membership, else snd_pcm_drain()
+	 * leaves its on-stack wait entry queued on a member's sleep list
+	 */
+	snd_pcm_group_for_each_entry(s, substream)
+		wake_up(&s->runtime->sleep);
+
 	relink_to_local(substream);
 	refcount_dec(&group->refs);

--
2.55.0