[PATCH 5.10.y] ALSA: core: Serialize deferred fasync state checks

[Sasha Levin <sashal@kernel.org>]

From: Cássio Gabriel <cassiogabrielcontato@gmail.com>

[ Upstream commit 5337213381df578058e2e41da93cbd0e4639935f ]

snd_fasync_helper() updates fasync->on under snd_fasync_lock, and
snd_fasync_work_fn() now also evaluates fasync->on under the same
lock. snd_kill_fasync() still tests the flag before taking the lock,
leaving an unsynchronized read against FASYNC enable/disable updates.

Move the enabled-state check into the locked section.

Also clear fasync->on under snd_fasync_lock in snd_fasync_free()
before unlinking the pending entry. Together with the locked sender-side
check, this publishes teardown before flushing the deferred work and
prevents a racing sender from requeueing the entry after free has
started.

Fixes: ef34a0ae7a26 ("ALSA: core: Add async signal helpers")
Fixes: 8146cd333d23 ("ALSA: core: Fix potential data race at fasync handling")
Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260506-alsa-core-fasync-on-lock-v1-1-ea48c77d6ca4@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[ converted upstream guard()/scoped_guard() to explicit spin_lock_irqsave/spin_lock_irq and dropped the list_del_init() from a missing prerequisite ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 sound/core/misc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/sound/core/misc.c b/sound/core/misc.c
index c3f3d94b51970..51f175b2f9c8e 100644
--- a/sound/core/misc.c
+++ b/sound/core/misc.c
@@ -219,9 +219,13 @@ void snd_kill_fasync(struct snd_fasync *fasync, int signal, int poll)
 {
 	unsigned long flags;
 
-	if (!fasync || !fasync->on)
+	if (!fasync)
 		return;
 	spin_lock_irqsave(&snd_fasync_lock, flags);
+	if (!fasync->on) {
+		spin_unlock_irqrestore(&snd_fasync_lock, flags);
+		return;
+	}
 	fasync->signal = signal;
 	fasync->poll = poll;
 	list_move(&fasync->list, &snd_fasync_list);
@@ -234,7 +238,9 @@ void snd_fasync_free(struct snd_fasync *fasync)
 {
 	if (!fasync)
 		return;
+	spin_lock_irq(&snd_fasync_lock);
 	fasync->on = 0;
+	spin_unlock_irq(&snd_fasync_lock);
 	flush_work(&snd_fasync_work);
 	kfree(fasync);
 }
-- 
2.53.0