[PATCH] ALSA: core: Serialize snd_refcount put and sync

[Takashi Iwai <tiwai@suse.de>]

When a thread tries to sync-and-destroy the resource with
snd_refcount_sync(), it might lead to a UAF in a very slight time
window.  Namely, snd_refount_put() calls wake_up() after
atomic_dec_and_test(), but wait_event() in snd_refcount_sync() checks
the condition via atomic_test() without serialization, hence the
thread that called snd_refcount_sync() can continue immediately to
release the resource before another thread calls wake_up().

For avoiding the race above, introduce the spinlock and take it at
snd_refcount_put() and snd_refcount_sync() in order to serialize the
operations.

Fixes: dcca9b6064c3 ("ALSA: Add simple refcount helper functions")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20261015071838.31082-1-tiwai%40suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 include/sound/core.h | 1 +
 sound/core/misc.c    | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/sound/core.h b/include/sound/core.h
index daa91a46a16b..657b12e1e95b 100644
--- a/include/sound/core.h
+++ b/include/sound/core.h
@@ -84,6 +84,7 @@ struct snd_device {
 struct snd_refcount {
 	atomic_t count;
 	wait_queue_head_t waiter;
+	spinlock_t lock;
 };
 
 void snd_refcount_init(struct snd_refcount *ref);
diff --git a/sound/core/misc.c b/sound/core/misc.c
index 066fb9ecdcdc..6fe199e76e83 100644
--- a/sound/core/misc.c
+++ b/sound/core/misc.c
@@ -183,11 +183,13 @@ void snd_refcount_init(struct snd_refcount *ref)
 {
 	atomic_set(&ref->count, 0);
 	init_waitqueue_head(&ref->waiter);
+	spin_lock_init(&ref->lock);
 }
 EXPORT_SYMBOL_GPL(snd_refcount_init);
 
 void snd_refcount_put(struct snd_refcount *ref)
 {
+	guard(spinlock_irqsave)(&ref->lock);
 	if (atomic_dec_and_test(&ref->count))
 		wake_up(&ref->waiter);
 }
@@ -195,6 +197,7 @@ EXPORT_SYMBOL_GPL(snd_refcount_put);
 
 void snd_refcount_sync(struct snd_refcount *ref)
 {
-	wait_event(ref->waiter, !atomic_read(&ref->count));
+	guard(spinlock_irq)(&ref->lock);
+	wait_event_lock_irq(ref->waiter, !atomic_read(&ref->count), ref->lock);
 }
 EXPORT_SYMBOL_GPL(snd_refcount_sync);
-- 
2.55.0