[PATCH] ALSA: core: Fix potential data race at fasync handling

[Takashi Iwai]

In snd_fasync_work_fn(), which is the offload work for traversing and
processing the pending fasync list, the call of kill_fasync() is done
outside the snd_fasync_lock for avoiding deadlocks.  The problem is
that its the references of fasync->on, fasync->signal and fasync->poll
are done there also outside the lock.  Since these may be modified by
snd_kill_fasync() call concurrently from other process, inconsistent
values might be passed to kill_fasync().  Although there shouldn't be
critical UAF, it's still better to be addressed.

This patch moves the kill_fasync() argument evaluations inside the
snd_fasync_lock for avoiding the data races above.  The handling in
fasync->on flag is optimized in the loop to skip directly.

Also, for more clarity, snd_fasync_free() takes the lock and unlink
the pending entry more directly instead of clearing fasync->on flag.

Reported-by: Jake Lamberson <lamberson.jake@xxxxxxxxx>
Fixes: ef34a0ae7a26 ("ALSA: core: Add async signal helpers")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 sound/core/misc.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/sound/core/misc.c b/sound/core/misc.c
index 88d9e1f9a6e9..5aca09edf971 100644
--- a/sound/core/misc.c
+++ b/sound/core/misc.c
@@ -100,14 +100,18 @@ static LIST_HEAD(snd_fasync_list);
 static void snd_fasync_work_fn(struct work_struct *work)
 {
 	struct snd_fasync *fasync;
+	int signal, poll;
 
 	spin_lock_irq(&snd_fasync_lock);
 	while (!list_empty(&snd_fasync_list)) {
 		fasync = list_first_entry(&snd_fasync_list, struct snd_fasync, list);
 		list_del_init(&fasync->list);
+		if (!fasync->on)
+			continue;
+		signal = fasync->signal;
+		poll = fasync->poll;
 		spin_unlock_irq(&snd_fasync_lock);
-		if (fasync->on)
-			kill_fasync(&fasync->fasync, fasync->signal, fasync->poll);
+		kill_fasync(&fasync->fasync, signal, poll);
 		spin_lock_irq(&snd_fasync_lock);
 	}
 	spin_unlock_irq(&snd_fasync_lock);
@@ -158,7 +162,10 @@ void snd_fasync_free(struct snd_fasync *fasync)
 {
 	if (!fasync)
 		return;
-	fasync->on = 0;
+
+	scoped_guard(spinlock_irq, &snd_fasync_lock)
+		list_del_init(&fasync->list);
+
 	flush_work(&snd_fasync_work);
 	kfree(fasync);
 }
-- 
2.53.0