[PATCH 5.12 049/173] ALSA: seq: Fix race of snd_seq_timer_open()

[Greg Kroah-Hartman]

From: Takashi Iwai <tiwai@xxxxxxx>

commit 83e197a8414c0ba545e7e3916ce05f836f349273 upstream.

The timer instance per queue is exclusive, and snd_seq_timer_open()
should have managed the concurrent accesses.  It looks as if it's
checking the already existing timer instance at the beginning, but
it's not right, because there is no protection, hence any later
concurrent call of snd_seq_timer_open() may override the timer
instance easily.  This may result in UAF, as the leftover timer
instance can keep running while the queue itself gets closed, as
spotted by syzkaller recently.

For avoiding the race, add a proper check at the assignment of
tmr->timeri again, and return -EBUSY if it's been already registered.

Reported-by: syzbot+ddc1260a83ed1cbf6fb5@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/000000000000dce34f05c42f110c@xxxxxxxxxx
Link: https://lore.kernel.org/r/20210610152059.24633-1-tiwai@xxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 sound/core/seq/seq_timer.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/sound/core/seq/seq_timer.c
+++ b/sound/core/seq/seq_timer.c
@@ -297,8 +297,16 @@ int snd_seq_timer_open(struct snd_seq_qu
 		return err;
 	}
 	spin_lock_irq(&tmr->lock);
-	tmr->timeri = t;
+	if (tmr->timeri)
+		err = -EBUSY;
+	else
+		tmr->timeri = t;
 	spin_unlock_irq(&tmr->lock);
+	if (err < 0) {
+		snd_timer_close(t);
+		snd_timer_instance_free(t);
+		return err;
+	}
 	return 0;
 }