From 83e197a8414c0ba545e7e3916ce05f836f349273 Mon Sep 17 00:00:00 2001
From: Takashi Iwai <tiwai@suse.de>
Date: Thu, 10 Jun 2021 17:20:59 +0200
Subject: [PATCH] ALSA: seq: Fix race of snd_seq_timer_open()
Git-commit: 83e197a8414c0ba545e7e3916ce05f836f349273
Patch-mainline: v5.13-rc6
References: git-fixes

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@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/000000000000dce34f05c42f110c@google.com
Link: https://lore.kernel.org/r/20210610152059.24633-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

---
 sound/core/seq/seq_timer.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/sound/core/seq/seq_timer.c b/sound/core/seq/seq_timer.c
index 1645e4142e30..9863be6fd43e 100644
--- 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_queue *q)
 		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;
 }
 
-- 
2.26.2