[PATCH 4.9 041/159] ALSA: oss: Fix potential deadlock at unregistration

[Greg Kroah-Hartman]

From: Takashi Iwai <tiwai@xxxxxxx>

commit 97d917879d7f92df09c3f21fd54609a8bcd654b2 upstream.

We took sound_oss_mutex around the calls of unregister_sound_special()
at unregistering OSS devices.  This may, however, lead to a deadlock,
because we manage the card release via the card's device object, and
the release may happen at unregister_sound_special() call -- which
will take sound_oss_mutex again in turn.

Although the deadlock might be fixed by relaxing the rawmidi mutex in
the previous commit, it's safer to move unregister_sound_special()
calls themselves out of the sound_oss_mutex, too.  The call is
race-safe as the function has a spinlock protection by itself.

Link: https://lore.kernel.org/r/CAB7eexJP7w1B0mVgDF0dQ+gWor7UdkiwPczmL7pn91xx8xpzOA@xxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20221011070147.7611-2-tiwai@xxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 sound/core/sound_oss.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

--- a/sound/core/sound_oss.c
+++ b/sound/core/sound_oss.c
@@ -179,7 +179,6 @@ int snd_unregister_oss_device(int type,
 		mutex_unlock(&sound_oss_mutex);
 		return -ENOENT;
 	}
-	unregister_sound_special(minor);
 	switch (SNDRV_MINOR_OSS_DEVICE(minor)) {
 	case SNDRV_MINOR_OSS_PCM:
 		track2 = SNDRV_MINOR_OSS(cidx, SNDRV_MINOR_OSS_AUDIO);
@@ -191,12 +190,18 @@ int snd_unregister_oss_device(int type,
 		track2 = SNDRV_MINOR_OSS(cidx, SNDRV_MINOR_OSS_DMMIDI1);
 		break;
 	}
-	if (track2 >= 0) {
-		unregister_sound_special(track2);
+	if (track2 >= 0)
 		snd_oss_minors[track2] = NULL;
-	}
 	snd_oss_minors[minor] = NULL;
 	mutex_unlock(&sound_oss_mutex);
+
+	/* call unregister_sound_special() outside sound_oss_mutex;
+	 * otherwise may deadlock, as it can trigger the release of a card
+	 */
+	unregister_sound_special(minor);
+	if (track2 >= 0)
+		unregister_sound_special(track2);
+
 	kfree(mptr);
 	return 0;
 }