[PATCH 5.19 002/717] ALSA: oss: Fix potential deadlock at unregistration

Greg Kroah-Hartman
2022-10-22T07:35:41Z

From: Takashi Iwai <tiwai@suse.de>

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@mail.gmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20221011070147.7611-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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
@@ -162,7 +162,6 @@ 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);
@@ -174,12 +173,18 @@ 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;
}