Re: possible deadlock in snd_rawmidi_free

[Takashi Iwai]

On Mon, 19 Sep 2022 14:46:13 +0200,
Rondreis wrote:
> 
> Hello,
> 
> When fuzzing the Linux kernel driver v6.0-rc4, the following crash was
> triggered.
> 
> HEAD commit: 7e18e42e4b280c85b76967a9106a13ca61c16179
> git tree: upstream
> 
> kernel config: https://pastebin.com/raw/xtrgsXP3
> console output: https://pastebin.com/raw/9tabWDtu
> 
> Sorry for failing to extract the reproducer, and the crash occurred at
> the moment of disconnecting the midi device. On other versions of
> Linux, I also triggered this crash.
> 
> I would appreciate it if you have any idea how to solve this bug.

I think there are two ways to work around it.

The first one is to move the unregister_sound*() calls out of the
sound_oss_mutex, something like:
-- 8< --

--- a/sound/core/sound_oss.c
+++ b/sound/core/sound_oss.c
@@ -162,7 +162,6 @@ int snd_unregister_oss_device(int type, struct snd_card *card, int dev)
 		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, struct snd_card *card, int dev)
 		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;
 }
-- 8< --

This should be OK, as the unregister_sound_*() itself can be called
concurrently.

Another workaround would be just to remove the register_mutex call at
snd_rawmidi_free(), e.g. something like:

-- 8< --
--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -1899,10 +1899,8 @@ static int snd_rawmidi_free(struct snd_rawmidi *rmidi)
 
 	snd_info_free_entry(rmidi->proc_entry);
 	rmidi->proc_entry = NULL;
-	mutex_lock(&register_mutex);
 	if (rmidi->ops && rmidi->ops->dev_unregister)
 		rmidi->ops->dev_unregister(rmidi);
-	mutex_unlock(&register_mutex);
 
 	snd_rawmidi_free_substreams(&rmidi->streams[SNDRV_RAWMIDI_STREAM_INPUT]);
 	snd_rawmidi_free_substreams(&rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT]);
-- 8< --

This register_mutex there should be superfluous since the device has
been already processed and detached by snd_rawmidi_dev_disconnect()
beforehand.  But if the first one is confirmed to work, the second one
can be left untouched.


thanks,

Takashi