[PATCH] ALSA: rawmidi: Fix potential UAF from sequencer destruction

[John Keeping <john@metanate.com>]

If the sequencer device outlives the rawmidi device, then
snd_rawmidi_dev_seq_free() will run after release_rawmidi_device() has
freed the snd_rawmidi structure.

This can easily be reproduced with CONFIG_DEBUG_KOBJECT_RELEASE.

Keep a reference to the rawmidi device until the sequencer has been
destroyed in order to avoid this.

Signed-off-by: John Keeping <john@metanate.com>
---
 sound/core/rawmidi.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/core/rawmidi.c b/sound/core/rawmidi.c
index 6f30231bdb88..b015f5f69175 100644
--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -1860,6 +1860,7 @@ static void snd_rawmidi_dev_seq_free(struct snd_seq_device *device)
 	struct snd_rawmidi *rmidi = device->private_data;
 
 	rmidi->seq_dev = NULL;
+	put_device(&rmidi->dev);
 }
 #endif
 
@@ -1936,6 +1937,9 @@ static int snd_rawmidi_dev_register(struct snd_device *device)
 #if IS_ENABLED(CONFIG_SND_SEQUENCER)
 	if (!rmidi->ops || !rmidi->ops->dev_register) { /* own registration mechanism */
 		if (snd_seq_device_new(rmidi->card, rmidi->device, SNDRV_SEQ_DEV_ID_MIDISYNTH, 0, &rmidi->seq_dev) >= 0) {
+			/* Ensure we outlive the sequencer (see snd_rawmidi_dev_seq_free). */
+			get_device(&rmidi->dev);
+
 			rmidi->seq_dev->private_data = rmidi;
 			rmidi->seq_dev->private_free = snd_rawmidi_dev_seq_free;
 			sprintf(rmidi->seq_dev->name, "MIDI %d-%d", rmidi->card->number, rmidi->device);
-- 
2.33.0