[PATCH] ALSA: usb: Fix UAF at delayed release of MIDI2 EPs

The recent fix for UAF in ump_to_endpoint() caused another UAF because
it tries to dereference the UMP endpoint object, but this might be
executed at a delayed context where the endpoint has been already
released.

Add private_free to clear the associated data for avoiding the further
dereference for delayed releases.

Fixes: 4a05b2d1b464 ("ALSA: usb-audio: fix use-after-free in ump_to_endpoint()")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=565b1138cfbe549d4422
Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>
---
 sound/usb/midi2.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/sound/usb/midi2.c b/sound/usb/midi2.c
index 83980fb83ac8..1dc78f536ef9 100644
--- a/sound/usb/midi2.c
+++ b/sound/usb/midi2.c
@@ -677,6 +677,14 @@ static int parse_midi_2_0_endpoints(struct snd_usb_midi2_interface *umidi)
 	return 0;
 }
 
+static void free_ump_private_data(struct snd_ump_endpoint *ump)
+{
+	struct snd_usb_midi2_ump *rmidi = ump->private_data;
+
+	if (rmidi)
+		rmidi->ump = NULL;
+}
+
 static void free_all_midi2_umps(struct snd_usb_midi2_interface *umidi)
 {
 	struct snd_usb_midi2_ump *rmidi;
@@ -727,6 +735,7 @@ static int create_midi2_ump(struct snd_usb_midi2_interface *umidi,
 
 	ump->private_data = rmidi;
 	ump->ops = &snd_usb_midi_v2_ump_ops;
+	ump->private_free = free_ump_private_data;
 
 	rmidi->eps[STR_IN] = ep_in;
 	rmidi->eps[STR_OUT] = ep_out;
-- 
2.55.0