ALSA: usb-audio: fix use-after-free in ump_to_endpoint()

create_midi2_ump() registers a card-owned snd_ump_endpoint and stores a
back-pointer to its per-interface snd_usb_midi2_ump object in
ump->private_data, but it never installs an ump->private_free hook and
never clears that pointer.

If a later step of snd_usb_midi_v2_create() fails, its error path calls
free_all_midi2_umps(), which kfree()s the snd_usb_midi2_ump object while
the already-registered endpoint keeps pointing at it.  The created
/dev/snd/umpC*D* node stays exposed, so the first operation of any UMP
open, ump_to_endpoint(), dereferences the dangling ump->private_data and
reads rmidi->eps[dir] out of freed memory.

A malicious USB MIDI 2.0 device that makes creation fail after the
endpoint is registered can thus trigger a slab use-after-free read on a
subsequent open of the UMP node.

Clear the endpoint's back-pointer before freeing the object, and let
ump_to_endpoint() tolerate a NULL private_data so the open/close/trigger
callbacks fail cleanly (their callers already handle a NULL endpoint)
instead of dereferencing a stale pointer.

Discovered by XBOW, triaged by Baul Lee <[email protected]>

Fixes: ff49d1df79 ("ALSA: usb-audio: USB MIDI 2.0 UMP support")
Reported-by: Federico Kirschbaum <[email protected]>
Reported-by: Baul Lee <[email protected]>
Cc: [email protected]
Signed-off-by: Baul Lee <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

sound/usb/midi2.c

@@ -329,7 +329,7 @@ ump_to_endpoint(struct snd_ump_endpoint *ump, int dir)
 {
 	struct snd_usb_midi2_ump *rmidi = ump->private_data;
 
-	return rmidi->eps[dir];
+	return rmidi ? rmidi->eps[dir] : NULL;
 }
 
 /* ump open callback */
@@ -685,6 +685,8 @@ static void free_all_midi2_umps(struct snd_usb_midi2_interface *umidi)
 		rmidi = list_first_entry(&umidi->rawmidi_list,
 					 struct snd_usb_midi2_ump, list);
 		list_del(&rmidi->list);
+		if (rmidi->ump)
+			rmidi->ump->private_data = NULL;
 		kfree(rmidi);
 	}
 }