ALSA: usb-audio: Fix use-after-free in snd_usb_mixer_free()

When snd_usb_create_mixer() fails, snd_usb_mixer_free() frees
mixer->id_elems but the controls already added to the card still
reference the freed memory. Later when snd_card_register() runs,
the OSS mixer layer calls their callbacks and hits a use-after-free read.

Call trace:
  get_ctl_value+0x63f/0x820 sound/usb/mixer.c:411
  get_min_max_with_quirks.isra.0+0x240/0x1f40 sound/usb/mixer.c:1241
  mixer_ctl_feature_info+0x26b/0x490 sound/usb/mixer.c:1381
  snd_mixer_oss_build_test+0x174/0x3a0 sound/core/oss/mixer_oss.c:887
  ...
  snd_card_register+0x4ed/0x6d0 sound/core/init.c:923
  usb_audio_probe+0x5ef/0x2a90 sound/usb/card.c:1025

Fix by calling snd_ctl_remove() for all mixer controls before freeing
id_elems. We save the next pointer first because snd_ctl_remove()
frees the current element.

Fixes: 6639b6c ("[ALSA] usb-audio - add mixer control notifications")
Cc: stable@vger.kernel.org
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Berk Cem Goksel <berkcgoksel@gmail.com>
Link: https://patch.msgid.link/20260120102855.7300-1-berkcgoksel@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: berkgoksel

sound/usb/mixer.c

@@ -2945,10 +2945,23 @@ static int parse_audio_unit(struct mixer_build *state, int unitid)
 
 static void snd_usb_mixer_free(struct usb_mixer_interface *mixer)
 {
+	struct usb_mixer_elem_list *list, *next;
+	int id;
+
 	/* kill pending URBs */
 	snd_usb_mixer_disconnect(mixer);
 
-	kfree(mixer->id_elems);
+	/* Unregister controls first, snd_ctl_remove() frees the element */
+	if (mixer->id_elems) {
+		for (id = 0; id < MAX_ID_ELEMS; id++) {
+			for (list = mixer->id_elems[id]; list; list = next) {
+				next = list->next_id_elem;
+				if (list->kctl)
+					snd_ctl_remove(mixer->chip->card, list->kctl);
+			}
+		}
+		kfree(mixer->id_elems);
+	}
 	if (mixer->urb) {
 		kfree(mixer->urb->transfer_buffer);
 		usb_free_urb(mixer->urb);