sound: snd_ctl_remove_user_ctl: prevent removal of kernel controls

Ensure that userspace can remove only user controls.  Controls created
by kernel drivers must not be removed because they might be referenced
in calls to snd_ctl_notify().

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/control.c

@@ -436,6 +436,10 @@ static int snd_ctl_remove_user_ctl(struct snd_ctl_file * file,
 		ret = -ENOENT;
 		goto error;
 	}
+	if (!(kctl->vd[0].access & SNDRV_CTL_ELEM_ACCESS_USER)) {
+		ret = -EINVAL;
+		goto error;
+	}
 	for (idx = 0; idx < kctl->count; idx++)
 		if (kctl->vd[idx].owner != NULL && kctl->vd[idx].owner != file) {
 			ret = -EBUSY;