ALSA: line6: Fix racy initialization of LINE6 MIDI

The initialization of MIDI devices that are found on some LINE6
drivers are currently done in a racy way; namely, the MIDI buffer
instance is allocated and initialized in each private_init callback
while the communication with the interface is already started via
line6_init_cap_control() call before that point.  This may lead to
Oops in line6_data_received() when a spurious event is received, as
reported by syzkaller.

This patch moves the MIDI initialization to line6_init_cap_control()
as well instead of the too-lately-called private_init for avoiding the
race.  Also this reduces slightly more lines, so it's a win-win
change.

Reported-by: syzbot+0d2b3feb0a2887862e06@syzkallerlkml..appspotmail.com
Link: https://lore.kernel.org/r/000000000000a4be9405c28520de@google.com
Link: https://lore.kernel.org/r/20210517132725.GA50495@hyeyoo
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210518083939.1927-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/line6/driver.c

@@ -699,6 +699,10 @@ static int line6_init_cap_control(struct usb_line6 *line6)
 		line6->buffer_message = kmalloc(LINE6_MIDI_MESSAGE_MAXLEN, GFP_KERNEL);
 		if (!line6->buffer_message)
 			return -ENOMEM;
+
+		ret = line6_init_midi(line6);
+		if (ret < 0)
+			return ret;
 	} else {
 		ret = line6_hwdep_init(line6);
 		if (ret < 0)

sound/usb/line6/pod.c

@@ -376,11 +376,6 @@ static int pod_init(struct usb_line6 *line6,
 	if (err < 0)
 		return err;
 
-	/* initialize MIDI subsystem: */
-	err = line6_init_midi(line6);
-	if (err < 0)
-		return err;
-
 	/* initialize PCM subsystem: */
 	err = line6_init_pcm(line6, &pod_pcm_properties);
 	if (err < 0)

sound/usb/line6/variax.c

@@ -159,7 +159,6 @@ static int variax_init(struct usb_line6 *line6,
 		       const struct usb_device_id *id)
 {
 	struct usb_line6_variax *variax = line6_to_variax(line6);
-	int err;
 
 	line6->process_message = line6_variax_process_message;
 	line6->disconnect = line6_variax_disconnect;
@@ -172,11 +171,6 @@ static int variax_init(struct usb_line6 *line6,
 	if (variax->buffer_activate == NULL)
 		return -ENOMEM;
 
-	/* initialize MIDI subsystem: */
-	err = line6_init_midi(&variax->line6);
-	if (err < 0)
-		return err;
-
 	/* initiate startup procedure: */
 	schedule_delayed_work(&line6->startup_work,
 			      msecs_to_jiffies(VARIAX_STARTUP_DELAY1));