ALSA: line6: Fix racy access to midibuf

There can be concurrent accesses to line6 midibuf from both the URB
completion callback and the rawmidi API access.  This could be a cause
of KMSAN warning triggered by syzkaller below (so put as reported-by
here).

This patch protects the midibuf call of the former code path with a
spinlock for avoiding the possible races.

Reported-by: syzbot+78eccfb8b3c9a85fc6c5@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/00000000000000949c061df288c5@google.com
Cc: <stable@vger.kernel.org>
Link: https://patch.msgid.link/20240805130129.10872-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/line6/driver.c

@@ -286,12 +286,14 @@ static void line6_data_received(struct urb *urb)
 {
 	struct usb_line6 *line6 = (struct usb_line6 *)urb->context;
 	struct midi_buffer *mb = &line6->line6midi->midibuf_in;
+	unsigned long flags;
 	int done;
 
 	if (urb->status == -ESHUTDOWN)
 		return;
 
 	if (line6->properties->capabilities & LINE6_CAP_CONTROL_MIDI) {
+		spin_lock_irqsave(&line6->line6midi->lock, flags);
 		done =
 			line6_midibuf_write(mb, urb->transfer_buffer, urb->actual_length);
 
@@ -300,12 +302,15 @@ static void line6_data_received(struct urb *urb)
 			dev_dbg(line6->ifcdev, "%d %d buffer overflow - message skipped\n",
 				done, urb->actual_length);
 		}
+		spin_unlock_irqrestore(&line6->line6midi->lock, flags);
 
 		for (;;) {
+			spin_lock_irqsave(&line6->line6midi->lock, flags);
 			done =
 				line6_midibuf_read(mb, line6->buffer_message,
 						   LINE6_MIDI_MESSAGE_MAXLEN,
 						   LINE6_MIDIBUF_READ_RX);
+			spin_unlock_irqrestore(&line6->line6midi->lock, flags);
 
 			if (done <= 0)
 				break;