staging: most: usb: fix usb_disconnect race condition

The functions usb_disconnect and usb_sndbulkpipe are racing for the struct
usb_device, which might cause a null pointer dereference exception. This
patch fixes this race condition by protecting the critical section inside
the function hdm_enque with the io_mutex.
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/most/usb/usb.c

@@ -565,14 +565,19 @@ static int hdm_enqueue(struct most_interface *iface, int channel,
 
 	mdev = to_mdev(iface);
 	conf = &mdev->conf[channel];
-	dev = &mdev->usb_device->dev;
 
-	if (!mdev->usb_device)
-		return -ENODEV;
+	mutex_lock(&mdev->io_mutex);
+	if (!mdev->usb_device) {
+		retval = -ENODEV;
+		goto _exit;
+	}
 
+	dev = &mdev->usb_device->dev;
 	urb = usb_alloc_urb(NO_ISOCHRONOUS_URB, GFP_ATOMIC);
-	if (!urb)
-		return -ENOMEM;
+	if (!urb) {
+		retval = -ENOMEM;
+		goto _exit;
+	}
 
 	if ((conf->direction & MOST_CH_TX) && mdev->padding_active[channel] &&
 	    hdm_add_padding(mdev, channel, mbo)) {
@@ -613,12 +618,14 @@ static int hdm_enqueue(struct most_interface *iface, int channel,
 		dev_err(dev, "URB submit failed with error %d.\n", retval);
 		goto _error_1;
 	}
-	return 0;
+	goto _exit;
 
 _error_1:
 	usb_unanchor_urb(urb);
 _error:
 	usb_free_urb(urb);
+_exit:
+	mutex_unlock(&mdev->io_mutex);
 	return retval;
 }