USB: appledisplay: close race between probe and completion handler

There is a small window during probing when IO is running
but the backlight is not registered. Processing events
during that time will crash. The completion handler
needs to check for a backlight before scheduling work.

The bug is as old as the driver.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
CC: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240912123317.1026049-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: oneukum

drivers/usb/misc/appledisplay.c

@@ -107,7 +107,12 @@ static void appledisplay_complete(struct urb *urb)
 	case ACD_BTN_BRIGHT_UP:
 	case ACD_BTN_BRIGHT_DOWN:
 		pdata->button_pressed = 1;
-		schedule_delayed_work(&pdata->work, 0);
+		/*
+		 * there is a window during which no device
+		 * is registered
+		 */
+		if (pdata->bd )
+			schedule_delayed_work(&pdata->work, 0);
 		break;
 	case ACD_BTN_NONE:
 	default:
@@ -202,6 +207,7 @@ static int appledisplay_probe(struct usb_interface *iface,
 	const struct usb_device_id *id)
 {
 	struct backlight_properties props;
+	struct backlight_device *backlight;
 	struct appledisplay *pdata;
 	struct usb_device *udev = interface_to_usbdev(iface);
 	struct usb_endpoint_descriptor *endpoint;
@@ -272,13 +278,14 @@ static int appledisplay_probe(struct usb_interface *iface,
 	memset(&props, 0, sizeof(struct backlight_properties));
 	props.type = BACKLIGHT_RAW;
 	props.max_brightness = 0xff;
-	pdata->bd = backlight_device_register(bl_name, NULL, pdata,
+	backlight = backlight_device_register(bl_name, NULL, pdata,
 					      &appledisplay_bl_data, &props);
-	if (IS_ERR(pdata->bd)) {
+	if (IS_ERR(backlight)) {
 		dev_err(&iface->dev, "Backlight registration failed\n");
-		retval = PTR_ERR(pdata->bd);
+		retval = PTR_ERR(backlight);
 		goto error;
 	}
+	pdata->bd = backlight;
 
 	/* Try to get brightness */
 	brightness = appledisplay_bl_get_brightness(pdata->bd);