net: can: ems_usb: fix use-after-free in ems_usb_disconnect()

commit ab4a0b8 upstream.

In ems_usb_disconnect() dev pointer, which is netdev private data, is
used after free_candev() call:
| 	if (dev) {
| 		unregister_netdev(dev->netdev);
| 		free_candev(dev->netdev);
|
| 		unlink_all_urbs(dev);
|
| 		usb_free_urb(dev->intr_urb);
|
| 		kfree(dev->intr_in_buffer);
| 		kfree(dev->tx_msg_buffer);
| 	}

Fix it by simply moving free_candev() at the end of the block.

Fail log:
| BUG: KASAN: use-after-free in ems_usb_disconnect
| Read of size 8 at addr ffff88804e041008 by task kworker/1:2/2895
|
| CPU: 1 PID: 2895 Comm: kworker/1:2 Not tainted 5.13.0-rc5+ #164
| Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.4
| Workqueue: usb_hub_wq hub_event
| Call Trace:
|     dump_stack (lib/dump_stack.c:122)
|     print_address_description.constprop.0.cold (mm/kasan/report.c:234)
|     kasan_report.cold (mm/kasan/report.c:420 mm/kasan/report.c:436)
|     ems_usb_disconnect (drivers/net/can/usb/ems_usb.c:683 drivers/net/can/usb/ems_usb.c:1058)

Fixes: 702171a ("ems_usb: Added support for EMS CPC-USB/ARM7 CAN/USB interface")
Link: https://lore.kernel.org/r/[email protected]
Cc: linux-stable <[email protected]>
Signed-off-by: Pavel Skripkin <[email protected]>
Signed-off-by: Marc Kleine-Budde <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/net/can/usb/ems_usb.c

@@ -1053,14 +1053,15 @@ static void ems_usb_disconnect(struct usb_interface *intf)
 
 	if (dev) {
 		unregister_netdev(dev->netdev);
-		free_candev(dev->netdev);
 
 		unlink_all_urbs(dev);
 
 		usb_free_urb(dev->intr_urb);
 
 		kfree(dev->intr_in_buffer);
 		kfree(dev->tx_msg_buffer);
+
+		free_candev(dev->netdev);
 	}
 }