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

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

BugLink: https://bugs.launchpad.net/bugs/1938199
commit ab4a0b8fcb9a95c02909b62049811bd2e586aaa4 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: 702171adeed3 ("ems_usb: Added support for EMS CPC-USB/ARM7 CAN/USB interface")
Link: https://lore.kernel.org/r/20210617185130.5834-1-paskripkin@gmail.com
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>