staging: rtl8712: fix use-after-free in rtl8712_dl_fw

In the Linux kernel, the following vulnerability has been resolved:

staging: rtl8712: fix use-after-free in rtl8712_dl_fw

Syzbot reported use-after-free in rtl8712_dl_fw(). The problem was in

race condition between r871xu_dev_remove() ->ndo_open() callback.

It's easy to see from crash log, that driver accesses released firmware

in ->ndo_open() callback. It may happen, since driver was releasing

firmware _before_ unregistering netdev. Fix it by moving

unregister_netdev() before cleaning up resources.

Call Trace:

...

rtl871x_open_fw drivers/staging/rtl8712/hal_init.c:83 [inline]

rtl8712_dl_fw+0xd95/0xe10 drivers/staging/rtl8712/hal_init.c:170

rtl8712_hal_init drivers/staging/rtl8712/hal_init.c:330 [inline]

rtl871x_hal_init+0xae/0x180 drivers/staging/rtl8712/hal_init.c:394

netdev_open+0xe6/0x6c0 drivers/staging/rtl8712/os_intfs.c:380

__dev_open+0x2bc/0x4d0 net/core/dev.c:1484

Freed by task 1306:

...

release_firmware+0x1b/0x30 drivers/base/firmware_loader/main.c:1053

r871xu_dev_remove+0xcc/0x2c0 drivers/staging/rtl8712/usb_intf.c:599

usb_unbind_interface+0x1d8/0x8d0 drivers/usb/core/driver.c:458