usbip: vudc: Fix use after free bug in vudc_remove due to race condition

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

usbip: vudc: Fix use after free bug in vudc_remove due to race condition

This patch follows up Zheng Wang's 2023 report of a use-after-free in

vudc_remove(). The original thread stalled on Shuah Khan's request for

runtime testing of the unplug/unbind path. This patch supplies that

testing and keeps Zheng's original fix shape.

In vudc_probe(), v_init_timer() binds udc->tr_timer.timer to v_timer().

usbip_sockfd_store() starts the timer via v_start_timer()/v_kick_timer().

vudc_remove() can then free the containing struct vudc while the timer is

still pending or executing.

KASAN confirms the race on an unpatched x86_64 QEMU guest with

CONFIG_KASAN=y, CONFIG_USBIP_VUDC=y, CONFIG_USB_ZERO=y, and a tight loop

that repeatedly writes a socket fd to usbip_sockfd, closes the socket

pair, and unbinds/rebinds usbip-vudc.0:

BUG: KASAN: slab-use-after-free in __run_timer_base.part.0+0x8ba/0x8e0

Write of size 8 at addr ffff888001b80740 by task trigger_and_unb/239

Allocated by task 239:

vudc_probe+0x4d/0xaa0

Freed by task 239:

kfree+0x18f/0x520

device_release_driver_internal+0x388/0x540

unbind_store+0xd9/0x100

This lands in the timer core rather than v_timer() itself because the

embedded timer_list is being walked after its containing struct vudc has

already been freed. The underlying lifetime bug is the same one Zheng

reported.

With v_stop_timer() called from vudc_remove() and the timer deleted

synchronously, the same harness completed 5000 bind/unbind iterations

with no KASAN report.