usb: gadget: udc: fix use-after-free in usb_gadget_state_work

usb: gadget: udc: fix use-after-free in usb_gadget_state_work

A race condition during gadget teardown can lead to a use-after-free
in usb_gadget_state_work(), as reported by KASAN:

  BUG: KASAN: invalid-access in sysfs_notify+0x2c/0xd0
  Workqueue: events usb_gadget_state_work

The fundamental race occurs because a concurrent event (e.g., an
interrupt) can call usb_gadget_set_state() and schedule gadget->work
at any time during the cleanup process in usb_del_gadget().

Commit 399a45e5237c ("usb: gadget: core: flush gadget workqueue after
device removal") attempted to fix this by moving flush_work() to after
device_del(). However, this does not fully solve the race, as a new
work item can still be scheduled *after* flush_work() completes but
before the gadget's memory is freed, leading to the same use-after-free.

This patch fixes the race condition robustly by introducing a 'teardown'
flag and a 'state_lock' spinlock to the usb_gadget struct. The flag is
set during cleanup in usb_del_gadget() *before* calling flush_work() to
prevent any new work from being scheduled once cleanup has commenced.
The scheduling site, usb_gadget_set_state(), now checks this flag under
the lock before queueing the work, thus safely closing the race window.

Fixes: 5702f75375aa9 ("usb: gadget: udc-core: move sysfs_notify() to a workqueue")
Cc: stable <stable@kernel.org>
Signed-off-by: Jimmy Hu <hhhuuu@google.com>
Link: https://patch.msgid.link/20251023054945.233861-1-hhhuuu@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>