usb: gadget: u_serial: Fix race condition in TTY wakeup

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

usb: gadget: u_serial: Fix race condition in TTY wakeup

A race condition occurs when gs_start_io() calls either gs_start_rx() or

gs_start_tx(), as those functions briefly drop the port_lock for

usb_ep_queue(). This allows gs_close() and gserial_disconnect() to clear

port.tty and port_usb, respectively.

Use the null-safe TTY Port helper function to wake up TTY.

Example

CPU1: CPU2:

gserial_connect() // lock

gs_close() // await lock

gs_start_rx() // unlock

usb_ep_queue()

gs_close() // lock, reset port.tty and unlock

gs_start_rx() // lock

tty_wakeup() // NPE