usb: gadget: u_serial: Fix the issue that gs_start_io crashed due to accessing null pointer

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

usb: gadget: u_serial: Fix the issue that gs_start_io crashed due to accessing null pointer

Considering that in some extreme cases,

when u_serial driver is accessed by multiple threads,

Thread A is executing the open operation and calling the gs_open,

Thread B is executing the disconnect operation and calling the

gserial_disconnect function,The port->port_usb pointer will be set to NULL.

E.g.

Thread A Thread B

gs_open() gadget_unbind_driver()

gs_start_io() composite_disconnect()

gs_start_rx() gserial_disconnect()

... ...

spin_unlock(&port->port_lock)

status = usb_ep_queue() spin_lock(&port->port_lock)

spin_lock(&port->port_lock) port->port_usb = NULL

gs_free_requests(port->port_usb->in) spin_unlock(&port->port_lock)

Crash

This causes thread A to access a null pointer (port->port_usb is null)

when calling the gs_free_requests function, causing a crash.

If port_usb is NULL, the release request will be skipped as it

will be done by gserial_disconnect.

So add a null pointer check to gs_start_io before attempting

to access the value of the pointer port->port_usb.

Call trace:

gs_start_io+0x164/0x25c

gs_open+0x108/0x13c

tty_open+0x314/0x638

chrdev_open+0x1b8/0x258

do_dentry_open+0x2c4/0x700

vfs_open+0x2c/0x3c

path_openat+0xa64/0xc60

do_filp_open+0xb8/0x164

do_sys_openat2+0x84/0xf0

__arm64_sys_openat+0x70/0x9c

invoke_syscall+0x58/0x114

el0_svc_common+0x80/0xe0

do_el0_svc+0x1c/0x28

el0_svc+0x38/0x68