usb: gadget: u_ether: Fix race between gether_disconnect and eth_stop

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

usb: gadget: u_ether: Fix race between gether_disconnect and eth_stop

A race condition between gether_disconnect() and eth_stop() leads to a

NULL pointer dereference. Specifically, if eth_stop() is triggered

concurrently while gether_disconnect() is tearing down the endpoints,

eth_stop() attempts to access the cleared endpoint descriptor, causing

the following NPE:

Unable to handle kernel NULL pointer dereference

Call trace:

__dwc3_gadget_ep_enable+0x60/0x788

dwc3_gadget_ep_enable+0x70/0xe4

usb_ep_enable+0x60/0x15c

eth_stop+0xb8/0x108

Because eth_stop() crashes while holding the dev->lock, the thread

running gether_disconnect() fails to acquire the same lock and spins

forever, resulting in a hardlockup:

Core - Debugging Information for Hardlockup core(7)

Call trace:

queued_spin_lock_slowpath+0x94/0x488

_raw_spin_lock+0x64/0x6c

gether_disconnect+0x19c/0x1e8

ncm_set_alt+0x68/0x1a0

composite_setup+0x6a0/0xc50

The root cause is that the clearing of dev->port_usb in

gether_disconnect() is delayed until the end of the function.

Move the clearing of dev->port_usb to the very beginning of

gether_disconnect() while holding dev->lock. This cuts off the link

immediately, ensuring eth_stop() will see dev->port_usb as NULL and

safely bail out.