CVE-2026-98772: USB: serial: fix port tear down use-after-free

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

USB: serial: fix port tear down use-after-free

Some drivers for multiport devices access port driver data from completion handlers of shared URBs submitted at attach() or first open() and stopped at disconnect() or last close(), respectively.

A simple NULL check before accessing the driver data makes sure that a port state container has at least been allocated, but a completion handler can still race with port tear down.

Reorder the disconnect handling so that ports are not deregistered (and their driver data freed) until after all ports have been hung up and the driver disconnect() callback has run so that all I/O has been stopped.