AZL-99954

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

xhci: dbgtty: Fix unregister on tty_register_driver() failure

If tty_register_driver() fails, it drops the reference, but fails to set the global dbc_tty_driver to NULL, causing the unregister to be called again when module exits.

On module unload dbc_tty_exit() only gates its cleanup on the driver pointer being non-NULL, so it operates on the already-freed driver:

module_init(xhci_hcd_init)
  xhci_hcd_init()
    xhci_dbc_init()                       [return value ignored]
      dbc_tty_init()
        tty_register_driver() fails
          tty_driver_kref_put()           -> driver freed
          (dbc_tty_driver left dangling)
...
module_exit(xhci_hcd_fini)
  xhci_hcd_fini()
    xhci_dbc_exit()
      dbc_tty_exit()
        if (dbc_tty_driver)               -> true (dangling)
          tty_unregister_driver()         -> use-after-free