net: hso: fix NULL-deref on disconnect regression

Commit 8a12f88 ("net: hso: fix null-ptr-deref during tty device
unregistration") fixed the racy minor allocation reported by syzbot, but
introduced an unconditional NULL-pointer dereference on every disconnect
instead.

Specifically, the serial device table must no longer be accessed after
the minor has been released by hso_serial_tty_unregister().

Fixes: 8a12f88 ("net: hso: fix null-ptr-deref during tty device unregistration")
Cc: [email protected]
Cc: Anirudh Rayabharam <[email protected]>
Reported-by: Leonardo Antoniazzi <[email protected]>
Signed-off-by: Johan Hovold <[email protected]>
Reviewed-by: Anirudh Rayabharam <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

drivers/net/usb/hso.c

@@ -3104,7 +3104,7 @@ static void hso_free_interface(struct usb_interface *interface)
 			cancel_work_sync(&serial_table[i]->async_put_intf);
 			cancel_work_sync(&serial_table[i]->async_get_intf);
 			hso_serial_tty_unregister(serial);
-			kref_put(&serial_table[i]->ref, hso_serial_ref_free);
+			kref_put(&serial->parent->ref, hso_serial_ref_free);
 		}
 	}