I don't think it's related to signals, whole system freezes - Ctrl-Alt-F1 (switch to root console), Ctrl-Alt-Del, screen, mouse. Nothing works. Mouse does not move, screen does not update. For example if you turn on/off laptop charger or terminate wifi, charger/wifi icon will not update. It's reproducible on real hardware and on Virtual box VM (of course only guest system freezes), and confirmed by Ubuntu team member.