Do AskUbuntu - é sugerido adicionar o comando de bloqueio do protetor de tela:
I've been able to workaround this issue, by using the following command
sudo pm-hibernate; gnome-screensaver-command -l
Do note that, you should run this as a normal user and not as root user for locking to work, because by default in Ubuntu, there is no password for the root user so after resume, lock-screen can be unlocked by simply shaking the mouse.