This morning, I got a spare machine from of of the labmates. The OS is ubuntu 12.04. I could not log into the system because it is under other person's account.
Attack
Just enter into recovery mode when I start the computer.
select root.
passwd [username]
Then there is an error information. authentication token manipulation error.
mount -rw -o remount /
passwd [username]
Countermeasure
[hide recovery mode]
vi /etc/default/grub
update-grub
[set root password]
sudo passwd root