Linux error su: incorrect password

After backup directory /bin to my home with command :

cp -rf /bin /home/toto

I try to restore that directory with command :

cp -rf /home/toto/bin /

But when I try to use su (super user command) from konsole, I get error message su: incorrect password. The problem is that “su” has to be suid in order to read from the shadow file of your system to verify the password you have given. By issuing “chmod 777 *” you deleted the suid bit (777 = 0777). You have to reset this bit by executing :

chmod 4755 /bin/su

Source : http://www.linux-noob.com/forums/index.php?/topic/797-su-incorrect-password/