1. Click "e" when entering the grub
2. Add option " init=/bin/sh" to linux line. (/bin/sh can not be replace by /bin/bash , /bin/bash do not have passwd command)
3.Mount path (mount -o remount, rw /)
4.Change permission of passwd (chmod -v 4711 /usr/bin/passwd)
5.passwd root to change password
6.reboot target
How to reset password for unknow root
原文：http://www.cnblogs.com/root-wang/p/4616098.html