问题：
    sudo: /etc/sudoers is world writable
    sudo: no valid sudoers sources found, quitting
    sudo: unable to initialize policy plugin

    1. $ su root     #切换到root权限
    2. $ 输入你的密码
    3. sh-3.2# chmod 440 /etc/sudoers        #将当前文件权限修改为原来的权限