/etc/sudoers的权限必须是0440，错误修改成0777后导致sudo命令无法使用。
xxx@yyy:~$ sudo more /etc/sudoers.d/README
[sudo] password for xxx:
#
# As of Debian version 1.7.2p1-1, the default /etc/sudoers file created on
# installation of the package now includes the directive:
#
# #includedir /etc/sudoers.d
#
# This will cause sudo to read and parse any files in the /etc/sudoers.d
# directory that do not end in '~' or contain a '.' character.
#
# Note that there must be at least one file in the sudoers.d directory (this
# one will do), and all files in this directory should be mode 0440.
#
# Note also, that because sudoers contents can vary widely, no attempt is
# made to add this directive to existing sudoers files on upgrade. Feel free
# to add the above directive to the end of your /etc/sudoers file to enable
# this functionality for existing installations if you wish!
#
解决办法1：如果系统中有root用户，那么只需要登录root用户修改/etc/sudoers文件的权限。
su root
chmod 0440 /etc/sudoers
但如果此时未设置root用户或密码，无法登录root用户。
解决办法2：可在普通用户下通过pkexec命令来修改权限。
在重新返回通常模式下，执行pkexec chmod 0440 /etc/sudoers
xxx@yyy:~$ pkexec chmod 0440 /etc/sudoers
==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
Authentication is needed to run `/bin/chmod' as the super user
Authenticating as: xxx,,, (xxx)
Password:
==== AUTHENTICATION COMPLETE ===
解决办法3：
1、重启机器，按下SHIFT，选择recovery mode，蹦出来一个菜单，选择root进入root shell。
mount -o remount,rw /
chmod 0440 /etc/sudoers
2、重启进入普通状态下的系统，就能sudo了