默认 系统 root 登录 图形界面，出现 登录失败。解决方法如下：
1，登录普通用户, 打开终端执行命令, 使用su root或sudo -i切换到root用户(必须)
su root
按照提示输入密码即可切换用户。
若root账户密码忘记，使用如下命令重设后再切换。
sudo passwd root
2，在终端中输入如下命令，打开文件
gedit /etc/pam.d/gdm-autologin
将打开的文件中第三行前加#，如下注释掉语句 “auth required pam_succeed_if.so user != root quiet_success”
#%PAM-1.0
auth requisite pam_nologin.so
#auth required pam_succeed_if.so user != root quiet_success
auth optional pam_gdm.so
auth optional pam_gnome_keyring.so
auth required pam_permit.so
@include common-account
# SELinux needs to be the first session rule. This ensures that any
# lingering context has been cleared. Without this it is possible
# that a module could execute code in the wrong domain.
session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_selinux.so close
session required pam_loginuid.so
# SELinux needs to intervene at login time to ensure that the process
# starts in the proper default security context. Only sessions which are
# intended to run in the user's context should be run after this.
session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_selinux.so open
session optional pam_keyinit.so force revoke
session required pam_limits.so
session required pam_env.so readenv=1
session required pam_env.so readenv=1 user_readenv=1 envfile=/etc/default/locale
@include common-session
session optional pam_gnome_keyring.so auto_start
@include common-password
3，保存并退出,继续在终端执行
gedit /etc/pam.d/gdm-password
将打开的文件中第三行语句加#注释掉，如下图
#%PAM-1.0
auth requisite pam_nologin.so
#auth required pam_succeed_if.so user != root quiet_success
@include common-auth
auth optional pam_gnome_keyring.so
@include common-account
# SELinux needs to be the first session rule. This ensures that any
# lingering context has been cleared. Without this it is possible
# that a module could execute code in the wrong domain.
session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_selinux.so close
session required pam_loginuid.so
# SELinux needs to intervene at login time to ensure that the process
# starts in the proper default security context. Only sessions which are
# intended to run in the user's context should be run after this.
# pam_selinux.so changes the SELinux context of the used TTY and configures
# SELinux in order to transition to the user context with the next execve()
# call.
session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_selinux.so open
session optional pam_keyinit.so force revoke
session required pam_limits.so
session required pam_env.so readenv=1
session required pam_env.so readenv=1 user_readenv=1 envfile=/etc/default/locale
@include common-session
session optional pam_gnome_keyring.so auto_start
@include common-password
4，保存并退出。在终端继续执行
gedit /root/.profile
将打开的文件中“mesg n 2> /dev/null || true” 改为
tty -s&&mesg n 2> /dev/null || true
5，保存并退出。
6，重启电脑，即可以root用户登录。
