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。

参考文章： 解决Ubuntu使用ROOT账户登录图形界面报错认证失败_D&T的博客-CSDN博客