su - root正确输入密码但是登录不了系统，报错su: Permission denied

su - root输入了正确的密码但是一直报错，密码肯定是输入对的。这主要是因为权限的问题导致的。
Linux里面有一个文件，/etc/pam.d/su文件。

[chaofan@master ~]$ cat /etc/pam.d/su
#%PAM-1.0
auth		sufficient	pam_rootok.so
# Uncomment the following line to implicitly trust users in the "wheel" group.
#auth		sufficient	pam_wheel.so trust use_uid
# Uncomment the following line to require a user to be in the "wheel" group.
#auth		required	pam_wheel.so use_uid
auth		substack	system-auth
auth		include		postlogin
account		sufficient	pam_succeed_if.so uid = 0 use_uid quiet
account		include		system-auth
password	include		system-auth
session		include		system-auth
session		include		postlogin
session		optional	pam_xauth.so

主要是看第5、6这两行。如果把第6行注释打开了，那么只有那些加入了wheel组的普通用户才可以使用su - root来切换到root用户，比如如下所示：
现在有两个用户，一个是chaofeng，一个是chaofan。这两个用户中，chaofeng是加入到了wheel组里面的，如下所示：

[root@master ~]# id chaofeng
uid=1000(chaofeng) gid=1000(chaofeng) groups=1000(chaofeng),10(wheel)
[root@master ~]# id chaofan
uid=1001(chaofan) gid=1001(chaofan) groups=1001(chaofan)

然后现在我处于chaofeng这个用户登录的终端里面，执行su - root

[root@master ~]# su - chaofeng
Last login: Thu Jul  2 11:40:15 CST 2020 on pts/1
[chaofeng@master ~]$ su - root
Password: 
Last login: Thu Jul  2 11:40:21 CST 2020 on pts/1
Last failed login: Thu Jul  2 11:42:33 CST 2020 on pts/1
There were 3 failed login attempts since the last successful login.
[root@master ~]#                                                       # 看的出来我现在登录成功了

接着现在我处于chaofan这个用户登录的终端里面，再次执行su - root尝试

[root@master ~]# su - chaofan
Last login: Thu Jul  2 11:42:19 CST 2020 on pts/1
[chaofan@master ~]$ su - root
Password: 
su: Permission denied
[chaofan@master ~]$ 

我可以保证输入的root用户的密码是没有错误的，但是一直报错“su: Permission denied”。
所以从这里可以看出来/etc/pam.d/su我们去掉的这一行的作用所在了。