问题

linux系统中（ubuntu20），root用户下执行passwd命令，提示 passwd: Permission denied ，如下图：

排查

1.执行 ll /usr/bin/passwd ，查看文件权限是否正确，正常情况是 -rwsr-xr-x. root root，如果不是，请修改至正确权限

2.查看/etc/pam.d/passwd，常见正常内容如下图（centos系统中）

在另一台ubuntu20机器上查看内容如下：

#
# The PAM configuration file for the Shadow `passwd' service
#

@include common-password

发现此文件内容为 @include common-password
引用了一个common-password文件，此文件在 /etc/pam.d/目录下，然后查看 /etc/pam.d/common-password 文件内容，如下图：

#
# /etc/pam.d/common-password - password-related modules common to all services
#
# This file is included from other service-specific PAM config files,
# and should contain a list of modules that define the services to be
# used to change user passwords.  The default is pam_unix.

# Explanation of pam_unix options:
#
# The "sha512" option enables salted SHA512 passwords.  Without this option,
# the default is Unix crypt.  Prior releases used the option "md5".
#
# The "obscure" option replaces the old `OBSCURE_CHECKS_ENAB' option in
# login.defs.
#
# See the pam_unix manpage for other options.

# As of pam 1.0.1-6, this file is managed by pam-auth-update by default.
# To take advantage of this, it is recommended that you configure any
# local modules either before or after the default block, and use
# pam-auth-update to manage selection of other modules.  See
# pam-auth-update(8) for details.

# here are the per-package modules (the "Primary" block)
password        [success=1 default=ignore]      pam_unix.so obscure sha512
# here's the fallback if no module succeeds
password        requisite                       pam_deny.so
# prime the stack with a positive return value if there isn't one already;
# this avoids us returning an error just because nothing sets a success code
# since the modules above will each just jump around
password        required                        pam_permit.so
# and here are more per-package modules (the "Additional" block)
# end of pam-auth-update config
password        *                               pam_unix.so sha512
password        required                        remember=5

通过对 password 这几行测试，发现将最后两行注释后，即可使用passwd命令