不关闭SELinux解决权限问题

说起SELinux，多数Linux发行版缺省都激活了它，可见它对系统安全的重要性，可惜由于它本身有一定的复杂性，如果不熟悉的话往往会产生一些看似莫名其妙的问题，导致人们常常放弃使用它，为了不因噎废食，学学如何解决SELinux问题是很有必要的。

我们以CentOS环境为例重现一个非常常见的SELinux问题：

首先需要确认SELinux处于激活状态，可以使用getenforce或sestatus命令：

$ getenforce
Enforcing

shell> sestatus
SELinux status:                 enabled
SELinuxfs mount:                /selinux
Current mode:                   enforcing
Mode from config file:          enforcing
Policy version:                 24
Policy from config file:        targeted

注：关于SELinux的基础知识介绍请参考鸟哥的Linux私房菜中相关的介绍。

我们还需要确认系统已经安装并启动了Apache，没有的话就YUM装一个，这很简单，就不多说了，接着在root目录创建一个测试文件test.html，如下：

$ cat /root/test.html
hello, world.

然后把这个测试文件拷贝到Apache的DocumentRoot目录，我的Apache是通过YUM安装的话，缺省是/var/www/html目录，如下：

$ cp /root/test.html /var/www/html

接着浏览一下，如果没出什么幺蛾子，应该一切都在意料之中，如下：

$ curl
hello, world.

看到这，你可能觉得我废话连篇，别着急，下面就是见证奇迹的时候了：

同样还是那个测试文件test.html，不过这次不再是拷贝，而是剪切，如下：

$ mv /root/test.html /var/www/html

接着浏览一下，怎么样，结果很出人意料吧，竟然提示权限错误，如下：

$ curl

Forbidden

You don't have permission to access /test.html
on this server.

当然，我们现在知道这个问题是由于SELinux引起的，但还不知其所以然，实际上问题的原因此时已经被audit进程记录到了相应的日志里，可以这样查看：

$ audit2why < /var/log/audit/audit.log

结果信息可读性不是很好，如果看不懂的话，推荐安装setroubleshoot套件：

$ yum install setroubleshoot

它本身是一个GUI套件，不过其中包含的一个sealert命令对我们命令行用户很有用：

$ sealert -a /var/log/audit/audit.log
Summary:

SELinux is preventing /usr/sbin/httpd "getattr" access to
/var/www/html/test.html.

Detailed Description:

SELinux denied access requested by httpd. /var/www/html/test.html may be a
mislabeled. /var/www/html/test.html default SELinux type is httpd_sys_content_t,
but its current type is admin_home_t. Changing this file back to the default
type, may fix your problem.

File contexts can be assigned to a file in the following ways.

  * Files created in a directory receive the file context of the parent
    directory by default.
  * The SELinux policy might override the default label inherited from the
    parent directory by specifying a process running in context A which creates
    a file in a directory labeled B will instead create the file with label C.
    An example of this would be the dhcp client running with the dhclient_t type
    and creating a file in the directory /etc. This file would normally receive
    the etc_t type due to parental inheritance but instead the file is labeled
    with the net_conf_t type because the SELinux policy specifies this.
  * Users can change the file context on a file using tools such as chcon, or
    restorecon.

This file could have been mislabeled either by user error, or if an normally
confined application was run under the wrong domain.

However, this might also indicate a bug in SELinux because the file should not
have been labeled with this type.

If you believe this is a bug, please file a bug report against this package.

Allowing Access:

You can restore the default system context to this file by executing the
restorecon command. restorecon '/var/www/html/test.html', if this file is a
directory, you can recursively restore using restorecon -R
'/var/www/html/test.html'.

Fix Command:

/sbin/restorecon '/var/www/html/test.html'

这次应该看懂了吧！原因是说Apache下文件上下文类型应该是httpd_sys_content_t，但是现在是admin_home_t，所以权限错误，并且在结尾处给出了修复命令。

可httpd_sys_content_t，admin_home_t都怎么看啊？很简单，借助ls命令的-Z参数即可：

$ ls -Z /root/test.html
unconfined_u:object_r:admin_home_t:s0 /root/test.html

$ ls -Z /var/www/html/test.html
system_u:object_r:httpd_sys_content_t:s0 /var/www/html/test.html

说明：回到问题的开始，拷贝之所以没出现问题，是因为cp自动修改上下文属性，而剪切之所以出现问题是因为mv保留原文件的上下文属性。

知道了如何解决SELinux问题，以后在遇到类似的情况不要急着武断的关闭SELinux。