SELinux mount options


Sven Vermeulen Wed 01 May 2013

When you read through the Gentoo Hardened SELinux handbook, you'll notice that we sometimes update /etc/fstab with some SELinux-specific settings. So, what are these settings about and are there more of them?

First of all, let's look at a particular example from the installation instructions so you see what I am talking about:

tmpfs  /tmp  tmpfs  defaults,noexec,nosuid,rootcontext=system_u:object_r:tmp_t  0 0

What the rootcontext= option does here is to set the context of the "root" of that file system (meaning, the context of /tmp in the example) to the specified context before the file system is made visible to the userspace. Because we do it soon, the file system is known as tmp_t throughout its life cycle (not just after the mount or so).

Another option that you'll frequently see on the Internet is the context= option. This option is most frequently used for file systems that do not support extended attributes, and as such cannot store the context of files on the file system. With the context= mount option set, all files on that file system get the specified context. For instance, context=system_u:object_r:removable_t.

If the file system does support extended attributes, you might find some benefit in using the defcontext= option. When set, the context of files and directories (and other resources on that file system) that do not have a SELinux context set yet will use this default context. However, once a context is set, it will use that context instead.

The last context-related mount option is fscontext=. With this option, you set the context of the "filesystem" class object of the file system rather than the mount itself (or the files). Within SELinux, "filesystem" is one of the resource classes that can get a context. Remember the /tmp mount example from before? Well, even though the files are labeled tmp_t, the file system context itself is still tmpfs_t.

It is important to know that, if you use one of these mount options, context= is mutually exclusive to the other options as it "forces" the context on all resources (including the filesystem class).