File labels are the most common aspect of a SELinux system that users and administrators will need to care for. As SELinux policy decisions are based on the label of a resource, making sure that the file labels are correctly set is the most important part of maintaining SELinux systems.
The term label is used for the SELinux context of a file or other object on a system. Whenever a document talks about a file context or file label, both actually mean the same thing. The term comes from the SELinux permissions relabelfrom and relabelto which inform the policy if a relabel operation (change of context) is allowed from a particular label (context) or towards a particular label (context).
Label values are also often abbreviated in documents. A file with
user_home_t label (or context) actually needs to have an entire context assigned, but in the document itself the type part of the context is the most important one. As such, instead of talking about a file with
user_u:object_r:user_home_t:s0 label (and having to explain that the SELinux user part can be different) the type
user_home_t alone is used.
Labels are extended attributes
On most file systems, SELinux labels are stored as extended attributes. This is not always the case though - some file systems do not support extended attributes. In these cases, all files on the file system get assigned the same context, usually provided through the mount option of the file system.
On a SELinux system, everything needs to have a context / label assigned. Even resources such as TCP and UDP ports get a label. These labels are assigned by SELinux itself through policy definitions, although users can still manipulate the assigned port types if no specific port type is used yet.
Managing file labels
Although file labels are set as extended attributes in most cases, managing file labels goes further than just ensuring that that particular extended attribute is correctly set. The SELinux user space maintains a database of file path expressions together with the file context to be assigned to these resources. This allows administrators to verify if a file context is correct or not, and to reset the context of a file.
Listing file labels
The Linux core utilities support displaying of SELinux labels.
For instance, using
ls -Z (the
-Z option is often used for displaying SELinux contexts):
ls -lZ /etc/resolv.conf
-rw-r--r--. 1 root root system_u:object_r:net_conf_t 68 May 4 18:14 /etc/resolv.conf
stat command displays the label as well, here on an MLS-enabled system:
File: '/etc/resolv.conf' Size: 149 Blocks: 16 IO Block: 4096 regular file Device: fb02h/64258d Inode: 394204 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) Context: system_u:object_r:net_conf_t:s0 Access: 2013-01-13 13:24:01.402000000 +0100 Modify: 2014-05-04 19:43:03.563000000 +0200 Change: 2014-05-04 19:43:03.573000000 +0200
The context of a file can also be directly read through the
getfattr command, used to obtain a file's extended attributes:
getfattr -m . -d /etc/resolv.conf
getfattr: Removing leading '/' from absolute path names # file: etc/resolv.conf security.evm=0sAoTjX3aOeDQdWxbOfOUV930tWoDA security.ima=0sAYC508o0Lz4iAA9ucVAVsvK02tV/ security.selinux="system_u:object_r:net_conf_t:s0"
In the above output, next to the integrity related extended attributes, the
security.selinux attribute is shown, which contains the SELinux context of the file.
Relabeling files means that the context of one or more files is reset towards the definition stored by the SELinux user space utilities.
Such a relabeling operation is usually done through the
To relabel the entire file system, use
rlpkg (which is a Gentoo-specific tool):
rlpkg -a -r
rlpkg application also supports relabeling all files provided by a single Gentoo package. For instance, to relabel all files provided by the
Temporarily modifying file labels
To set a file context, the
chcon command can be used:
chcon -t net_conf_t /etc/puppet-resolv.conf
chcon tool does not update the SELinux user space definition list. As a result, when the administrator relabels a file, it will be reset to the original value.
Permanently modifying file labels
To ensure that a new file context remains, even after a relabeling operation, the administrator needs to update the SELinux user space definition list first.
The definition list uses PCRE-based regular expressions and can be managed through
For instance, to add /etc/puppet-resolv.conf to the list and assign it the
semanage fcontext -a -t net_conf_t "/etc/puppet-resolv\.conf"
The use of PCRE expressions allows for a denser set of rules that matches entire subdirectories easily:
semanage fcontext -a -t bin_t "/usr/lib/portage/bin(/.*)?"
After updating the definition list, the context of the file(s) isn't set yet. For this, the files need to be relabeled:
restorecon -R /etc/puppet-resolv.conf /usr/lib/portage/bin
Managing TCP and UDP port labels
TCP and UDP ports also get assigned a particular label.
Listing port labels
seinfo, the context for a particular port can be viewed:
portcon tcp 80 system_u:object_r:http_port_t portcon tcp 1-511 system_u:object_r:reserved_port_t portcon udp 1-511 system_u:object_r:reserved_port_t
It is also possible to list all port definition declarations using
semanage port -l | grep http
http_cache_port_t tcp 3128, 8080, 8118, 10001-10010 http_cache_port_t udp 3130 http_port_t tcp 8888, 80, 443, 488, 8008, 8009, 8443 pegasus_http_port_t tcp 5988 pegasus_https_port_t tcp 5989
Modifying port labels
If a port is not only assigned the
hi_reserved_port_t label, then the context for that port cannot be modified.
However, if the port only has one of these types assigned, then it is possible to set a specific type.
For instance, to assign the type
http_port_t to port 9980, first check if it is possible:
portcon udp 1024-65535 system_u:object_r:unreserved_port_t portcon tcp 1024-65535 system_u:object_r:unreserved_port_t
It is possible, so now use
semanage port to assign it:
semanage port -a -t http_port_t -p tcp 9980
Managing process labels
The label of a process is decided by the SELinux policy, and is called a domain.
It is not possible to relabel a process. To have a process run in a different domain, it might be possible to launch the command using the
runcon command. However, in the majority of cases, this will not be allowed by the SELinux policy. The domain in which a process runs is fully policy-driven (with a small configurable aspect provided for SELinux-aware applications).
Listing process labels
The standard core utilities can display the context of a process.
For instance, to get the context of the
ps -eZ | grep init
system_u:system_r:init_t 1 ? 00:00:01 init
Label management is one of the most core competences that a system administrator on a SELinux system must have. As SELinux decisions are based on labels (contexts), ensuring that the right context is used on all resources is a primary concern for all administrators.