Appendix A. Brief Background and History of SELinux
SELinux was originally a development project from the National Security Agency
(NSA) and others. It is an implementation of the
Flask operating system security
architecture. The Flask architecture implements MAC, which focuses on
providing an administratively-defined security policy that can control all
subjects and objects, basing decisions on all security-relevant information.
In addition, Flask focuses on the concept of least
privilege, which gives a process exactly the rights it needs
to perform it's given task.
The Flask model allows you to express a security policy in a naturally
flowing manner, so that parts of the security rules are like parts in a
sentence. In Flask, changes are supported so you can tune your policy.
Added to this architecture in the security server are TE and RBAC security
models, providing fine-grained controls that can be transparent to users and
applications.
As a next step in the evolution of SELinux, the NSA integrated SELinux into the
Linux kernel using the Linux Security Modules
(LSM) framework. SELinux
motivated the creation of LSM, at the suggestion of Linus
Torvalds, who wanted a modular approach to security instead of accepting
just SELinux into the kernel.
Originally, the SELinux implementation used persistent security
IDs (PSIDs) stored in an unused field of the
ext2 inode. These numerical representations (i.e., non-human-readable) were
mapped by SELinux to a security context label. Unfortunately, this required
modifying each file system type to support PSIDs, so was not a scalable
solution or one that would be supported upstream in the Linux kernel.
The next evolution of SELinux was as a loadable kernel module for the
2.4.<x> series of Linux kernels. This
module stored PSIDs in a normal file, and SELinux was able to support more
file systems. This solution was not optimal for performance, and was
inconsistent across platforms. Finally, the SELinux code was integrated
upstream to the 2.6.x kernel, which has full support for LSM and has
extended attributes
(xattrs) in the ext3 file system.
SELinux was moved to using xattrs to store security context information. The
xattr namespace provides useful separation for multiple security modules
existing on the same system.
Much of the work to get the kernel ready for upstream, as well as subsequent
SELinux development, has been a joint effort between the NSA, Red Hat, and the
community of SELinux developers.
For more information about the history of SELinux, the definitive website is
https://www.nsa.gov/selinux/.
