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/.
