Networking in NSA Security-Enhanced Linux

Break through the complexity of SELinux with a working example that shows how to add SELinux protection to a simple network server.

In this article, we take a look at how SELinux can help increase the security of networked systems, as well as the design and implementation of its network-specific security controls. We then walk through an example of using SELinux policy to lock down a simple network application.

Overview: SELinux Roles, Types and Domains

SELinux provides strong general security for networked systems. It allows systems to be locked down tightly so that services have only the minimum set of rights required to operate. This implementation of the principle of least privilege helps contain security breaches arising from buggy code, malicious code, user error and malicious users.

For example, an externally facing Web server normally might be hardened in a variety of ways, including:

  • Disabling unnecessary services.

  • Running server software in chroot jails.

  • Local packet filtering with iptables.

  • Privilege management with sudo.

  • Locking down configuration files.

This is a good, multilayered approach to security, implementing the principle of defense in depth.

SELinux adds another security layer, mandatory access control (MAC). Standard SELinux implements MAC via type enforcement (TE) combined with role-based access control (RBAC), under the control of a centrally managed security policy, enforced by the kernel. Unlike traditional UNIX security, normal users do not have any control over SELinux security policy (hence the term mandatory), while the superuser, root, can be split into isolated administrative roles for authorized users, called separation of duty.

Traditional discretionary access control (DAC) is further restricted by the TE model, which assigns types to operating system objects such as processes, files and network resources, then defines rules for interactions between them. (The type of a process usually is referred to as a domain.) This allows for fine-grained access control, extending the principle of least privilege well beyond the scope of typical OS hardening.

SELinux Network Access-Control Architecture

SELinux is built upon the LSM (Linux Security Modules) and Netfilter APIs in the 2.6 kernel. LSM and Netfilter are both access-control frameworks consisting of strategically located hook points within the kernel. Kernel flow is redirected from these hooks to security modules such as SELinux, which perform access-control calculations and return a verdict to the hook. A hook uses the verdict returned from the security module either to allow normal kernel flow to continue or prevent it.

One of the core design principles of SELinux is that it mediates access at the OS object level. Rather than a naive approach where a security monitor decides whether a program can execute a particular system call with certain arguments, SELinux looks at the full security context of the program during execution, the security label attached to the object being accessed and the action being taken. For example, ls run by the system administrator is different from ls run by a normal user.

The general form of an SELinux permission is:

action (source context)
(target context):(target object classes)
permissions

Here's an example from SELinux policy:

allow bluetooth_t self:socket listen;

This provides the bluetooth_t domain with the listen permission for sockets labeled with its own security context. So, a process running in the bluetooth_t domain is allowed to invoke listen() on a socket that it owns.

The self designator is simply a shorthand way of making the target context the same as the source context. This commonly is used in policy relating to sockets as they typically are labeled with the same security context as the creating process.

Network Object Labeling

Under SELinux, objects are labeled with a security context of the following form:

user:role:type

For example:

root:staff_r:staff_t

is the context of a process being run by root via the staff_r role in the staff_t domain. The label associated with port 80 is:

system_u:object_r:http_port_t

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Total bewilderment

derekfountain's picture

What a wonderful example of why kernel hackers shouldn't write magazine articles! I really wanted to understand this SELinux stuff, but this is an impenetrable mess.

Here's how not to do it: first, ensure you don't define your audience. I initially thought this was aimed at developers, but now I think perhaps it's aimed at systems administrators. Second, throw in a whole load of acronyms, right the beginning: MAC, TE, RBAC DAC, LSM (trying to ensure you never use them again in the rest of the piece). Then make life hard for the reader by giving a "general form" containing 3 lines, followed by an example containing 1 line. Still with us? Great. Throw in a few more undefined terms, like "security context", "target context" and "source context", then a few references to system calls (a couple of dozen should be enough). Next present a config file that is longer than the source of the program it is supposed to protect, plus tweaks to 3 other obscure files, the purpose of each remaining unexplained. Then throw in a "make" command without explaining that either. Finish up with a set of obscure commands to prove the example works, carefully labelling it as a "simple demonstration". Job done!

Perhaps I'm in a flippant mood this morning. Perhaps it's because I'm an application developer, not a sysadmin. Perhaps I haven't had enough caffeine. Or perhaps, just possibly, this article is written from the inside out, and is therefore only accessible to those who already understand what the heck it's on about.

If you want to understand mor

James M's picture

If you want to understand more about the underlying concepts, I'd suggest looking at the Faye Coker article listed in the resources, http://www.linuxjournal.com/article/7864

The article is aimed at anyone interested in how SELinux works underneath, and documents a lot of previously undocumented aspects of the networking. I guess it may have been better to drop the introductory section (instead referring to other resources) and include a short glossary.

There is no Faye Cocker article

Anonymous's picture

I looked at that link, http://www.linuxjournal.com/article/7864

There is no Faye Coker article listed on that page. I did a find on the entire web page for "faye" and "coker", nothing.

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