Networking in NSA Security-Enhanced Linux

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.

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:

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 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.

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