SELinux System Administration - Second Edition

4.5 (8 reviews total)
By Sven Vermeulen
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  1. Fundamental SELinux Concepts

About this book

Do you have the crucial job of protecting your private and company systems from malicious attacks and undefined application behavior? Are you looking to secure your Linux systems with improved access controls? Look no further, intrepid administrator! This book will show you how to enhance your system’s secure state across Linux distributions, helping you keep application vulnerabilities at bay.

This book covers the core SELinux concepts and shows you how to leverage SELinux to improve the protection measures of a Linux system. You will learn the SELinux fundamentals and all of SELinux’s configuration handles including conditional policies, constraints, policy types, and audit capabilities. These topics are paired with genuine examples of situations and issues you may come across as an administrator. In addition, you will learn how to further harden the virtualization offering of both libvirt (sVirt) and Docker through SELinux.

By the end of the book you will know how SELinux works and how you can tune it to meet your needs.

Publication date:
December 2016
Publisher
Packt
Pages
300
ISBN
9781787126954

 

Chapter 1. Fundamental SELinux Concepts

Security Enhanced Linux (SELinux) brings additional security measures to your Linux system to further protect its resources.

In this chapter, we will cover:

  • Why SELinux uses labels to identify resources

  • How SELinux differs from traditional Linux access controls by enforcing security rules

  • How the access control rules enforced by SELinux are provided through policy files

In the end, we will cover an overview of the differences between SELinux implementations across Linux distributions.

 

Providing more security to Linux


Seasoned Linux administrators and security engineers already know that they need to put some trust in the users and processes on their system in order for the system to remain secure. This is partially because users can attempt to exploit vulnerabilities found in the software running on the system, but a large contribution to this trust level is because the secure state of the system depends on the behavior of the users. A Linux user with access to sensitive information could easily leak that out to the public, manipulate the behavior of the applications he or she launches, and do many other things that affect the security of the system. The default access controls that are active on a regular Linux system are discretionary; it is up to the users how the access controls should behave.

The Linux discretionary access control (DAC) mechanism is based on the user and/or group information of the process and is matched against the user and/or group information of...

 

Labeling all resources and objects


When SELinux has to decide whether it has to allow or deny a particular action, it makes a decision based on the context of both the subject (which is initiating the action) and the object (which is the target of the action). These contexts (or parts of the context) are mentioned in the policy rules that SELinux enforces.

The context of a process is what identifies the process to SELinux. SELinux has no notion of Linux process ownership and, once running, does not care how the process is called, which process ID it has, and what account the process runs as. All it wants to know is what the context of that process is, which is represented to users and administrators as a label. Label and context are often used interchangeably, and although there is a technical distinction (one is a representation of the other), we will not dwell on that much.

Let's look at an example label: the context of the current user (try it out yourself if you are on a SELinux-enabled...

 

Defining and distributing policies


Enabling SELinux does not automatically start the enforcement of access. If SELinux is enabled and it cannot find a policy, it will refuse to start. That is because the policy defines the behavior of the system (what SELinux should allow). SELinux policies are generally distributed in a compiled form (just like with software) as policy modules. These modules are then aggregated into a single policy store and loaded in memory to allow SELinux to enforce the policy rules on the system.

Note

Gentoo, being a source-based meta-distribution, distributes the SELinux policies as (source) code as well, which is compiled and built at install time, just like it does with other software.

The following diagram shows the relationship between policy rules, policy modules, and a policy package (which is often a one-to-one mapping towards a policy store):

Relationship between policy rules, policy modules, and policy store

Writing SELinux policies

A SELinux policy writer can ...

 

Distinguishing between policies


The most common SELinux policy store names are strict, targeted, mcs, and mls. None of the names assigned to policy stores are fixed, though, so it is a matter of convention. Hence, it is recommended to consult the distribution documentation to verify what the proper name of the policy should be. Still, the name often provides some information about the SELinux options that are enabled through the policy.

Supporting MLS

One of the options that can be enabled is MLS support. If it is disabled, then the SELinux context will not have a fourth field with sensitivity information in it, making the contexts of processes and files look as follows:

    staff_u:sysadm_r:sysadm_t 

To check whether or not MLS is enabled, it is sufficient to see if the context, indeed, doesn't contain such a fourth field, but it can also be acquired from the Policy MLS status line in the output of sestatus:

    # sestatus | grep MLS 
    Policy MLS Status:            disabled...
 

Summary


In this chapter, we saw that SELinux offers a more fine-grained access control mechanism on top of the Linux access controls. SELinux is implemented through Linux Security Modules and uses labels to identify its resources and processes based on ownership (user), role, type, and even the security sensitivity and categorization of the resource. We covered how SELinux policies are handled within a SELinux-enabled system and briefly touched upon how policy writers structure policies.

Linux distributions implement SELinux policies, which might be a bit different from each other based on supporting features, such as sensitivity labels, default behavior for unknown permissions, support for confinement levels, or specific constraints put in place such as UBAC. However, most of the policy rules themselves are similar and are even based on the same upstream reference policy project.

Switching between SELinux enforcement modes and understanding the log events that SELinux creates when it prohibits...

About the Author

  • Sven Vermeulen

    Sven Vermeulen (sjvermeu on Twitter) is a long-term contributor to various free software projects and the author of several online guides and resources, including the Gentoo Handbook. He got his first taste of free software in 1997 and never looked back. Within SELinux, Sven contributed several policies to the Reference Policy project, and actively participated in policy development and user space development projects. In his daily job, Sven is an enterprise architect in a European financial institution as well as a self-employed solution engineer and consultant. Prior to this, he graduated with an MSE in computer engineering from Ghent University and an MSc in ICT enterprise architecture from IC Institute.

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Latest Reviews

(8 reviews total)
Good informative tome. Learned quite a few things
very good book, good contain ans structure
Whole pages where "marked up" with a yellow marker pen, making it very hard to read. The content it self is informative and well written.

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