SELinux for Developers: Integrating Security into Your Workflow
Unlocking the Power of Secure Linux
What is SELinux?
Security-Enhanced Linux(SELinux) is a security architecture for Linux system that allows administrators to have more control over who can access the system.
It was originally developed by US National Security Agency (NSA) as a series of patches to the Linux Kernal using Linux Security Modules (LSM).
SELinux was released to the open source community in 2000, and was integrated into the upstream Linux kernal in 2003.
How does SELniux work ?
SELinux defines access controls for the applications, processes, and files on a system. It uses security policies, which are a set of rules that tell SELinux what can or can’t be accessed, to enforce the access allowed by a policy.
When an application or process, known as a subject, makes a request to access an object, like a file, SELinux checks with an access vector cache (AVC), where permissions are cached for subjects and objects.
If SELinux is unable to make a decision about access based on the cached permissions, it sends the request to the security server. The security server checks for the security context of the app or process and the file. Security context is applied from the SELinux policy database. Permission is then granted or denied.
SELinux Configuration
There are number of ways that you can configure SELinux to protect your system.
The most common are targeted policy or multi-level security (MLS).
MLS can be very complicated and is typically only used by government organizations.
You can tell what your system is supposed to be running at by looking at the /etc/sysconfig/selinux file. The file will have a section that shows you whether SELinux is in permissive mode, enforcing mode, or disabled, and which policy is supposed to be loaded.
Discretionary Access Control (DAC) vs Mandatory Access Control (MAC)
| DAC | MAC |
| DAC stands for Discretionary Access Control. | MAC stands for Mandatory Access Control. |
| DAC is easier to implement. | MAC is difficult to implement. |
| DAC is less secure to use. | MAC is more secure to use. |
| In DAC, the owner can determine the access and privileges and can restrict the resources based on the identity of the users. | In MAC, the system only determines the access and the resources will be restricted based on the clearance of the subjects. |
| DAC has extra labor-intensive properties. | MAC has no labor-intensive property. |
| Users will be provided access based on their identity and not using levels. | Users will be restricted based on their power and level of hierarchy. |
| DAC has high flexibility with no rules and regulations. | MAC is not flexible as it contains lots of strict rules and regulations. |
| DAC has complete trust in users. | MAC has trust only in administrators. |
| Decisions will be based only on user ID and ownership. | Decisions will be based on objects and tasks, and they can have their own ids. |
| Information flow is impossible to control. | Information flow can be easily controlled. |
| DAC is supported by commercial DBMSs. | MAC is not supported by commercial DBMSs. |
| DAC can be applied in all domains. | MAC can be applied in the military, government, and intelligence. |
| DAC is vulnerable to trojan horses. | MAC prevents virus flow from a higher level to a lower level. |
SELinux Modes -
Enforce - SELinux security policy is enforced.
Permissive - SELinux prints warnings instead of enforcing.
Disabled - No SELinux policy is loaded.
# ls -lZ → It will show SELinux context on file and folder.
# /etc/selinux/config → this is actual file and folder.
# /etc/sysconfig/selinux → this is soft link.
# setenforce 0 → set in permissive mode.
# setenforce 1 → set in enforcing mode.
# getenforce → to know in which mode selinux is.
How to handle SELinux errors
When you get an error in SELinux there is something that needs to be addressed. It is likely 1 of these 4 common problems:
The labels are wrong. If your labeling is incorrect you can use the tools to fix the labels.
A policy needs to be fixed. This could mean that you need to inform SELinux about a change you’ve made, or you might need to adjust a policy. You can fix it using booleans or policy modules.
There is a bug in the policy. It could be that a bug exists in the policy that needs to be addressed.
The system has been broken in to. Although SELinux can protect your systems in many scenarios, the possibility for a system to be compromised still exists. If you suspect that this is the case, take action immediately.
How to change the Context -
# chon -t httpd_sys_content_t anaconda-ks.cfg
To check the label of a process (ex:httpd)
To check label of the socket (ex:httpd)
Checking errors related to SELinux
What are booleans?
Booleans are on/off settings for functions in SELinux. There are hundreds of settings that can turn SELinux capabilities on or off, and many are already predefined. You can find out which booleans have already been set in your system by running getsebool -a.
Conclusion
SELinux is a powerful security mechanism that plays a crucial role in hardening Linux systems against unauthorized access and potential threats. While it may seem complex at first, understanding its foundational concepts, such as contexts, policies, and modes, can significantly enhance your ability to manage and secure Linux environments effectively.
By investing time in mastering SELinux, you can unlock a deeper understanding of Linux security and take a proactive approach to protecting your systems. Remember, security is not just about reacting to threats but about building a robust and resilient defense.
Whether you’re an administrator, developer, or enthusiast, SELinux is a tool worth integrating into your skill set. As you explore and implement its capabilities, you’ll not only secure your Linux systems but also gain valuable insights into how modern security measures work in practice.