In this lesson, you will learn how to implement hierarchical security levels in SELinux to enforce strict data classification standards across your system.
The Concept of Multi-Level Security (MLS)
While Type Enforcement (TE) manages what a process can do based on its role and type, Multi-Level Security (MLS) manages access based on the sensitivity level of the data and the clearance of the user. Think of this as a "top-secret" filing system: a user with "Secret" clearance can read "Secret" or "Unclassified" data, but they are strictly forbidden from accessing "Top Secret" files.
In SELinux, these levels are defined as a sensitivity (e.g., s0, s1, s2) optionally followed by categories (e.g., :c0.c1023).
Enforcing Hierarchical Data Access
To implement this, you assign security contexts to both users and files that include an MLS range. A process running at a specific sensitivity level will automatically be blocked from reading data at a higher level (the "No Read Up" rule) or writing data to a lower level (the "No Write Down" rule to prevent data leakage).
When you label a file, the context includes these ranges. For example:
# Setting a file to a specific sensitivity level
chcon -l s1:c0 file_classified_level1.txt
# Checking the context of a process or file
ls -Z file_classified_level1.txt
# Output: unconfined_u:object_r:user_home_t:s1:c0 file_classified_level1.txt
If a user session is logged in with a sensitivity of s0, and they attempt to access file_classified_level1.txt (set to s1), the kernel will deny the request because the process's current clearance is lower than the file's classification.
Why Use This?
MLS is essential in environments where information sensitivity is strictly regulated, such as government or enterprise environments handling trade secrets. By layering MLS over your existing Type Enforcement policies, you add a secondary "vertical" defense that prevents unauthorized data exposure even if a process’s primary type is otherwise permitted to access the file system.
Key Takeaway: Multi-Level Security enforces hierarchical data classification by matching sensitivity labels on processes against sensitivity ranges on objects, preventing unauthorized information flow across security tiers.