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49.4. Multi-Category Security (MCS)

49.4.1. Introduction

Multi-Category Security (MCS) is an enhancement to SELinux, and allows users to label files with categories. These categories are used to further constrain Discretionary Access Control (DAC) and Type Enforcement (TE) logic. They may also be used when displaying or printing files. An example of a category is "Company_Confidential". Only users with access to this category can access files labeled with the category, assuming the existing DAC and TE rules also permit access.
The term categories refers to the same non-hierarchical categories used by Multi-Level Security (MLS). Under MLS, objects and subjects are labeled with Security Levels. These Security Levels consist of a hierarchical sensitivity value (such as "Top Secret") and zero or more non-hierarchical categories (such as "Crypto"). Categories provide compartments within sensitivity levels and enforce the need-to-know security principle. Refer to Section 49.6, “Multi-Level Security (MLS)” for more information about Multi-Level Security. What is Multi-Category Security?

MCS is an adaptation of MLS. From a technical point of view, MCS is a policy change, combined with a few userland modifications to hide some of the unneeded MLS technology. Some kernel changes were also made, but only relating to making it easy to upgrade to MCS (or MLS) without invoking a full file system relabel.
The hope is to improve the quality of the system as a whole, reduce costs, leverage the open source process, increase transparency, and make the technology base useful to more than a small group of extremely special-case users.