This section provides an introduction to using MCS labels to extend the Mandatory Access Control (MAC) capabilities of SELinux. It discusses MCS categories, SELinux user identities, and how they apply to Linux user accounts and files. It builds on the conceptual information provided in Section 49.4, “Multi-Category Security (MCS)”
, and introduces some basic examples of usage.
MCS labeling from a user and system administrator standpoint is straightforward. It consists of configuring a set of categories, which are simply text labels, such as "Company_Confidential" or "Medical_Records", and then assigning users to those categories. The system administrator first configures the categories, then assigns users to them as required. The users can then use the labels as they see fit.
The names of the categories and their meanings are set by the system administrator, and can be set to whatever is required for the specific deployment. A system in a home environment may have only one category of "Private", and be configured so that only trusted local users are assigned to this category.
In a corporate environment, categories could be used to identify documents confidential to specific departments. Categories could be established for "Finance", "Payroll", "Marketing", and "Personnel". Only users assigned to those categories can access resources labeled with the same category.
After users have been assigned to categories, they can label any of their own files with any of the categories to which they have been assigned. For example, a home user in the system described above could label all of their personal files as "Private", and no service such as Apache or vsftp would ever be able to access those files, because they don't have access to the "Private" category.
MCS works on a simple principle: to access a file, a user needs to be assigned to all of the categories with which the file is labeled. The MCS check is applied after normal Linux Discretionary Access Control (DAC) and Type Enforcement (TE) rules, so it can only further restrict security.