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Central to the Multilevel Security policy is the MLS label, also known as a level. Each subject (e.g., a DBMS session open on behalf of a user) and object (e.g., a DBMS row) is assigned an MLS label.
The MLS label assigned to a subject determines which operations the subject may perform. The subject label is also assigned to objects created by the subject. The MLS label assigned to an object determines which operations may be performed upon the object.
Generally, a subject may read an object if the subject's label dominates (is "higher" than) the object's label and may update an object if the subject's label equals the object's label.
MLS label's are composed of two distinct parts: the sensitivity (objects)/clearance (subjects) and the compartments. Compartments may also be known as categories.
The sensitivity/clearance component of a label represents the degree to which information is security-wise sensitive (object's only) and the user's trustworthiness (subjects only). It frequently takes on a string form like "Top Secret" or "Unclassified."
The compartments component of a label represents a user's "need to know." This usually refers to the information a user needs to operationally perform her duties. While they may be "cleared" or "trusted" to see other information, they have no operational need to see such data to perform their job responsibilities and, taking a conservative approach to security, the MLS policy denies them access.
Trusted RUBIX labels its DBMS objects with the same MLS labels used by the underlying operating system.
The following diagram gives sample MLS labels in Solaris 10 Trusted Extensions (TX) and SELinux formats. For SELinux, the MLS level is one of four components within its security context. The left-most part of each MLS label represents the sensitivity/clearance component while the right-most component represents the compartments.
In both cases the MLS label may be configured to have a user-friendly string representation. SELinux MLS label compartments may be specified as a comma separated list (e.g., c1, c5, c9) and as a range (e.g., c1.c3). Also note that a MLS label need not contain any compartments.