Type enforcement

The concept of type enforcement (TE) in the field of information technology is related to access control. Implementing TE gives priority to “mandatory access control” (MAC) over “discretionary access control” (DAC). Access clearance is first given to a subject (e.g. process) accessing objects (e.g. files, records, messages) based on rules defined in an attached security context. A security context in a domain is defined by a domain security policy. In the Linux security module (LSM) in SELinux, the security context is an extended attribute. Type enforcement implementation is a prerequisite for MAC, and a first step before “multi-level security” (MLS) or its replacement “multi categories security” (MCS). It is a complement of “role based access control” (RBAC).

Control

Type enforcement implies fine grained control over the operating system, not only to have control over process execution, but also over “domain transition” or authorization scheme. This is why it is best implemented as a kernel module, as is the case with SELinux. Using type enforcement is a way to implement the FLASK architecture.

Access

Using type enforcement, users may (as in Microsoft Active Directory) or may not (as in SELinux) be associated with a Kerberos realm, although the original type enforcement model implies so. It is always necessary to define a TE access matrix containing rules about clearance granted to a given security context, or subject's rights over objects according to an authorization scheme.

Security

Practically, type enforcement evaluates a set of rules from the source security context of a subject, against a set of rules from the target security context of the object. A clearance decision occurs depending on the TE access description (matrix). Then, DAC or other access control mechanisms (MLS / MCS, …) apply.

History

Type enforcement was introduced in the Secure Ada Target architecture in the late 1980s with a full implementation developed in the LOCK system. The Sidewinder Internet Firewall was implemented on a custom version of Unix that incorporated type enforcement.

A variant called domain type enforcement was developed in the Trusted MACH system.

The original type enforcement model stated that labels should be attached to subject and object: a “domain label” for a subject and a “type label ” for an object. This implementation mechanism was improved by the FLASK architecture, substituting complex structures and implicit relationship. Also, the original TE access matrix was extended to others structures: lattice-based, history-based, environment-based, policy logic… This is a matter of implementation of TE by the various operating systems. In SELinux, TE implementation does not internally distinguish TE-domain from TE-types. It should be considered a weakness of TE original model to specify detailed implementation aspects such as labels and matrix, especially using the terms “domain” and “types” which have other, more generic, widely accepted meanings.

References

• P. A. Loscocco, S. D. Smalley, P. A. Muckelbauer, R. C. Taylor, S. J. Turner, and J. F. Farrell. The Inevitability of Failure: The Flawed Assumption of Security in Modern Computing Environments. In Proceedings of the 21st National Information Systems Security Conference, pages 303–314, October 1998. [1]
• L. Badger, D. F. Sterne, D. L. Sherman, K. M. Walker and S. A. Haghighat, A Domain and Type Enforcement UNIX Prototype, In Proceedings of the 5th USENIX UNIX Security Symposium, June 1995. [2]
• W. E. Boebert and R. Y. Kain, A Practical Alternative to Hierarchical Integrity Policies, In Proceedings of the 8th National Computer Security Conference, page 18, 1985.
• LOCK - A trusted computing system

• See Earl Boebert Oral history interview 28 April 2015, Charles Babbage Institute, University of Minnesota>
• Richard Y. Kain Oral history interview, 27 May 2015, Charles Babbage Institute, University of Minnesota