TCB, MAC, DAC

Trusted Computing Base. The programs (more technically, objects and processes) that are trusted to do what they are supposed to do (and only what they are supposed to do), and that would put the system at risk if compromised. On traditional Unix systems, any program run by root is part of the TCB (whether or not you should be trusting it). That's because traditional Unix is a DAC (Discretionary Access Control) system, but "root" is allowed to bypass all controls.

MAC (Mandatory Access Control) systems don't necessarily allow a user like root that can bypass anything. However, because it is difficult to administer a system without an overall administrator with great privilege, MAC systems tend to concentrate on specific areas where power is limited. For example, Windows systems have the concept of ownership where not even the administrator can necessarily access a file owned by someone else. The administrator can take ownership for itself, but that's a one way trap door: once taken, the administrator can't give it back.

In the Linux world, SELinux s a MAC system. It is imported to understand that a MAC system doesn't necessarily mean "trusted". From the SELinux FAQ:

The phrase "Trusted Operating System" generally refers to an
operating system that provides sufficient support for multilevel
security and evidence of correctness to meet a particular set of
government requirements. Security-enhanced Linux incorporates useful
ideas from these systems but focuses upon mandatory access controls.
It is expected that this work would be combined with other efforts
(e.g., auditing and documentation) to construct a "trusted" system.
The initial focus of Security-enhanced Linux development has been
to create useful functionality that delivers tangible protection
benefits in a wide range of real-world environments in order to
demonstrate the technology.
 

1 comment

More Articles by Tony Lawrence - Find me on Google+

Jump to Comments

Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them. I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.

Specific links that take you to pages that allow you to purchase the item I reviewed are very likely to pay me a commission. Many of the books I review were given to me by the publishers specifically for the purpose of writing a review. These gifts and referral fees do not affect my opinions; I often give bad reviews anyway.

We use Google third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, click here.