Page 554 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

from crossing between security levels. One user might be using a
multistate system to process secret information, while another user is

processing top-secret information at the same time. Technical
mechanisms prevent information from crossing between the two users
and thereby crossing between security levels.

In actual practice, multistate systems are relatively uncommon owing
to the expense of implementing the necessary technical mechanisms.
This expense is sometimes justified; however, when you’re dealing

with a very expensive resource, such as a massively parallel system,
the cost of obtaining multiple systems far exceeds the cost of
implementing the additional security controls necessary to enable
multistate operation on a single such system.

Protection Mechanisms

If a computer isn’t running, it’s an inert lump of plastic, silicon, and
metal doing nothing. When a computer is running, it operates a
runtime environment that represents the combination of the operating

system and whatever applications may be active. When running, the
computer also has the capability to access files and other data as the
user’s security permissions allow. Within that runtime environment,
it’s necessary to integrate security information and controls to protect
the integrity of the operating system itself, to manage which users are
allowed to access specific data items, to authorize or deny operations
requested against such data, and so forth. The ways in which running

computers implement and handle security at runtime may be broadly
described as a collection of protection mechanisms. What follows are
descriptions of various protection mechanisms such as protection
rings, operational states, and security modes.

Because the ways in which computers implement and use

protection mechanisms are so important to maintaining and
controlling security, you should understand how all three
mechanisms covered here—rings, operational states, and security
modes—are defined and how they behave. Don’t be surprised to

see exam questions about specifics in all three areas because this is