2 cissp ® Official Study Guide Eighth Edition

Process Isolation
Process isolation
requires that the operating system provide separate memory spaces for 
each process’s instructions and data. It also requires that the operating system enforce those 
boundaries, preventing one process from reading or writing data that belongs to another 
process. There are two major advantages to using this technique:
■
It prevents unauthorized data access. Process isolation is one of the fundamental 
requirements in a multilevel security mode system.
■
It protects the integrity of processes. Without such controls, a poorly designed process 
could go haywire and write data to memory spaces allocated to other processes, caus-
ing the entire system to become unstable rather than affecting only the execution of the 
errant process. In a more malicious vein, processes could attempt (and perhaps even 
succeed at) reading or writing to memory spaces outside their scope, intruding on or 
attacking other processes.
Many modern operating systems address the need for process isolation by implement-
ing virtual machines on a per-user or per-process basis. A virtual machine presents a user 
or process with a processing environment—including memory, address space, and other 
key system resources and services—that allows that user or process to behave as though 
they have sole, exclusive access to the entire computer. This allows each user or process to 
operate independently without requiring it to take cognizance of other users or processes 
that might be active simultaneously on the same machine. As part of the mediated access to 
the system that the operating system provides, it maps virtual resources and access in user 
mode so that they use supervisory mode calls to access corresponding real resources. This 
not only makes things easier for programmers, it also protects individual users and pro-
cesses from one another.