As interesting as the image might be, most users don't execute computation themselves. They use computers for that.
Earlier, we defined security as keeping the adversary from subverting the important properties of your system. "System" consists primarily of "computation." In order to subvert computation, an adversary must somehow have a way to get into it, such as via some type of input or some other computation. Computers use an operating system to bring some order and control to the computation they host. Consequently, operating systems become a natural arena for security attacks and defenses.
In this chapter, we explore the arena of OS security.