Hands-On System Programming with Linux

CPU privilege levels

As mentioned earlier in this chapter, the essential job of the CPU is to read in machine instructions from memory, decipher, and execute them. In the early days of computing, this is pretty much all the processor did. But then, engineers, thinking deeper on it, realized that there is a critical issue with this: if a programmer can feed an arbitrary stream of machine instructions to the processor, which it, in turn, blindly and obediently executes, herein lies scope to do damage, to hack the machine! How? Recall from the previous section the Intel processor's CR0 control register: Contains system control flags that control operating mode and states of the processor. If one has unlimited (read/write) access to the CR0 ...