\(9.5.\)Memory Protection

• A user process shouldn't be allowed to modify any virtual pages that are shared with other processes, unless all parties explicitly allow it(via calls to explicit interprocess communication system calls).

• It is straightforward to control access to the contents of a virtual page by adding some additional permission bits to the PTE.

• The SUP bit indicates whether processes must be running in kernel(supervisor) mode to access the page.

• The READ and WRITE bits control read and write access to the page.

• If an instruction violates these permissions, then the CPU triggers a general protection fault that transfers control to an exception handler in the kernel, which sends a SIGSEGV signal to the offending process. Linux shells typically report this exception as a "segmentation fault".