18. Alice can read and write to the file x, can read the file y, and can execute the file z. Bob can read x, can read and write to y, and cannot access z. (a) Write a set of access control lists for this situation. Which list is associated with which file? (b) Write a set of capability lists for this situation. With what is each list associated? 19. Briefly describe Trusted Computing Base (TCB) and its objective. 20. Briefly describe reference monitor as an access control enforcement mechanism, its properties, and components. 21. Why does operating system need to control processes' access to memory and how does it enforce security? 22. Why do Unix's syscalls for accessing shared-memory resources ignores execute flag? 23. What's a buffer-overflow attack and the two steps involved in this attack? 24. What's the purpose of using "nops/nop slide" by an attacker, in a buffer overflow attack? 25. How are stack canaries used to prevent change of program's control flow in buffer- overflow attacks? 26. What countermeasure does OS use to prevent attacker's code from being injected and executed, in buffer-overflow attacks? 27. Briefly describe Return to libc and defenses against this attack