1. Is Multilevel Security policy mandatory or not, why?

2. Suppose we are running Multics operating system. One process, running in ring 5, is with clearance level of TOP SECRET and with the category of {MIL, INTEL}. During its execution, it needs to access one segment with sensitivity level of Secret and with the category of {INTEL}. The segment descriptor is as follows:

Address 0x2f453e40

Length 512

R1 1

R2 3

R3 6

R 1

W 0

E 1

Gate Offset 0

Can the process invoke the code at the very beginning of the segment or not (call the code at the 0x0 of this segment), why? (Note, you don't need to consider "read before execute". )

3. Compare the access control policy of Windows (token and access control list) and UNIX (mode bits) we discussed in class. Which one do you think is more powerful, which one is more complicated, and which one is more ease-of-use? Please give the reasons.

4. Read the following pseudo code, and think about if attacker can still launch TOCTTOU attack or not, why?

if (access(/home/tdata, W_OK) != 0) { exit(1); } inode1 = stat(/home/tdata); fd = open(/home/tdata, O_WRONLY); inode2 = fstat(fd); if (inode 1 == inode 2) { write(fd, buffer, 100); }