Text exercise

1.     Give an example of the use of physical separation for security in a computing environment.

2.     Give an example of the use of temporal separation for security in a computing environment.

3.     Give an example of an object whose sensitivity may change during execution.

4.     Respond to the allegation “An operating system requires no protection for its executable code (in memory) because that code is a duplicate of code maintained on disk.”

5. Explain how a fence register is used for relocating a user’s program.

6. Can any number of concurrent processes be protected from one another by just one pair of base/bounds registers?

7. The discussion of base/bounds registers implies that program code is execute only and that data areas are read-write-only. Is this ever not the case? Explain your answer.

8. A design using tag bits presupposes that adjacent memory locations hold dissimilar things: a line of code, a piece of data, a line of code, two pieces of data, and so forth. Most programs do not look like that. How can tag bits be appropriate in a situation in which programs have the more conventional arrangement of code and data?

9. What are some other modes of access that users might want to apply to code or data, in addition to the common read, write, and execute permission?

10. If two users share access to a segment, they must do so by the same name. Must their protection rights to it be the same? Why or why not?

11. A problem with either segmented or paged address translation is timing. Suppose a user wants to read some data from an input device into memory. For efficiency during data transfer, often the actual memory address at which the data are to be placed is provided to an I/O device. The real address is passed so that time-consuming address translation does not have to be performed during a very fast data transfer. What security problems does this approach bring?

12. A directory is also an object to which access should be controlled. Why is it not appropriate to allow users to modify their own directories?

13. Why should the directory of one user not be generally accessible to other users (not even for read-only access)?

14. File access control relates largely to the secrecy dimension of security. What is the relationship between an access control matrix and the integrity of the objects to which access is being controlled?

15. One feature of a capability-based protection system is the ability of one process to transfer a copy of a capability to another process. Describe a situation in which one process should be able to transfer a capability to another.

16. Describe a mechanism by which an operating system can enforce limited transfer of capabilities. That is, process A might transfer a capability to process B, but A wants to prevent B from transferring the capability to any other processes. Your design should include a description of the activities to be performed by A and B, as well as the activities performed by and the information maintained by the operating system.

17. List two disadvantages of using physical separation in a computing system. List two disadvantages of using temporal separation in a computing system.

18. Explain why asynchronous I/O activity is a problem with many memory protection schemes, including base/bounds and paging. Suggest a solution to the problem.

19. Suggest an efficient scheme for maintaining a per-user protection scheme. That is, the system maintains one directory per user, and that directory lists all the objects to which the user is allowed access. Your design should address the needs of a system with 1000 users, of whom no more than 20 are active at any time. Each user has an average of 200 permitted objects; there are 50,000 total objects in the system.

20. A flaw in the protection system of many operating systems is argument passing. Often a common shared stack is used by all nested routines for arguments as well as for the remainder of the context of each calling process. Explain what vulnerabilities this flaw presents. Explain how the flaw can be controlled. The shared stack is still to be used for passing arguments and storing context.