Understanding Shared Objects in Operating Systems

1. Secure Operating Systems Lesson 5: Shared Objects

2. Where are we? • We have got more of the fundamental security structures of our OS in our heads • But now we have to face a real challenge: shared objects

3. The OS doesn’t HAVE TO… • I’ve used that heading before, but it’s true • There’s no requirement for our OS to support sharing between users and processes… but it sure comes in handy • Once again, we have a tension between performance and security

4. Two Parts of the Problem • Sharing actual information • Synchronizing between threads and/or processes

5. Peterson’s Solution • Two shared variables: • int turn;boolean flag[2] • Code: • flag[i] = TRUE;turn = j;while (flag[j] && turn == j); // Do Critical Sectionflag[i] = FALSE;

6. Peterson’s Solution II PROCESS 0 PROCESS 1 flag[1] = TRUE;turn = 0;while(flag[0] && turn == 0); // Critical Sectionflag[1] = FALSE; • flag[0] = TRUE;turn = 1;while(flag[1] && turn == 1); // Critical Sectionflag[0] = FALSE;

7. Hardware Support • The challenge of disabling interrupts is that it’s expensive • Many OS provide a hardware “test and set” instruction, which allows atomic access to a chunk of memory • Swap: void Swap(boolean *a, boolean *b) {boolean temp = *a; *a = *b; *b = temp; }

8. Implemented as… • do { key = TRUE; while (key == TRUE) swap(&lock, &key); // Critical Section lock = FALSE;} while (TRUE); • Mutual-exclusion with Swap…

9. Semaphores • wait(S) { while (S <= 0); //nop S--;} • signal(S) { S++;} • This really looks like a spinlock…

10. Semaphores • wait(semaphore *S) { S->value--; if (S->value < 0) { add this proc to S->list; block(); // SLEEP }} // This will halt until we own the semaphore

11. Deadlocks • P0 • wait(S);wait(Q);…signal(S);signal(Q); • P1 • wait(Q);wait(S);…signal(Q);signal(S);

12. Priority Inversion • Imagine we have three procii, L, M and H, where L is Low Priority, M, medium, and H, High • L is holding a resource which is blocking H, but gets swapped out for M • This is known as Priority Inversion… and it’s a real problem! • Probably we should talk about different scheduling approaches

13. Mars Sojourner • Long running, medium priority Comms task • Low priority weather task • High priority information bus thread • Low priority wx task acquires a mutex for the bus… gets interrupted by the Comms task (long running), blocking the high priority bus thread… tada! Priority Inversion • Can be a security issue too! • Can be solved by priority inheritance

14. Atomicity • Making sure something is atomic is pretty easy on a single core system • On a more complex system it can get REALLY hard • One approach is transactional memory – move the problem to the memory not the programmer • None of this has even touched on how we SHARE information between processes…

15. Race Conditions • Poor synchronization can lead to race conditions – a subset of which is called TOCTOU • Race conditions arise from interdependence that is unrealized or incorrectly implemented

16. Things to Do • Read “An Investigation of the Therac-25 Accidents”, Nancy Leveson, Clark S. Turner

17. Questions & Comments • What do you want to know?