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Verifying the Safety of User Pointer Dereferences

Verifying the Safety of User Pointer Dereferences. Suhabe Bugrara suhabe@stanford.edu Stanford University Joint work with Alex Aiken. Unchecked User Pointer Dereferences. Security property of operating systems Two types of pointers in operating systems

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Verifying the Safety of User Pointer Dereferences

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  1. Verifying the Safety of User Pointer Dereferences Suhabe Bugrara suhabe@stanford.edu Stanford University Joint work with Alex Aiken

  2. Unchecked User Pointer Dereferences • Security property of operating systems • Two types of pointers in operating systems • kernel pointer: pointer created by the operating system • user pointer: pointer created by a user application and passed to the operating system via an entry point such as a system call • Must check that a user pointer points into userspace before dereferencing it

  3. Unchecked User PointerDereferences 1: static ssize_t read_port(…, char * __user buf, …) { 2: unsigned long i = *ppos; 3: char * __user tmp = buf; 4:

  4. Unchecked User PointerDereferences 1: static ssize_t read_port(…, char * __user buf, …) { 2: unsigned long i = *ppos; 3: char * __user tmp = buf; 4: 7: 8: while (count-- > 0 && i < 65536) { 9: if (__put_user(inb(i),tmp) < 0) //deref 10: return -EFAULT; 11: i++; 12: tmp++; 13: } 14: 15: *ppos = i; 16: return tmp-buf; 17: }

  5. Unchecked User PointerDereferences 1: static ssize_t read_port(…, char * __user buf, …) { 2: unsigned long i = *ppos; 3: char * __user tmp = buf; 4: 5: if (!access_ok(..,buf,...)) //check 6: return -EFAULT; 7: 8: while (count-- > 0 && i < 65536) { 9: if (__put_user(inb(i),tmp) < 0) //deref 10: return -EFAULT; 11: i++; 12: tmp++; 13: } 14: 15: *ppos = i; 16: return tmp-buf; 17: }

  6. Security Vulnerability • Malicious user could • Take control of the operating system • Overwrite kernel data structures • Read sensitive data out of kernel memory • Crash machine by corrupting data

  7. Goal • Design a program analysis to prove statically that no unchecked user pointer dereferences exist in the entire operating system

  8. Challenges • Verification • provide guarantee of correctness • Precision • report low number of false alarms • Scalability • analyze more than 6 MLOC

  9. Verification

  10. Verification • Soundness • If the program analysis reports that no vulnerabilities exist, then the program contains none

  11. Verification • Soundness • If the program analysis reports that no vulnerabilities exist, then the program contains none • Completeness • If the program analysis reports that a vulnerability exists, then program contains one

  12. Verification • Soundness • If the program analysis reports that no vulnerabilities exist, then the program contains none • Completeness • If the program analysis reports that a vulnerability exists, then program contains one • Impossible for a program analysis to be both sound and complete

  13. Sound and Incomplete Verifier • Proves the absence of vulnerabilities • May report false alarms

  14. Soundness Caveats • Unsafe memory operations • Concurrency • Inline assembly • Analysis fails to analyze some procedures

  15. Precision • Minimize the number of false alarms • Reasoning more deeply about program • Computationally expensive • High precision inhibits scalability

  16. Example 1: void sys_call (int *u, const int cmd) { //u is user pointer 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { //check u 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; //dereference u 12: }

  17. One Possible Approach 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)

  18. One Possible Approach 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user) (*u,user)

  19. One Possible Approach 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user) (*u,user) (*u,user)

  20. One Possible Approach 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user) (*u,user) (*u,user) (*u,user) (*u,checked)

  21. One Possible Approach 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user) (*u,user) (*u,user) (*u,user) (*u,checked) (*u,user) lost precision!

  22. One Possible Approach 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user) (*u,user) (*u,user) (*u,user) (*u,checked) (*u,user) lost precision! (*u,user) (*u,error) emit warning! …, but, procedure does not contain any vulnerabilities!

  23. Path Sensitivity • Ability to reason about branch correlations • Programs use substantial amount of branch correlation in practice • Important for reducing the number of false alarms

  24. Example 1: void sys_call (int *u, int cmd) { //u is user pointer 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { //check u 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; //dereference u 12: }

  25. Path Sensitivity 1: void sys_call (int *u, int cmd) { //u is user pointer 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { //check u 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; //dereference u 12: } Valid Path

  26. Path Sensitivity 1: void sys_call (int *u, const int cmd) { //u is user pointer 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { //check u 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; //dereference u 12: } Valid Path

  27. Path Sensitivity 1: void sys_call (int *u, const int cmd) { //u is user pointer 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { //check u 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; //dereference u 12: } Valid Path

  28. Path Sensitivity 1: void sys_call (int *u, const int cmd) { //u is user pointer 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { //check u 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; //dereference u 12: } Invalid Path!

  29. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: }

  30. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true

  31. Path Sensitive Analysis “guard” 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true

  32. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true

  33. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true

  34. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1

  35. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1

  36. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  . . .

  37. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  cmd == 1 && . . .

  38. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  cmd == 1 && !(cmd == 1) && . . .

  39. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  cmd == 1 && !(cmd == 1) && true . . .

  40. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  cmd == 1 && !(cmd == 1) && true  false

  41. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  false

  42. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  false

  43. Scalability • Abstraction • Throw away guards at procedure boundaries • Compositionality • Analyze each procedure in isolation

  44. Path Sensitive Analysis 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  false

  45. Abstraction (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  false initial summary

  46. Abstraction α = (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  false abstraction function initial summary

  47. Abstraction α = (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  false (*u,user)  true (*u,checked)  false (*u,error)  false abstraction function initial summary final summary

  48. Abstraction 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,error)  false

  49. Abstraction 1: void sys_call (int *u, const int cmd) { 2: int x; 3: 4: if (cmd == 1) { 5: if (!access_ok(u)) { 6: return; 7: } 8: } 9: … 10: if (cmd == 1) 11: x = *u; 12: } (*u,user)  true (*u,user)  true (*u,user)  true (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  cmd == 1 (*u,user)  true (*u,checked)  false (*u,error)  false

  50. Compositionality 1: int get (int *v) { 2: int x; 3: 4: x = *v; 5: 6: return x; 7: }

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