An introduction to ROP
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Slides from my workshop at Hack.LU 2010 in Luxembourg. This workshop introduced the basic concepts of Return Oriented Programming with some hands-on exercises.
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An introduction to ROP
- 1. net-square Introduction to ROP a first look at return oriented programming Saumil Shah, Net-Square Hack.LU 2010 - Luxembourg
- 2. net-square Introduction • DEP • EIP control • Ret2LibC • Return Oriented Programming
- 3. net-square DEP • Hardware enforced (NX). • Data areas marked non-executable. – Stack marked non-executable. – Heap marked non-executable. • You can load your shellcode in the stack or the heap... • ...but you can't jump to it.
- 4. net-square EIP control • Stack - forbidden • Heap - forbidden • Binary - OK • DLLs - OK Program Image Heap Stack DLL DLL DLL
- 5. net-square Ret2LibC • Return to LibC. • Pioneered by Solar Designer in 1997. • EIP made to "return to a function". • Need control of the stack memory. – We usually have it.
- 6. net-square Ret2LibC - how does it work? • Create a fake frame on the stack. • After an overflowed function returns... • ...set the EIP return address to the new function. • Append the fake frame. • New function executes. – parameters consumed from the fake frame. • system("/bin/sh")
- 7. net-square Return Oriented Programming • Series of function returns. • Chained frames. • Transform EIP based primitives into stubs that can be "returned into". • ESP is the new EIP!
- 8. net-square Topics • Function calls and returns • Stack overflows revisited • Creating stack frames • Chaining frames • ESP control
- 9. net-square Calling a function • Add two ints x, y. • add(3,4) • What does the calling frame look like? void add(int x, int y) { int sum; sum = x + y; printf("%dn", sum); } int main() { add(3, 4); }
- 10. net-square Stack frame for add(3,4) frame for add() return address from add() 3 4 call add
- 11. net-square Return from add(3,4) • add() is about to return. • RET after epilogue of add(). • Where does ESP point to? – immediately before the RET • What does the stack look like?
- 12. net-square Before the RET return address from add() 3 4 ESP
- 13. net-square Another function • Stack overflow in func1. • Can we call add(5, 6) after returning from func1? void func1(char *s) { char buffer[128]; strcpy(buffer, s); } int main() { func1(argv[1]); }
- 14. net-square Stack frame for func1() buffer return address from func1 s
- 15. net-square strcpy(buffer, s) buffer return address from func1 s AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAA AAAA AAAA AAAA
- 16. net-square Before the RET buffer return address from func1 s AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAA AAAA AAAA AAAA ESP
- 17. net-square After the RET buffer return address from func1 s AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAA AAAA AAAA AAAA EIP = 0x41414141 ESP
- 18. net-square Return to add() • Insert a fake frame in the buffer. • Make func1() return to: add(01010101, 02020202) • What does the stack frame look like?
- 19. net-square strcpy(buffer, s) buffer return address from func1 s AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA address of add return address from add 01010101 02020202
- 20. net-square Before func1() returns buffer return address from func1 s AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA address of add return address from add 01010101 02020202 ESP
- 21. net-square Return to add() buffer return address from func1 s AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAA address of add return address from add 01010101 02020202 ESP EIP = add()
- 22. net-square Return to add() • By carefully creating a frame... • ...we can make the program "return to our function". • We control the parameters. • We also control where to jump to after our function returns.
- 23. net-square victim2.c int main(int argc, char *argv[]) { add(3, 4); func1(argv[1]); } void func1(char *s) { char buffer[128]; strcpy(buffer, s); } void print_hello(void) { printf("Hello Worldn"); } void add(int x, int y) { int sum; sum = x + y; printf("%d + %d = %dn", x, y, sum); } stack overflow lurks here!
- 24. net-square victim2.c • Compile victim2.c • Run victim2 under a debugger • Study the stack frame before add(3, 4) gcc victim2.c -o victim2 gdb victim2
- 25. net-square gdb'ing add • Set a breakpoint before add(3, 4) 0x804836c <main>: push %ebp 0x804836d <main+1>: mov %esp,%ebp 0x804836f <main+3>: sub $0x8,%esp 0x8048372 <main+6>: and $0xfffffff0,%esp 0x8048375 <main+9>: mov $0x0,%eax 0x804837a <main+14>: sub %eax,%esp 0x804837c <main+16>: sub $0x8,%esp 0x804837f <main+19>: push $0x4 0x8048381 <main+21>: push $0x3 0x8048383 <main+23>: call 0x80483de <add> 0x8048388 <main+28>: add $0x10,%esp 0x804838b <main+31>: cmpl $0x1,0x8(%ebp)
- 26. net-square Stack frame before add(3, 4) • Dump the stack • Continue (gdb) x/64 $esp 0xbffff8ac: 0x08048388 0x00000003 0x00000004 0xbffff8c8 0xbffff8bc: 0x0804841c 0x40148f50 0x40012780 0xbffff8e8 0x08048388 3 4
- 27. net-square Overflowing func1() • Overflow func1 and... ...return to add(01010101, 02020202) • Create a fake frame. • Overwrite stack. • frame1.pl return from func1 param1 param2 return from add 0x080483de 0x01010101 0x02020202 0x42424242
- 28. net-square frame1.pl • Creates the overflow buffer as follows: • Set this in an environment variable EGG and run victim2 with $EGG: 080483deAAAAAA...140...AAAAAA 42424242 01010101 02020202 distance to EIP address of add return from add param1 param2 export EGG=`./frame1.pl` gdb victim2 (gdb) run $EGG
- 29. net-square ESP • Where will ESP be after returning from add? • Verify 080483deAAAAAA...140...AAAAAA 42424242 01010101 02020202 (gdb) x/64 $esp 0xbffff824: 0x01010101 0x02020202 0x08048400 0x40148f50 0xbffff834: 0x40012780 0xbffff858 0x4002e7f7 0x00000002 ESP
- 30. net-square Chaining functions • After add(01010101, 02020202), we want to run add(03030303, 04040404). • How should we set up the frames? • First, study the frame after add() returns.
- 31. net-square After add() returns AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA address of add 42424242 01010101 02020202 ESP EIP = 42424242
- 32. net-square Where does the new frame go? AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA address of add 42424242 01010101 02020202 address of add ?? 03030303 04040404
- 33. net-square Where does the new frame go? • We get only ONE chance at strcpy. • How do we preserve params 01010101 and 02020202? • We can only APPEND the second frame below our first frame. • We have to UNWIND the first frame before returning to the second frame. • Return to epilogue!
- 34. net-square Chaining the frames AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA address of add address of POP/POP/RET 01010101 02020202 address of add 42424242 03030303 04040404 add(01010101, 02020202) add(03030303, 04040404)
- 35. net-square Keeping ESP in control AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA address of add address of POP/POP/RET 01010101 02020202 address of add 42424242 03030303 04040404 Return from func1 Return to add() Return to POP/POP/RET POP POP ESP RET - Return to add() Finally EIP = 0x42424242
- 36. net-square frame2.pl • Creates the overflow buffer as follows: 080483deAAAAAA...140...AAAAAA 0804843d 01010101 02020202 distance to EIP address of add POP/POP /RET param1 param2 080483de 42424242 03030303 04040404 address of add return from add param1 param2 Use msfelfscan to find the address of POP/POP/RET from victim2 binary.
- 37. net-square frame2.pl • Set this in an environment variable EGG and run victim2 with $EGG: export EGG=`./frame2.pl` gdb victim2 (gdb) run $EGG Starting program: /home/user0/victim2 $EGG 3 + 4 = 7 1010101 + 2020202 = 3030303 3030303 + 4040404 = 7070707 Program received signal SIGSEGV, Segmentation fault. 0x42424242 in ?? ()
- 38. net-square It's all about ESP! • ESP is the new EIP. • ROP involves keeping the ESP moving through the frames on the stack. • Frames can be chained by returning to epilogues of functions. – to appropriately unwind the parameters pushed on the stack. • We must never lose sight of RET.
- 39. net-square ROP frames - generic approach f1(A, B) f2(X) f1(C, D) f3(P, Q, R, S) f2(Y) : : & f1() & POP/POP/RET A B & f2() & POP/RET X & f3() & POPAD/RET P Q R & f1() & POP/POP/RET C D junk junk junk & f2() & POP/RET Y S
- 40. net-square Topics • Transforming classic EIP code to ROP • ROP vs. classic programming • Thinking in ROP terms • Assembling frames • Gadgets • Searching for gadgets • Generic techniques
- 41. net-square EIP vs. ESP Classic EIP code • N ops = N instructions. • EIP increments. • ESP fluctuates. • The CPU increments EIP automatically. ROP code • N ops = N frames. • ESP increments. • EIP fluctuates. • We have to control ESP through RET instructions.
- 42. net-square Transform EIP code to ROP • Load two registers • Call a function – with params 3,4 • How does this translate in ROP terms? mov eax, 14 mov ecx, 02500000 push 3 push 4 call 77fe3210
- 43. net-square Thinking in ROP terms ?? AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA 14 ESP Put 14 in the next stack word, and POP it into EAX. RET function body epilogue prologue vulnerable function EIP mov eax, 14 RET POP EAX POP EBX functionX 700344fe 700344fd 700344ff
- 44. net-square Thinking in ROP terms 700344fe AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA 14ESP RET function body epilogue prologue vulnerable function EIP Jump to address of "POP EAX; RET". Search function epilogues. RET POP EAX POP EBX functionX mov eax, 14
- 45. net-square Thinking in ROP terms 700344fe ?? AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA 14 02500000 ESP RET function body epilogue prologue vulnerable function EIP Place 02500000 as the next stack word, to be loaded into ECX. RET POP EAX POP EBX functionX eax = 00000014 mov ecx, 02500000
- 46. net-square Thinking in ROP terms 700344fe 6d894430 AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA 14 02500000 RET function body epilogue prologue vulnerable function We now need an address of a "POP ECX; RET" sequence. RET POP EAX POP EBX functionX RET POP ECX functionY ESP EIP eax = 00000014 mov ecx, 02500000 6d894430 6d894431
- 47. net-square Thinking in ROP terms 700344fe 6d894430 AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA 14 02500000ESP RET function body epilogue prologue vulnerable function 02500000 is popped into ECX. RET POP EAX POP EBX functionX EIP RET POP ECX functionY eax = 00000014 mov ecx, 02500000
- 48. net-square Thinking in ROP terms 700344fe 6d894430 AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA 14 02500000 ??ESP RET function body epilogue prologue vulnerable function We now need to call a function at 77fe3210, with parameters 3, 4 RET POP EAX POP EBX functionX EIP RET POP ECX functionY push 3; push 4; call 77fe3210 eax = 00000014 ecx = 02500000
- 49. net-square Gadget Dictionary POP EAX; RET Load value into register value POP ECX; RET Read memory at address address ADD EAX,n; RET Add MOV EAX,[ECX]; RET POP EAX; RET Write value at address address POP ECX; RET value MOV [EAX],ECX; RET INC EAX; RET Increment address of function Call a function param cleanup param 1 param 2 RET NOP param N POP; POP ... RET ADD ESP,24; RET POPAD; RET POP EAX; RET Call a function pointer address CALL [EAX]; RET XCHG EAX,ESP; RET Stack flip ESP=EAX LEAVE; RET Stack flip ESP=EBP POP ESP; RET Stack flip ESP=addr address
- 50. net-square Instruction Opcodes Instruction Opcodes RET C3 RET n C2 16bits POP EAX 58 POP ECX 59 MOV EAX,[ECX] 8B 01 MOV [EAX],ECX 89 08 MOV [ECX],EAX 89 01 INC EAX 40 ADD EAX, n 83 C0 8bits
- 51. net-square Instruction Opcodes Instruction Opcodes POP EBX/EDX/ESI/EDI/EBP 5B/5A/5E/5F/5D POPAD 61 ADD ESP,24 83 C4 18 CALL [EAX] FF 10 XCHG EAX,ESP 94 LEAVE C9 (mov esp,ebp; pop ebp) POP ESP 5C
- 52. net-square Searching for Gadgets • Use msfpescan's regex search. • Example: MOV EAX,[ECX]; RET msfpescan -D -r 'x8Bx01xC3' <file> msfpescan -D -r 'x8Bx01xC2.x00' <file> msfpescan -D -r 'x8Bx01.xC3' <file> msfpescan -D -r 'x8Bx01..xC3' <file> • Sometimes you may need to improvise.
- 53. net-square Generic Techniques • Run arbitrary shellcode. • Difficult to transform entire shellcode to ROP frames. • Create a ROP loader: – Allocate RWX memory – Copy classic shellcode to this memory – Jump to shellcode • Load and run any shellcode.
- 54. net-square DEMO TIME! • Java JNLP docbase overflow • Exploiting IE8 using ROP
- 55. net-square KTHXBAI! saumil@net-square.com linked-in: saumilshah no LOLcats were harmed in preparation of these slides