ropc

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ROPC

ROPC is an example of a Turing complete ROP compiler. It's not supposed to be practical -- generated payloads are too big to be useful (emulating a stack is very space consuming). Generated programs are pure -- nothing is ever executed from executable memory.

Programs for ROPC are written in a 'high-level' language ROPL. Its features include:

• conditional jumps ,
• functions (even recursive),
• local variables,
• labels, pointers, memory derefencing, nested arithmetic operations, etc.

For examples, look in examples-ropl/.

ROPC is a POC, and should be treated as such -- don't expect it to work on apps from /usr/bin. Test it on the included synthetic example (test.asm, test.c).

Compiling

Compile BAP (bap-0.4) as explained in its README/INSTALL files (you will need to install some dependencies and die a little inside during this process).

Compile all of ROPC's utils with make test-all in ropc/ dir. Expected output:

./a.out compiled.bin buf=0x09f34170 roundup buf=0x09f40000 0 1 1 2 3 5 8 13 21 34 55 Segmentation fault

(it's supposed to segfault.)

Utils

gadget - extracts and dynamically classifies gadgets found in a binary. Usage: ./gadget

verify - verifies the semantics of gadgets returned by the 'gadget' util. Usage: ./verify . You will need the STP SAT solver to use it.

ropc - actual compiler. Usage; ./ropc . The compiled ROP payload is saved to 'compiled.bin'.

a.out - a synthetic example with gadgets defined in 'test.asm'. Logic is defined in 'test.c'. Usage: ./a.out <compiled ROP payload file> - this will allocate new stack, copy the payload there and start executing it. It's useful for testing compiled ROP programs :).

dumper - dumps contents of gadget files created by 'gadget' or 'verify'. Usage: ./dumper .

How it works

ROPC is based on Q [1]. Gadget discovery, classification and verification algorithms are identical. The approach to compilation is very different though.

Overview:

• byte sequences (possible gadget candidates) ending in RET are dynamically classified (like in Q),
• semantics of found candidates are verified with a SAT solver (like in Q),
• the AST of the ROPL source file is simplified and flattened to a list of pseudoinstructions (PI). PIs express complex logic using simpler PIs, or gadgets.
• ROPC unrolls all PIs to a list of gadgets, making shure that there are no register conflicts and gadgets will not overwrite registers used by other gadgets.

All of the magic happens during the last two steps. Stack is emulated in the .data section of the targeted application. Conditional jumps are implemented using lahf / add esp, gadgets.

See gdtr.wordpress.com for more details.

Performance

Emulating x86 from ocaml is very slow. Classifing all gadgets from the included examples takes ~30 secs.

The compilation process on the other hand is optimal in the sense that its running time is O(n), where n is the number of ROPL instructions. There's a slight problem with the constant hidden in O(), but it's a bit too convoluted to describe here.

The Fibonacci example: examples-ropl/fib.ropl compiles in 2 seconds.

BAP

ROPC relies heavily on BAP [2] for IR handling, symbolic execution and interfacing with SAT solvers. At least you can use ROPC as an example of how to use BAP's API :). BAP has so much stuff implemented, that it's a crime not to use it :P.

I'd like to thank Edward J. Schwartz from the CMU team, for this helpful comments regarding BAP, Q, and other things.

Author

p_k twitter.com/pa_kt gdtr.wordpress.com

References

1. Q: Exploit Hardening Made Easy, http://users.ece.cmu.edu/~ejschwar/papers/usenix11.pdf
2. BAP: The Next-Generation Binary Analysis Platform, http://http://bap.ece.cmu.edu/