pakt / ropc

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 <exe file> <output file with gadget candidates>

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

ropc - actual compiler. Usage; ./ropc <ropl src file> <verified gadgets file>.
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 <gadget file>.


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, <reg> 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/