Patcherex

Patcherex is the component used to create patched binaries in our CRS.

Installation

sudo apt-get install nasm clang

sudo apt-get install clang-10 gcc-avr binutils-avr avr-libc # (optional) for AVR patching (see patcherex/backends/detourbackends/avr.py)

# mkvirtualenv cgc # create and activate a proper virtual env in which other CRS components have been installed (see setup.py)

git clone https://github.com/angr/patcherex.git
cd patcherex
pip install -e .

Usage

There are three fundamental concepts in patcherex:

• patches
• techniques
• backends

patches

A patch is a single modification to a binary.

Different types of patches exist, for instance:

• InsertCodePatch: add some code that is going to be executed before an instruction at a specific address
• AddEntryPointPatch: add some code that is going to be executed before the original entry point of the binary.
• AddCodePatch: add some code that other patches can use.
• AddRWData: add some RW data that other patches can use.
• ...

See patcherex/patches.py for the full list of available patches.

Every patch has a name and it is possible to refer from a patch to another patch using its name.

backends

A backend is the component responsible to "inject" a list of patches in an existing binary and produce a new binary.

There are two backends:

• DetourBackend: it adds patches by inserting jumps inside the original code.
• ReassemblerBacked: it adds code by disassembling and then reassembling the original binary.

The DetourBackend generates bigger and slower binaries (and in some rare cases it cannot insert some patches), however it is slightly more reliable than the ReassemblerBackend (i.e., it breaks slightly less binaries).

techniques

A technique is a component analyzing a binary and returning a list of patches.

For instance:

• StackRetEncryption: it encrypts the return pointers of "unsafe" functions.
• Backdoor: it adds a backdoor to a binary.
• ...

Examples

IPython usage

Patcherex can be used with IPython.

The following example modifies the binary CADET_00003 so that it prints "HI!" every time a new string is entered by the user.

import patcherex
from patcherex.backends.detourbackend import DetourBackend
from patcherex.backends.reassembler_backend import ReassemblerBackend
from patcherex.patches import *

# the detour backend can be used as well:
# backend = DetourBackend("test_binaries/CADET_00003")
backend = ReassemblerBackend("test_binaries/CADET_00003")
patches = []

transmit_code = '''
  ; eax is the transmitted buffer
  ; ebx is the length
  pusha
  mov ecx,eax
  mov edx,ebx
  mov eax,0x2
  mov ebx,0x1
  mov esi,0x0
  int 0x80
  popa
  ret
  '''
patches.append(AddCodePatch(transmit_code, name="transmit_function"))
patches.append(AddRODataPatch(b"HI!\x00", name="transmitted_string"))
# the following code is going to be executed just before the original instruction at 0x8048166
injected_code = '''
; at this code location, it is fine to clobber eax and ebx
mov eax, {transmitted_string} ; a patch can refer to another patch address, by putting its name between curly brackets
mov ebx, 4
call {transmit_function}
'''
patches.append(InsertCodePatch(0x8048166,injected_code,name="injected_code_after_receive"))

# now we ask to the backend to inject all our patches
backend.apply_patches(patches)
# and then we save the file
backend.save("/tmp/CADET_00003_mod1")
# at this point you can try to run /tmp/CADET_00003_mod1 inside the DECREE VM or using our modified version of QEMU

Command line usage

Any method of the class PatchMaster (in patch_master.py) called generate_something_binary can be directly invoked from the command line.

The syntax is the following:

./patch_master.py single <input_file> <method> <output_file>

For instance, running the following command:

./patch_master.py single ../test_binaries/CADET_00003 stackretencryption  /tmp/CADET_00003_stackretencryption

will execute the following code:

def generate_stackretencryption_binary(self, test_bin=None):
    backend = ReassemblerBackend(self.infile)
    patches = []
    patches.extend(StackRetEncryption(self.infile, backend).get_patches())
    backend.apply_patches(patches)
    final_content = backend.get_final_content()
    return (final_content, "")

patch_master.py contains also methods to patch multiple binaries in parallel and quickly test them.