simple_test_harness.py

""" 
   Simple test harness for AFL's Unicorn Mode.

   This loads the simple_target.bin binary (precompiled as MIPS code) into
   Unicorn's memory map for emulation, places the specified input into
   simple_target's buffer (hardcoded to be at 0x300000), and executes 'main()'.
   If any crashes occur during emulation, this script throws a matching signal
   to tell AFL that a crash occurred.

   Run under AFL as follows:

   $ cd <afl_path>/unicorn_mode/samples/simple/
   $ ../../../afl-fuzz -U -m none -i ./sample_inputs -o ./output -- python simple_test_harness.py @@ 
"""

import argparse
import os
import signal

from unicorn import *
from unicorn.mips_const import *

# Path to the file containing the binary to emulate
BINARY_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), 'simple_target.bin')

# Memory map for the code to be tested
CODE_ADDRESS  = 0x00100000  # Arbitrary address where code to test will be loaded
CODE_SIZE_MAX = 0x00010000  # Max size for the code (64kb)
STACK_ADDRESS = 0x00200000  # Address of the stack (arbitrarily chosen)
STACK_SIZE    = 0x00010000  # Size of the stack (arbitrarily chosen)
DATA_ADDRESS  = 0x00300000  # Address where mutated data will be placed
DATA_SIZE_MAX = 0x00010000  # Maximum allowable size of mutated data

try:
    # If Capstone is installed then we'll dump disassembly, otherwise just dump the binary.
    from capstone import *
    cs = Cs(CS_ARCH_MIPS, CS_MODE_MIPS32 + CS_MODE_BIG_ENDIAN)
    def unicorn_debug_instruction(uc, address, size, user_data):
        mem = uc.mem_read(address, size)
        for (cs_address, cs_size, cs_mnemonic, cs_opstr) in cs.disasm_lite(bytes(mem), size):
            print("    Instr: {:#016x}:\t{}\t{}".format(address, cs_mnemonic, cs_opstr))
except ImportError:
    def unicorn_debug_instruction(uc, address, size, user_data):
        print("    Instr: addr=0x{0:016x}, size=0x{1:016x}".format(address, size))    

def unicorn_debug_block(uc, address, size, user_data):
    print("Basic Block: addr=0x{0:016x}, size=0x{1:016x}".format(address, size))
    
def unicorn_debug_mem_access(uc, access, address, size, value, user_data):
    if access == UC_MEM_WRITE:
        print("        >>> Write: addr=0x{0:016x} size={1} data=0x{2:016x}".format(address, size, value))
    else:
        print("        >>> Read: addr=0x{0:016x} size={1}".format(address, size))    

def unicorn_debug_mem_invalid_access(uc, access, address, size, value, user_data):
    if access == UC_MEM_WRITE_UNMAPPED:
        print("        >>> INVALID Write: addr=0x{0:016x} size={1} data=0x{2:016x}".format(address, size, value))
    else:
        print("        >>> INVALID Read: addr=0x{0:016x} size={1}".format(address, size))   

def force_crash(uc_error):
    # This function should be called to indicate to AFL that a crash occurred during emulation.
    # Pass in the exception received from Uc.emu_start()
    mem_errors = [
        UC_ERR_READ_UNMAPPED, UC_ERR_READ_PROT, UC_ERR_READ_UNALIGNED,
        UC_ERR_WRITE_UNMAPPED, UC_ERR_WRITE_PROT, UC_ERR_WRITE_UNALIGNED,
        UC_ERR_FETCH_UNMAPPED, UC_ERR_FETCH_PROT, UC_ERR_FETCH_UNALIGNED,
    ]
    if uc_error.errno in mem_errors:
        # Memory error - throw SIGSEGV
        os.kill(os.getpid(), signal.SIGSEGV)
    elif uc_error.errno == UC_ERR_INSN_INVALID:
        # Invalid instruction - throw SIGILL
        os.kill(os.getpid(), signal.SIGILL)
    else:
        # Not sure what happened - throw SIGABRT
        os.kill(os.getpid(), signal.SIGABRT)

def main():

    parser = argparse.ArgumentParser(description="Test harness for simple_target.bin")
    parser.add_argument('input_file', type=str, help="Path to the file containing the mutated input to load")
    parser.add_argument('-d', '--debug', default=False, action="store_true", help="Enables debug tracing")
    args = parser.parse_args()

    # Instantiate a MIPS32 big endian Unicorn Engine instance
    uc = Uc(UC_ARCH_MIPS, UC_MODE_MIPS32 + UC_MODE_BIG_ENDIAN)

    if args.debug:
        uc.hook_add(UC_HOOK_BLOCK, unicorn_debug_block)
        uc.hook_add(UC_HOOK_CODE, unicorn_debug_instruction)
        uc.hook_add(UC_HOOK_MEM_WRITE | UC_HOOK_MEM_READ, unicorn_debug_mem_access)
        uc.hook_add(UC_HOOK_MEM_WRITE_UNMAPPED | UC_HOOK_MEM_READ_INVALID, unicorn_debug_mem_invalid_access)

    #---------------------------------------------------
    # Load the binary to emulate and map it into memory

    print("Loading data input from {}".format(args.input_file))
    binary_file = open(BINARY_FILE, 'rb')
    binary_code = binary_file.read()
    binary_file.close()

    # Apply constraints to the mutated input
    if len(binary_code) > CODE_SIZE_MAX:
        print("Binary code is too large (> {} bytes)".format(CODE_SIZE_MAX))
        return

    # Write the mutated command into the data buffer
    uc.mem_map(CODE_ADDRESS, CODE_SIZE_MAX)
    uc.mem_write(CODE_ADDRESS, binary_code)

    # Set the program counter to the start of the code
    start_address = CODE_ADDRESS          # Address of entry point of main()
    end_address   = CODE_ADDRESS + 0xf4   # Address of last instruction in main()
    uc.reg_write(UC_MIPS_REG_PC, start_address)

    #-----------------
    # Setup the stack

    uc.mem_map(STACK_ADDRESS, STACK_SIZE)
    uc.reg_write(UC_MIPS_REG_SP, STACK_ADDRESS + STACK_SIZE)

    #-----------------------------------------------------
    # Emulate 1 instruction to kick off AFL's fork server
    #   THIS MUST BE DONE BEFORE LOADING USER DATA! 
    #   If this isn't done every single run, the AFL fork server 
    #   will not be started appropriately and you'll get erratic results!
    #   It doesn't matter what this returns with, it just has to execute at
    #   least one instruction in order to get the fork server started.

    # Execute 1 instruction just to startup the forkserver
    print("Starting the AFL forkserver by executing 1 instruction")
    try:
        uc.emu_start(uc.reg_read(UC_MIPS_REG_PC), 0, 0, count=1)
    except UcError as e:
        print("ERROR: Failed to execute a single instruction (error: {})!".format(e))
        return

    #-----------------------------------------------
    # Load the mutated input and map it into memory

    # Load the mutated input from disk
    print("Loading data input from {}".format(args.input_file))
    input_file = open(args.input_file, 'rb')
    input = input_file.read()
    input_file.close()

    # Apply constraints to the mutated input
    if len(input) > DATA_SIZE_MAX:
        print("Test input is too long (> {} bytes)".format(DATA_SIZE_MAX))
        return

    # Write the mutated command into the data buffer
    uc.mem_map(DATA_ADDRESS, DATA_SIZE_MAX)
    uc.mem_write(DATA_ADDRESS, input)

    #------------------------------------------------------------
    # Emulate the code, allowing it to process the mutated input

    print("Executing until a crash or execution reaches 0x{0:016x}".format(end_address))
    try:
        result = uc.emu_start(uc.reg_read(UC_MIPS_REG_PC), end_address, timeout=0, count=0)
    except UcError as e:
        print("Execution failed with error: {}".format(e))
        force_crash(e)

    print("Done.")

if __name__ == "__main__":
    main()