unicorn模拟执行学习

前言

unicorn 是一个模拟执行软件，用于模拟执行各种平台的二进制文件，前几天在 twitter 上看到一篇文章，这里做个记录。

正文

记录系统调用

首先是一个简单的示例

e8ffffffffc05d6a055b29dd83c54e89e96a02030c245b31d266ba12008b39c1e710c1ef1081e9feffffff8b4500c1e010c1e81089c309fb21f8f7d021d86689450083c5024a85d20f85cfffffffec37755d7a0528ed24ed24ed0b887feb509838f95c962b9670fec6ffc6ff9f321f581e00d380

这是一段 x86_32 的 shellcode，可以用 radare2 反汇编它

rasm2 -a x86 -b 32 -d e8ffffffffc05d6a055b29dd83c54e89e96a02030c245b31d266ba12008b39c1e710c1ef1081e9feffffff8b4500c1e010c1e81089c309fb21f8f7d021d86689450083c5024a85d20f85cfffffffec37755d7a0528ed24ed24ed0b887feb509838f95c962b9670fec6ffc6ff9f321f581e00d380

paste image

这里的目标是记录他的系统调用，在 32 中使用 int 80 来执行系统调用，所以我们在执行 int 80 前记录它的寄存器信息，就可以记录系统调用了。


from unicorn import *
from unicorn.x86_const import *


shellcode = "e8ffffffffc05d6a055b29dd83c54e89e96a02030c245b31d266ba12008b39c1e710c1ef1081e9feffffff8b4500c1e010c1e81089c309fb21f8f7d021d86689450083c5024a85d20f85cfffffffec37755d7a0528ed24ed24ed0b887feb509838f95c962b9670fec6ffc6ff9f321f581e00d380".decode("hex")

BASE = 0x400000
STACK_ADDR = 0x0
STACK_SIZE = 1024*1024

mu = Uc (UC_ARCH_X86, UC_MODE_32)

mu.mem_map(BASE, 1024*1024)
mu.mem_map(STACK_ADDR, STACK_SIZE)


mu.mem_write(BASE, shellcode)
mu.reg_write(UC_X86_REG_ESP, STACK_ADDR + STACK_SIZE/2)

def syscall_num_to_name(num):
    syscalls = {1: "sys_exit", 15: "sys_chmod"}
    return syscalls[num]

def hook_code(mu, address, size, user_data):
    #print('>>> Tracing instruction at 0x%x, instruction size = 0x%x' %(address, size))  
    
    machine_code = mu.mem_read(address, size)
    if machine_code == "\xcd\x80":
        
        r_eax = mu.reg_read(UC_X86_REG_EAX)
        r_ebx = mu.reg_read(UC_X86_REG_EBX)
        r_ecx = mu.reg_read(UC_X86_REG_ECX)
        r_edx = mu.reg_read(UC_X86_REG_EDX)
        syscall_name = syscall_num_to_name(r_eax)
        
        print "--------------"
        print "We intercepted system call: "+syscall_name
        
        if syscall_name == "sys_chmod":
            s = mu.mem_read(r_ebx, 20).split("\x00")[0]
            print "arg0 = 0x%x -> %s" % (r_ebx, s)
            print "arg1 = " + oct(r_ecx)
        elif syscall_name == "sys_exit":
            print "arg0 = " + hex(r_ebx)
            exit()
        
        mu.reg_write(UC_X86_REG_EIP, address + size)
        
mu.hook_add(UC_HOOK_CODE, hook_code)

mu.emu_start(BASE, BASE-1)

关键就是使用 mu.hook_add, 使得在 unicorn 执行一条指令之前会先执行 hook_code 并且传入了与程序运行状态相关的参数，便于我们对程序状态进行操纵。在这里就是获取了寄存器的值，然后根据系统调用号解析参数。

arm代码模拟执行

测试程序位于

http://t.cn/RQ6viS6

其实就是执行一个递归函数，最后打印返回值

paste image

我们的目标是加速程序的执行，可以加速的原理在于，这里是递归调用，对于的参数，返回值确定，所以我们就可以对已经执行过的参数，直接设置返回值，进而加速程序的运行。

from unicorn import *
from unicorn.arm_const import *
import struct

def read(name):
    with open(name) as f:
        return f.read()
        
def u32(data):
    return struct.unpack("I", data)[0]
    
def p32(num):
    return struct.pack("I", num)


mu = Uc(UC_ARCH_ARM, UC_MODE_LITTLE_ENDIAN)


BASE = 0x10000
STACK_ADDR = 
STACK_SIZE = 1024*10240x300000

mu.mem_map(BASE, 1024*1024)
mu.mem_map(STACK_ADDR, STACK_SIZE)


mu.mem_write(BASE, read("./task4"))
mu.reg_write(UC_ARM_REG_SP, STACK_ADDR + STACK_SIZE/2)

instructions_skip_list = []

CCC_ENTRY = 0x000104D0
CCC_END = 0x00010580

stack = []                                          # Stack for storing the arguments
d = {}                                              # Dictionary that holds return values for given function arguments 

def hook_code(mu, address, size, user_data):  
    #print('>>> Tracing instruction at 0x%x, instruction size = 0x%x' %(address, size))
    if address == CCC_ENTRY:                        # Are we at the beginning of ccc function?
        arg0 = mu.reg_read(UC_ARM_REG_R0)           # Read the first argument. it is passed by R0
        
        if arg0 in d:                               # Check whether return value for this function is already saved.
            ret = d[arg0]
            mu.reg_write(UC_ARM_REG_R0, ret)        # Set return value in R0
            mu.reg_write(UC_ARM_REG_PC, 0x105BC)    # Set PC to point at "BX LR" instruction. We want to return from fibonacci function
            
        else:
            stack.append(arg0)                      # If return value is not saved for this argument, add it to stack.
        
    elif address == CCC_END:
        arg0 = stack.pop()                          # We know arguments when exiting the function
        
        ret = mu.reg_read(UC_ARM_REG_R0)            # Read the return value (R0)
        d[arg0] = ret                               # Remember the return value for this argument
        
mu.hook_add(UC_HOOK_CODE, hook_code)
mu.emu_start(0x00010584, 0x000105A8)
return_value = mu.reg_read(UC_ARM_REG_R1)           # We end the emulation at printf("%d\n", ccc(x)).
print "The return value is %d" % return_value

关键点，用一个数组存储了 参数：返回值 对，从而规避一些冗余的运算。

参考

http://eternal.red/2018/unicorn-engine-tutorial/

unicorn模拟执行学习

前言

正文

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