Reverse Engineering

Flare-Emu – Emulation features for the x86, x86_64, ARM, ARM64

September 27, 2019
14 Min Read
Flare-Emu - Powered by IDA Pro and the Unicorn emulation framework that provides scriptable emulation features for the x86, x86_64, ARM, and ARM64 architectures to reverse engineers

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flare-emu marries IDA Pro’s binary analysis capabilities with Unicorn’s emulation framework to provide the user with an easy to use and flexible interface for scripting emulation tasks.

It is designed to handle all the housekeeping of setting up a flexible and robust emulator for its supported architectures so that you can focus on solving your code analysis problems. Currently, flare-emu supports the x86, x86_64, ARM, and ARM64 architectures.

It currently provides four different interfaces to serve your emulation needs, along with a slew of related helper and utility functions.

  1. emulateRange – This API is used to emulate a range of instructions, or a function, within a user-specified context. It provides options for user-defined hooks for both individual instructions and for when “call” instructions are encountered. The user can decide whether the emulator will skip over, or call into function calls. This interface provides an easy way for the user to specify values for given registers and stack arguments. If a bytestring is specified, it is written to the emulator’s memory and the pointer is written to the register or stack variable. After emulation, the user can make use of flare-emu’s utility functions to read data from the emulated memory or registers, or use the Unicorn emulation object that is returned for direct probing. A small wrapper function for emulateRange, named emulateSelection, can be used to emulate the range of instructions currently highlighted in IDA Pro.
  2. iterate – This API is used to force emulation down specific branches within a function in order to reach a given target. The user can specify a list of target addresses, or the address of a function from which a list of cross-references to the function is used as the targets, along with a callback for when a target is reached. The targets will be reached, regardless of conditions during emulation that may have caused different branches to be taken. Like the emulateRange API, options for user-defined hooks for both individual instructions and for when “call” instructions are encountered are provided. An example use of the iterate API is to achieve something similar to what our argtracker tool does.
  3. iterateAllPaths – This API is much like iterate, except that instead of providing a target address or addresses, you provide a target function that it will attempt to find all paths through and emulate. This is useful when you are performing code analysis that wants to reach every basic block of a function.
  4. emulateBytes – This API provides a way to simply emulate a blob of extraneous shellcode. The provided bytes are not added to the IDB and are simply emulated as is. This can be useful for preparing the emulation environment. For example, flare-emu itself uses this API to manipulate a Model Specific Register (MSR) for the ARM64 CPU that is not exposed by Unicorn in order to enable Vector Floating Point (VFP) instructions and register access. The Unicorn emulation object is returned for further probing by the user.
  5. emulateFrom – This API is useful in cases where function boundaries are not clearly defined as is often the case with obfuscated binaries or shellcode. You provide a starting address, and it will emulate until there is nothing left to emulate or you stop emulation in one of your hooks. This can be called with the strict parameter set to False to enable dynamic code discovery; flare-emu will have IDA Pro make instructions as they are encountered during emulation.

Installation
To install flare-emu, simply drop flare_emu.py and flare_emu_hooks.py into your IDA Pro’s python directory and import it as a module in your IDApython scripts. flare-emu depends on Unicorn and its Python bindings.
IMPORTANT NOTE
flare-emu was written using the new IDA Pro 7x API, it is not backwards compatible with previous versions of IDA Pro.

Usage
While flare-emu can be used to solve many different code analysis problems, one of its more common uses is to aid in decrypting strings in malware binaries. FLOSS is a great tool than can often do this automatically for you by attempting to identify the string decrypting function(s) and using emulation to decrypt the strings passed in at every cross-reference to it. However, it is not possible for FLOSS to always be able to identify these functions and emulate them properly using its generic approaches. Sometimes you have to do a little more work, and this is where flare-emu can save you a lot of time once you are comfortable with it. Let’s walk through a common scenario a malware analyst encounters when dealing with encrypted strings.

Easy String Decryption Scenario
You’ve identified the function to decrypt all the strings in an x86_64 binary. This function is called all over the place and decrypts many different strings. In IDA Pro, you name this function decryptString. Here is your flare-emu script to decrypt all these strings and place comments with the decrypted strings at each function call as well as logging each decrypted string and the address it is decrypted at.

from __future__ import print_function
import idc
import idaapi
import idautils
import flare_emu

def decrypt(argv):
    myEH = flare_emu.EmuHelper()
    myEH.emulateRange(idc.get_name_ea_simple("decryptString"), registers = {"arg1":argv[0], "arg2":argv[1], 
                           "arg3":argv[2], "arg4":argv[3]})
    return myEH.getEmuString(argv[0])
    
def iterateCallback(eh, address, argv, userData):
    s = decrypt(argv)
    print("%016X: %s" % (address, s))
    idc.set_cmt(address, s, 0)
    
if __name__ == '__main__':   
    eh = flare_emu.EmuHelper()
    eh.iterate(idc.get_name_ea_simple("decryptString"), iterateCallback)

In __main__, we begin by creating an instance of the EmuHelper class from flare-emu. This is the class we use to do everything with flare-emu. Next, we use the iterate API, giving it the address of our decryptString function and the name of our callback function that EmuHelper will call for each cross-reference emulated up to.
The iterateCallback function receives the EmuHelper instance, named eh here, along with the address of the cross-reference, the arguments passed to this particular call, and a special dictionary named userData here. userData is not used in this simple example, but think of it as a persistent context to your emulator where you can store your own custom data. Be careful though, because flare-emu itself also uses this dictionary to store critical information it needs to perform its tasks. One such piece of data is the EmuHelper instance itself, stored in the “EmuHelper” key. If you are interested, search the source code to learn more about this dictionary. This callback function simply calls the decrypt function, prints the decrypted string and creates a comment for it at the address of that call to decryptString.
decrypt creates a second instance of EmuHelper that is used to emulate the decryptString function itself, which will decrypt the string for us. The prototype of this decryptString function is as follows: char * decryptString(char *text, int textLength, char *key, int keyLength). It simply decrypts the string in place. Our decrypt function passes in the arguments as received by the iterateCallback function to our call to EmuHelper‘s emulateRange API. Since this is an x86_64 binary, the calling convention uses registers to pass arguments and not the stack. flare-emu automatically determines which registers represent which arguments based on the architecture and file format of the binary as determined by IDA Pro, allowing you to write at least somewhat architecture agnostic code. If this were 32-bit x86, you would use the st ack argument to pass the arguments instead, like so: myEH.emulateRange(idc.get_name_ea_simple("decryptString"), stack = [0, argv[0], argv[1], argv[2], argv[3]]). The first stack value is the return address in x86, so we just use 0 as a placeholder value here. Once emulation is complete, we call the getEmuString API to retrieve the null-terminated string stored in the memory location pointed to by the first argument passed to the function.

Emulation Functions
emulateRange(startAddr, endAddr=None, registers=None, stack=None, instructionHook=None, callHook=None, memAccessHook=None, hookData=None, skipCalls=True, hookApis=True, strict=True, count=0) – Emulates the range of instructions starting at startAddress and ending at endAddress, not including the instruction at endAddress. If endAddress is None, emulation stops when a “return” type instruction is encountered within the same function that emulation began.

  • registers is a dictionary with keys being register names and values being register values. Some special register names are created by flare-emu and can be used here, such as arg1, arg2, etc., ret, and pc.
  • stack is an array of values to be pushed on the stack in reverse order, much like arguments to a function in x86 are. In x86, remember to account for the first value in this array being used as the return address for a function call and not the first argument of the function. flare-emu will initialize the emulated thread’s context and memory according to the values specified in the registers and stack arguments. If a string is specified for any of these values, it will be written to a location in memory and a pointer to that memory will be written to the specified register or stack location instead.
  • instructionHook can be a function you define to be called before each instruction is emulated. It has the following prototype: instructionHook(unicornObject, address, instructionSize, userData).
  • callHook can be a function you define to be called whenever a “call” type instruction is encountered during emulation. It has the following prototype: callHook(address, arguments, functionName, userData).
  • hookData is a dictionary containing user-defined data to be made available to your hook functions. It is a means to persist data throughout the emulation. flare-emu also uses this dictionary for its own purposes, so care must be taken not to define a key already defined. This variable is often named userData in user-defined hook functions due to its naming in Unicorn.
  • skipCalls will cause the emulator to skip over “call” type instructions and adjust the stack accordingly, defaults to True.
  • hookApis causes flare-emu to perform a naive implementation of some of the more common runtime and OS library functions it encounters during emulation. This frees you from having to be concerned about calls to functions such as memcpy, strcat, malloc, etc., and defaults to True.
  • memAccessHook can be a function you define to be called whenever memory is accessed for reading or writing. It has the following prototype: memAccessHook(unicornObject, accessType, memAccessAddress, memAccessSize, memValue, userData).
  • strict, when set to True (default), checks branch destinations to ensure the disassembler expects instructions. It otherwise skips the branch instruction. If set to False, flare-emu will make instructions in IDA Pro as it emulates them (DISABLE WITH CAUTION).
  • count is the maximum number of instructions to emulate, defaults to 0 which means no limit.

iterate(target, targetCallback, preEmuCallback=None, callHook=None, instructionHook=None, hookData=None, resetEmuMem=False, hookApis=True, memAccessHook=None) – For each target specified by target, a separate emulation is performed from the beginning of the containing function up to the target address. Emulation will be forced down the branches necessary to reach each target. target can be the address of a function, in which case the target list is populated with all the cross-references to the specified function. Or, target can be an explicit list of targets.

  • targetCallback is a function you create that will be called by flare-emu for each target that is reached during emulation. It has the following prototype: targetHook(emuHelper, address, arguments, userData).
  • preEmuCallback is a function you create that will be called before emulation for each target begins. You can implement some setup code here if needed.
  • resetEmuMem will cause flare-emu to reset the emulation memory before emulation of each target begins, defaults to False.

iterateAllPaths(target, targetCallback, preEmuCallback=None, callHook=None, instructionHook=None, hookData=None, resetEmuMem=False, hookApis=True, memAccessHook=None, maxPaths=MAXCODEPATHS, maxNodes=MAXNODESEARCH) – For the function containing the address target, a separate emulation is performed for each discovered path through it, up to maxPaths.

  • maxPaths – the max number of paths through the function that will be searched for and emulated. Some of the more complex functions can cause the graph search function to take a very long time or never finish; tweak this parameter to meet your needs in a reasonable amount of time.
  • maxNodes – the max number of basic blocks that will be searched when finding paths through the target function. This is a safety measure to prevent unreasonable search times and hangs and likely does not need to be changed.

emulateBytes(bytes, registers=None, stack=None, baseAddress=0x400000, instructionHook=None, hookData=None) – Writes the code contained in bytes to emulation memory at baseAddress if possible and emulates the instructions from the beginning to the end of bytes.
emulateFrom(startAddr, registers=None, stack=None, instructionHook=None, callHook=None, memAccessHook=None, hookData=None, skipCalls=True, hookApis=True, strict=True, count=0) – This API is useful in cases where function boundaries are not clearly defined as is often the case with obfuscated binaries or shellcode. You provide a starting address as startAddr, and it will emulate until there is nothing left to emulate or you stop emulation in one of your hooks. This can be called with the strict parameter set to False to enable dynamic code discovery; flare-emu will have IDA Pro make instructions as they are encountered during emulation.

Utility Functions
The following is an incomplete list of some of the useful utility functions provided by the EmuHelper class.

  • hexString(value) – Returns a hexadecimal formatted string for the value. Useful for logging and print statements.
  • getIDBString(address) – Returns the string of characters located at an address in the IDB, up to a null terminator. Characters are not necessarily printable. Useful for retrieving strings without an emulation context.
  • skipInstruction(userData, useIDA=False) – Call this from an emulation hook to skip the current instruction, moving the program counter to the next instruction. useIDA option was added to handle cases where IDA Pro folds multiple instructions into one pseudo instruction and you would like to skip all of them. This function cannot be called multiple times from a single instruction hook to skip multiple instructions. To skip multiple instructions, it is recommended not to write to the program counter directly if you are emulating ARM code as this might cause problems with thumb mode. Instead, try EmuHelper’s changeProgramCounter API (described below).
  • changeProgramCounter(userData, newAddress) – Call this from an emulation hook to change the value of the program counter register. This API takes care of thumb mode tracking for the ARM architecture.
  • getRegVal(registerName) – Retrieves the value of the specified register, being sensitive to sub-register addressing. For example, “ax” will return the lower 16 bits of the EAX/RAX register in x86.
  • stopEmulation(userData) – Call this from an emulation hook to stop emulation. Use this instead of calling the emu_stop Unicorn API so that the EmuHelper object can handle bookkeeping related to the iterate feature.
  • getEmuString(address) – Returns the string of characters located at an address in the emulated memory, up to a null terminator. Characters are not necessarily printable.
  • getEmuWideString(address) – Returns the string of “wide characters” located at an address in the emulated memory, up to a null terminator. “Wide characters” is meant loosely here to refer to any series of bytes containing a null byte every other byte, as would be the case for an ASCII string encoded in UTF-16 LE. Characters are not necessarily printable.
  • getEmuBytes(address, length) – Returns a string of bytes located at an address in the emulated memory.
  • getEmuPtr(address) – Returns the pointer value located at the given address.
  • writeEmuPtr(address) – Writes the pointer value at the given address in the emulated memory.
  • loadBytes(bytes, address=None) – Allocates memory in the emulator and writes the bytes to it.
  • isValidEmuPtr(address) – Returns True if the provided address points to valid emulated memory.
  • getEmuMemRegion(address) – Returns a tuple containing the start and end address of memory region containing the provided address, or None if the address is not valid.
  • getArgv() – Call this from an emulation hook at a “call” type instruction to receive an array of the arguments to the function.
  • addApiHook(apiName, hook) – Adds a new API hook for this instance of EmuHelper. Whenever a call instruction to apiName is encountered during emulation, EmuHelper will call the function specified by hook. If hook is a string, it is expected to be the name of an API already hooked by EmuHelper, in which case it will call its existing hook function. If hook is a function, it will call that function.
  • allocEmuMem(size, addr=None) – Allocates enough emulator memory to contain size bytes. It attempts to honor the requested address, but if it overlaps with an existing memory region, it will allocate in an unused memory region and return the new address. If address is not page-aligned, it will return an address that keeps the same page-aligned offset within the new region. For example, requesting address 0x1234 when 0x1000 is already allocated, may have it allocate at 0x2000 and return 0x2234 instead.

 


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