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反混淆学习-fla控制流平坦化篇

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2024/03/27

反混淆学习-fla控制流平坦化篇

感觉反混淆比混淆难多了。。。

现在也暂时不会怎么自己写ida-python脚本来反混淆

还是先学学看吧

deflat工具介绍

网上常用的反控制流平坦化的工具，可以直接在终端运行将经过fla编译的可执行文件转化为反混淆过的可执行文件，也支持arm架构。

下载地址：deflat: use angr to deobfuscation 

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$ python deflat.py --file 文件名 --addr 初始地址

需要一个python的angr库

因为要用到初始地址，所以就要求你的可执行文件没开PIE（随机地址）保护

（要求还挺苛刻的，估计只有比赛值得一用，实战估计全是保护全开，优点就是常用架构都能用）

流程

自己先用编译一个保护全关的经过fla过的文件

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opt -lowerswitch -S IR/TestProgram.ll -o IR/TestProgram_lowerswitch.ll
opt -load ../Build/LLVMObfuscator.so -fla -S -enable-new-pm=0 IR/TestProgram_lowerswitch.ll -o IR/TestProgram_fla.ll
clang IR/TestProgram_fla.ll -o Bin/TestProgram_fla -fno-stack-protector -z execstack -no-pie

image-20240327201723919

用ida看看，很常规的控制流平坦化

image-20240327201828945

用工具转化一下，得到了个TestProgram_fla_unprotect_recovered文件

image-20240327202021043

确实反混淆了，效果不错

image-20240327202141816

这下面有一堆nop看来是把一些跳转指令全去了，并进行了补全

代码分析

详细学习太复杂，简单概述一下大概流程

主函数

引入：

  1. 函数的开始地址为序言的地址
  2. 序言的后继为主分发器
  3. 后继为主分发器的块为预处理器
  4. 后继为预处理器的块为真实块
  5. 无后继的块为返回块
  6. 剩下的为无用块
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def main():
    parser = argparse.ArgumentParser(description="deflat control flow script")
    parser.add_argument("-f", "--file", help="binary to analyze")
    parser.add_argument(
        "--addr", help="address of target function in hex format")
    args = parser.parse_args()

    if args.file is None or args.addr is None:
        parser.print_help()
        sys.exit(0)

    filename = args.file
    start = int(args.addr, 16)

    project = angr.Project(filename, load_options={'auto_load_libs': False})
    # do normalize to avoid overlapping blocks, disable force_complete_scan to avoid possible "wrong" blocks
    # 获取控制流
    cfg = project.analyses.CFGFast(normalize=True, force_complete_scan=False)
    target_function = cfg.functions.get(start)
    # A super transition graph is a graph that looks like IDA Pro's CFG
    supergraph = am_graph.to_supergraph(target_function.transition_graph)

    base_addr = project.loader.main_object.mapped_base >> 12 << 12

    # get prologue_node and retn_node
    # 从控制流中获取返回块和序言
    prologue_node = None
    for node in supergraph.nodes():
        if supergraph.in_degree(node) == 0:
            prologue_node = node
        if supergraph.out_degree(node) == 0 and len(node.out_branches) == 0:
            retn_node = node

    if prologue_node is None or prologue_node.addr != start:
        print("Something must be wrong...")
        sys.exit(-1)
	
    # 主分发器，successors包括了所有正常的后续list
    main_dispatcher_node = list(supergraph.successors(prologue_node))[0]
    
    # 找到预处理器
    for node in supergraph.predecessors(main_dispatcher_node):
        if node.addr != prologue_node.addr:
            pre_dispatcher_node = node
            break
	# 找到真实块和预处理块（虚假块）
    relevant_nodes, nop_nodes = get_relevant_nop_nodes(
        supergraph, pre_dispatcher_node, prologue_node, retn_node)
    print('*******************relevant blocks************************')
    print('prologue: %#x' % start)
    print('main_dispatcher: %#x' % main_dispatcher_node.addr)
    print('pre_dispatcher: %#x' % pre_dispatcher_node.addr)
    print('retn: %#x' % retn_node.addr)
    relevant_block_addrs = [node.addr for node in relevant_nodes]
    print('relevant_blocks:', [hex(addr) for addr in relevant_block_addrs])
	# 打印相关块信息
    
    print('*******************symbolic execution*********************')
    relevants = relevant_nodes
    relevants.append(prologue_node)
    relevants_without_retn = list(relevants)
    relevants.append(retn_node)
    relevant_block_addrs.extend([prologue_node.addr, retn_node.addr])
	
    flow = defaultdict(list)
    patch_instrs = {}
    for relevant in relevants_without_retn:
        print('-------------------dse %#x---------------------' % relevant.addr)
        block = project.factory.block(relevant.addr, size=relevant.size)
        has_branches = False
        hook_addrs = set([])
        for ins in block.capstone.insns:
            # 不同架构的区分
            if project.arch.name in ARCH_X86:
                # 判断该真实块是否存在分支
                if ins.insn.mnemonic.startswith('cmov'):
                    # only record the first one
                    if relevant not in patch_instrs:
                        patch_instrs[relevant] = ins
                        has_branches = True
                # call指令对angr没影响，直接hook
                elif ins.insn.mnemonic.startswith('call'):
                    hook_addrs.add(ins.insn.address)
            elif project.arch.name in ARCH_ARM:
                if ins.insn.mnemonic != 'mov' and ins.insn.mnemonic.startswith('mov'):
                    if relevant not in patch_instrs:
                        patch_instrs[relevant] = ins
                        has_branches = True
                elif ins.insn.mnemonic in {'bl', 'blx'}:
                    hook_addrs.add(ins.insn.address)
            elif project.arch.name in ARCH_ARM64:
                if ins.insn.mnemonic.startswith('cset'):
                    if relevant not in patch_instrs:
                        patch_instrs[relevant] = ins
                        has_branches = True
                elif ins.insn.mnemonic in {'bl', 'blr'}:
                    hook_addrs.add(ins.insn.address)
                    
        # angr符号执行，对于两个分支的进行模拟，获取后续节点  
        if has_branches:
            tmp_addr = symbolic_execution(project, relevant_block_addrs,
                                                     relevant.addr, hook_addrs, claripy.BVV(1, 1), True)
            if tmp_addr is not None:
                flow[relevant].append(tmp_addr)
            tmp_addr = symbolic_execution(project, relevant_block_addrs,
                                                     relevant.addr, hook_addrs, claripy.BVV(0, 1), True)
            if tmp_addr is not None:
                flow[relevant].append(tmp_addr)
        else:
            tmp_addr = symbolic_execution(project, relevant_block_addrs,
                                                     relevant.addr, hook_addrs)
            if tmp_addr is not None:
                flow[relevant].append(tmp_addr)

    print('************************flow******************************')
    for k, v in flow.items():
        print('%#x: ' % k.addr, [hex(child) for child in v])

    print('%#x: ' % retn_node.addr, [])

    print('************************patch*****************************')
    with open(filename, 'rb') as origin:
        # Attention: can't transform to str by calling decode() directly. so use bytearray instead.
        origin_data = bytearray(origin.read())
        origin_data_len = len(origin_data)
        # 打开文件，记录长度

    recovery_file = filename + '_recovered'
    recovery = open(recovery_file, 'wb')
    # 重命名为_recovered文件并重新打开

    # patch irrelevant blocks
    for nop_node in nop_nodes:
        fill_nop(origin_data, nop_node.addr-base_addr,
                 nop_node.size, project.arch)
    # 如果在nop列表中全nop了

    # remove unnecessary control flows
    for parent, childs in flow.items():
        if len(childs) == 1:
            # 子节点为1的操作
            parent_block = project.factory.block(parent.addr, size=parent.size)
            last_instr = parent_block.capstone.insns[-1]
            file_offset = last_instr.address - base_addr
            # patch the last instruction to jmp
            if project.arch.name in ARCH_X86:
                fill_nop(origin_data, file_offset,
                         last_instr.size, project.arch)
                patch_value = ins_j_jmp_hex_x86(last_instr.address, childs[0], 'jmp')
            elif project.arch.name in ARCH_ARM:
                patch_value = ins_b_jmp_hex_arm(last_instr.address, childs[0], 'b')
                if project.arch.memory_endness == "Iend_BE":
                    patch_value = patch_value[::-1]
            elif project.arch.name in ARCH_ARM64:
                # FIXME: For aarch64/arm64, the last instruction of prologue seems useful in some cases, so patch the next instruction instead.
                if parent.addr == start:
                    file_offset += 4
                    patch_value = ins_b_jmp_hex_arm64(last_instr.address+4, childs[0], 'b')
                else:
                    patch_value = ins_b_jmp_hex_arm64(last_instr.address, childs[0], 'b')
                if project.arch.memory_endness == "Iend_BE":
                    patch_value = patch_value[::-1]
            patch_instruction(origin_data, file_offset, patch_value)
            # 针对不同架构将父节点的最后一条指令修补为跳转至子节点的跳转指令
        else:
            # 子节点多个的操作
            instr = patch_instrs[parent]
            file_offset = instr.address - base_addr
            # patch instructions starting from `cmovx` to the end of block
            fill_nop(origin_data, file_offset, parent.addr +
                     parent.size - base_addr - file_offset, project.arch)
            if project.arch.name in ARCH_X86:
                # patch the cmovx instruction to jx instruction
                patch_value = ins_j_jmp_hex_x86(instr.address, childs[0], instr.mnemonic[len('cmov'):])
                patch_instruction(origin_data, file_offset, patch_value)

                file_offset += 6
                # patch the next instruction to jmp instrcution
                patch_value = ins_j_jmp_hex_x86(instr.address+6, childs[1], 'jmp')
                patch_instruction(origin_data, file_offset, patch_value)
            elif project.arch.name in ARCH_ARM:
                # patch the movx instruction to bx instruction
                bx_cond = 'b' + instr.mnemonic[len('mov'):]
                patch_value = ins_b_jmp_hex_arm(instr.address, childs[0], bx_cond)
                if project.arch.memory_endness == 'Iend_BE':
                    patch_value = patch_value[::-1]
                patch_instruction(origin_data, file_offset, patch_value)

                file_offset += 4
                # patch the next instruction to b instrcution
                patch_value = ins_b_jmp_hex_arm(instr.address+4, childs[1], 'b')
                if project.arch.memory_endness == 'Iend_BE':
                    patch_value = patch_value[::-1]
                patch_instruction(origin_data, file_offset, patch_value)
            elif project.arch.name in ARCH_ARM64:
                # patch the cset.xx instruction to bx instruction
                bx_cond = instr.op_str.split(',')[-1].strip()
                patch_value = ins_b_jmp_hex_arm64(instr.address, childs[0], bx_cond)
                if project.arch.memory_endness == 'Iend_BE':
                    patch_value = patch_value[::-1]
                patch_instruction(origin_data, file_offset, patch_value)

                file_offset += 4
                # patch the next instruction to b instruction
                patch_value = ins_b_jmp_hex_arm64(instr.address+4, childs[1], 'b')
                if project.arch.memory_endness == 'Iend_BE':
                    patch_value = patch_value[::-1]
                patch_instruction(origin_data, file_offset, patch_value)
                #如果存在多个子节点，则需要进行更复杂的修补。对于x86架构，需要修改条件传送指令（如cmovx）为条件跳转指令（如jx）。对于ARM和ARM64架构，需要修改指令为条件跳转指令（如bx、b）

    assert len(origin_data) == origin_data_len, "Error: size of data changed!!!"
    #确保修复前后长度相同
    recovery.write(origin_data)
    recovery.close()
    print('Successful! The recovered file: %s' % recovery_file)

获取与控制流相关的NOP节点

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def get_relevant_nop_nodes(supergraph, pre_dispatcher_node, prologue_node, retn_node):
    # relevant_nodes = list(supergraph.predecessors(pre_dispatcher_node))
    relevant_nodes = []
    nop_nodes = []
    for node in supergraph.nodes():
        #使用块字节大小，来判断是否为真实块
        if supergraph.has_edge(node, pre_dispatcher_node) and node.size > 8:
            # XXX: use node.size is faster than to create a block
            relevant_nodes.append(node)
            continue
        #排除序言，返回块，预处理器
        if node.addr in (prologue_node.addr, retn_node.addr, pre_dispatcher_node.addr):
            continue
        nop_nodes.append(node)
        #其余都加入nop
    return relevant_nodes, nop_nodes

在给定的起始地址处执行符号执行

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def symbolic_execution(project, relevant_block_addrs, start_addr, hook_addrs=None, modify_value=None, inspect=False):
    
    #从状态中获取当前指令指针的值，并解除指定地址的挂钩
    def retn_procedure(state):
        ip = state.solver.eval(state.regs.ip)
        project.unhook(ip)
        return
    # 检查语句
    def statement_inspect(state):
        expressions = list(
            state.scratch.irsb.statements[state.inspect.statement].expressions)
        if len(expressions) != 0 and isinstance(expressions[0], pyvex.expr.ITE):
            state.scratch.temps[expressions[0].cond.tmp] = modify_value
            state.inspect._breakpoints['statement'] = []
	# 检查挂钩
    if hook_addrs is not None:
        skip_length = 4
        if project.arch.name in ARCH_X86:
            skip_length = 5

        for hook_addr in hook_addrs:
            project.hook(hook_addr, retn_procedure, length=skip_length)

    state = project.factory.blank_state(addr=start_addr, remove_options={
                                        angr.sim_options.LAZY_SOLVES})
    if inspect:
        state.inspect.b(
            'statement', when=angr.state_plugins.inspect.BP_BEFORE, action=statement_inspect)
    sm = project.factory.simulation_manager(state)
    sm.step()
    while len(sm.active) > 0:
        for active_state in sm.active:
            if active_state.addr in relevant_block_addrs:
                return active_state.addr
        sm.step()

    return None

原文作者：Straw

原文链接：https://straw-233.github.io/2024/03/27/%E5%8F%8D%E6%B7%B7%E6%B7%86%E5%AD%A6%E4%B9%A0-fla%E7%AF%87/

发表日期：March 27th 2024, 9:23:15 pm

更新日期：March 27th 2024, 9:43:28 pm

版权声明：本文采用知识共享署名-非商业性使用 4.0 国际许可协议进行许可

CATALOG
  1. 1. 反混淆学习-fla控制流平坦化篇
    1. 1.1. deflat工具介绍
    2. 1.2. 流程
    3. 1.3. 代码分析
      1. 1.3.1. 主函数
      2. 1.3.2. 获取与控制流相关的NOP节点
      3. 1.3.3. 在给定的起始地址处执行符号执行
Total : 23
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