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Android函数抽取壳

luoyesiqiu 人气:0

0x0 前言

函数抽取壳这个词不知道从哪起源的,但我理解的函数抽取壳是那种将dex文件中的函数代码给nop,然后在运行时再把字节码给填回dex的这么一种壳。

函数抽取前:

函数抽取后:

很早之前就想写这类的壳,最近终于把它做出来了,取名为dpt。现在将代码分享出来,欢迎把玩。项目地址:https://github.com/luoyesiqiu/dpt-shell

0x1 项目的结构

dpt代码分为两个部分,一个是proccessor,另一个是shell。

proccessor是可以将普通apk处理成加壳apk的模块。它的主要功能有:

流程如下:

shell模块最终生成的dex文件和so文件将被集成到需要加壳的apk中。它的要功能有:

流程如下:

0x2 proccessor

proccessor比较重要的逻辑两点,AndroidManiest.xml的处理和Codeitem的提取

(1)处理Androidmanifest.xml

我们处理AndroidManifest.xml的操作主要是备份原Application的类名和写入壳的代理Application的类名。备份原Application类名目的是在壳的流程执行完成后,调用我们原APK的Application。写入壳的代理Application类名的目的是在app启动时尽早的启动我们的代理Application,这样我们就可以做一些准备工作,比如自定义加载dex,Hook一些函数等。我们知道,AndroidManifest.xml在生成apk后它不是以普通xml文件的格式来存放的,而是以axml格式来存放的。不过幸运的是,已经有许多大佬写了对axml解析和编辑的库,我们直接拿来用就行。这里用到的axml处理的库是ManifestEditor

提取原Androidmanifest.xml Application完整类名代码如下,直接调用getApplicationName函数即可

 public static String getValue(String file,String tag,String ns,String attrName){
        byte[] axmlData = IoUtils.readFile(file);
        AxmlParser axmlParser = new AxmlParser(axmlData);
        try {
            while (axmlParser.next() != AxmlParser.END_FILE) {
                if (axmlParser.getAttrCount() != 0 && !axmlParser.getName().equals(tag)) {
                    continue;
                }
                for (int i = 0; i < axmlParser.getAttrCount(); i++) {
                    if (axmlParser.getNamespacePrefix().equals(ns) && axmlParser.getAttrName(i).equals(attrName)) {
                        return (String) axmlParser.getAttrValue(i);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    public static String getApplicationName(String file) {
        return getValue(file,"application","android","name");
    }

写入Application类名的代码如下:

 public static void writeApplicationName(String inManifestFile, String outManifestFile, String newApplicationName){
        ModificationProperty property = new ModificationProperty();
        property.addApplicationAttribute(new AttributeItem(NodeValue.Application.NAME,newApplicationName));

        FileProcesser.processManifestFile(inManifestFile, outManifestFile, property);

    }

(2) 提取CodeItem

CodeItem是dex文件中存放函数字节码相关数据的结构。下图显示的就是CodeItem大概的样子。

说是提取CodeItem,其实我们提取的是CodeItem中的insns,它里面存放的是函数真正的字节码。提取insns,我们使用的是Android源码中的dx工具,使用dx工具可以很方便的读取dex文件的各个部分。

下面的代码遍历所有ClassDef,并遍历其中的所有函数,再调用extractMethod对单个函数进行处理。

public static List<Instruction> extractAllMethods(File dexFile, File outDexFile) {
        List<Instruction> instructionList = new ArrayList<>();
        Dex dex = null;
        RandomAccessFile randomAccessFile = null;
        byte[] dexData = IoUtils.readFile(dexFile.getAbsolutePath());
        IoUtils.writeFile(outDexFile.getAbsolutePath(),dexData);

        try {
            dex = new Dex(dexFile);
            randomAccessFile = new RandomAccessFile(outDexFile, "rw");
            Iterable<ClassDef> classDefs = dex.classDefs();
            for (ClassDef classDef : classDefs) {
                
                ......
                
                if(classDef.getClassDataOffset() == 0){
                    String log = String.format("class '%s' data offset is zero",classDef.toString());
                    logger.warn(log);
                    continue;
                }

                ClassData classData = dex.readClassData(classDef);
                ClassData.Method[] directMethods = classData.getDirectMethods();
                ClassData.Method[] virtualMethods = classData.getVirtualMethods();
                for (ClassData.Method method : directMethods) {
                    Instruction instruction = extractMethod(dex,randomAccessFile,classDef,method);
                    if(instruction != null) {
                        instructionList.add(instruction);
                    }
                }

                for (ClassData.Method method : virtualMethods) {
                    Instruction instruction = extractMethod(dex, randomAccessFile,classDef, method);
                    if(instruction != null) {
                        instructionList.add(instruction);
                    }
                }
            }
        }
        catch (Exception e){
            e.printStackTrace();
        }
        finally {
            IoUtils.close(randomAccessFile);
        }

        return instructionList;
    }

处理函数的过程中发现没有代码(通常为native函数)或者insns的容量不足以填充return语句则跳过处理。这里就是对应函数抽取壳的抽取操作

private static Instruction extractMethod(Dex dex ,RandomAccessFile outRandomAccessFile,ClassDef classDef,ClassData.Method method)
            throws Exception{
        String returnTypeName = dex.typeNames().get(dex.protoIds().get(dex.methodIds().get(method.getMethodIndex()).getProtoIndex()).getReturnTypeIndex());
        String methodName = dex.strings().get(dex.methodIds().get(method.getMethodIndex()).getNameIndex());
        String className = dex.typeNames().get(classDef.getTypeIndex());
        //native函数
        if(method.getCodeOffset() == 0){
            String log = String.format("method code offset is zero,name =  %s.%s , returnType = %s",
                    TypeUtils.getHumanizeTypeName(className),
                    methodName,
                    TypeUtils.getHumanizeTypeName(returnTypeName));
            logger.warn(log);
            return null;
        }
        Instruction instruction = new Instruction();
        //16 = registers_size + ins_size + outs_size + tries_size + debug_info_off + insns_size
        int insnsOffset = method.getCodeOffset() + 16;
        Code code = dex.readCode(method);
        //容错处理
        if(code.getInstructions().length == 0){
            String log = String.format("method has no code,name =  %s.%s , returnType = %s",
                    TypeUtils.getHumanizeTypeName(className),
                    methodName,
                    TypeUtils.getHumanizeTypeName(returnTypeName));
            logger.warn(log);
            return null;
        }
        int insnsCapacity = code.getInstructions().length;
        //insns容量不足以存放return语句,跳过
        byte[] returnByteCodes = getReturnByteCodes(returnTypeName);
        if(insnsCapacity * 2 < returnByteCodes.length){
            logger.warn("The capacity of insns is not enough to store the return statement. {}.{}() -> {} insnsCapacity = {}byte(s),returnByteCodes = {}byte(s)",
                    TypeUtils.getHumanizeTypeName(className),
                    methodName,
                    TypeUtils.getHumanizeTypeName(returnTypeName),
                    insnsCapacity * 2,
                    returnByteCodes.length);

            return null;
        }
        instruction.setOffsetOfDex(insnsOffset);
        //这里的MethodIndex对应method_ids区的索引
        instruction.setMethodIndex(method.getMethodIndex());
        //注意:这里是数组的大小
        instruction.setInstructionDataSize(insnsCapacity * 2);
        byte[] byteCode = new byte[insnsCapacity * 2];
        //写入nop指令
        for (int i = 0; i < insnsCapacity; i++) {
            outRandomAccessFile.seek(insnsOffset + (i * 2));
            byteCode[i * 2] = outRandomAccessFile.readByte();
            byteCode[i * 2 + 1] = outRandomAccessFile.readByte();
            outRandomAccessFile.seek(insnsOffset + (i * 2));
            outRandomAccessFile.writeShort(0);
        }
        instruction.setInstructionsData(byteCode);
        outRandomAccessFile.seek(insnsOffset);
        //写出return语句
        outRandomAccessFile.write(returnByteCodes);

        return instruction;
    }

0x3 shell模块

shell模块是函数抽取壳的主要逻辑,它的功能我们上面已经讲过。

(1) Hook函数

Hook函数时机最好要早点,dpt在_init函数中开始进行一系列HOOK

extern "C" void _init(void) {
    dpt_hook();
}

Hook框架使用的Dobby,主要Hook两个函数:MapFileAtAddress和LoadMethod。

Hook MapFileAtAddress函数的目的是在我们加载dex能够修改dex的属性,让加载的dex可写,这样我们才能把字节码填回dex,有大佬详细的分析过,具体参考这篇文章

void* MapFileAtAddressAddr = DobbySymbolResolver(GetArtLibPath(),MapFileAtAddress_Sym());
DobbyHook(MapFileAtAddressAddr, (void *) MapFileAtAddress28,(void **) &g_originMapFileAtAddress28);

Hook到了之后,给prot参数追加PROT_WRITE属性

void* MapFileAtAddress28(uint8_t* expected_ptr,
              size_t byte_count,
              int prot,
              int flags,
              int fd,
              off_t start,
              bool low_4gb,
              bool reuse,
              const char* filename,
              std::string* error_msg){
    int new_prot = (prot | PROT_WRITE);
    if(nullptr != g_originMapFileAtAddress28) {
        return g_originMapFileAtAddress28(expected_ptr,byte_count,new_prot,flags,fd,start,low_4gb,reuse,filename,error_msg);
    }
}

在Hook LoadMethod函数之前,我们需要了解LoadMethod函数流程。为什么是这个LoadMethod函数,其他函数是否可行?

当一个类被加载的时候,它的调用链是这样的(部分流程已省略):

ClassLoader.java::loadClass -> DexPathList.java::findClass -> DexFile.java::defineClass -> class_linker.cc::LoadClass -> class_linker.cc::LoadClassMembers -> class_linker.cc::LoadMethod

也就是说,当一个类被加载,它是会去调用LoadMethod函数的,我们看一下它的函数原型:

void ClassLinker::LoadMethod(const DexFile& dex_file,
                             const ClassDataItemIterator& it,
                             Handle<mirror::Class> klass,
                             ArtMethod* dst);

这个函数太爆炸了,它有两个爆炸性的参数,DexFile和ClassDataItemIterator,我们可以从这个函数得到当前加载函数所在的DexFile结构和当前函数的一些信息,可以看一下ClassDataItemIterator结构:

class ClassDataItemIterator{
  
  ......
  
  // A decoded version of the method of a class_data_item
  struct ClassDataMethod {
    uint32_t method_idx_delta_;  // delta of index into the method_ids array for MethodId
    uint32_t access_flags_;
    uint32_t code_off_;
    ClassDataMethod() : method_idx_delta_(0), access_flags_(0), code_off_(0) {}

   private:
    DISALLOW_COPY_AND_ASSIGN(ClassDataMethod);
  };
  ClassDataMethod method_;

  // Read and decode a method from a class_data_item stream into method
  void ReadClassDataMethod();

  const DexFile& dex_file_;
  size_t pos_;  // integral number of items passed
  const uint8_t* ptr_pos_;  // pointer into stream of class_data_item
  uint32_t last_idx_;  // last read field or method index to apply delta to
  DISALLOW_IMPLICIT_CONSTRUCTORS(ClassDataItemIterator);
};

其中最重要的字段就是code_off_它的值是当前加载的函数的CodeItem相对于DexFile的偏移,当相应的函数被加载,我们就可以直接访问到它的CodeItem。其他函数是否也可以?在上面的流程中没有比LoadMethod更适合我们Hook的函数,所以它是最佳的Hook点。

Hook LoadMethod稍微复杂一些,倒不是Hook代码复杂,而是Hook触发后处理的代码比较复杂,我们要适配多个Android版本,每个版本LoadMethod函数的参数都可能有改变,幸运的是,LoadMethod改动也不是很大。那么,我们如何读取ClassDataItemIterator类中的code_off_呢?比较直接的做法是计算偏移,然后在代码中维护一份偏移。不过这样的做法不易阅读很容易出错。dpt的做法是把ClassDataItemIterator类拷过来,然后将ClassDataItemIterator引用直接转换为我们自定义的ClassDataItemIterator引用,这样就可以方便的读取字段的值。

下面是LoadMethod被调用后做的操作,逻辑是读取存在map中的insns,然后将它们填回指定位置。

void LoadMethod(void *thiz, void *self, const void *dex_file, const void *it, const void *method,
                void *klass, void *dst) {

    if (g_originLoadMethod25 != nullptr
        || g_originLoadMethod28 != nullptr
        || g_originLoadMethod29 != nullptr) {
        uint32_t location_offset = getDexFileLocationOffset();
        uint32_t begin_offset = getDataItemCodeItemOffset();
        callOriginLoadMethod(thiz, self, dex_file, it, method, klass, dst);

        ClassDataItemReader *classDataItemReader = getClassDataItemReader(it,method);


        uint8_t **begin_ptr = (uint8_t **) ((uint8_t *) dex_file + begin_offset);
        uint8_t *begin = *begin_ptr;
        // vtable(4|8) + prev_fields_size
        std::string *location = (reinterpret_cast<std::string *>((uint8_t *) dex_file +
                                                                 location_offset));
        if (location->find("base.apk") != std::string::npos) {

            //code_item_offset == 0说明是native方法或者没有代码
            if (classDataItemReader->GetMethodCodeItemOffset() == 0) {
                DLOGW("native method? = %s code_item_offset = 0x%x",
                      classDataItemReader->MemberIsNative() ? "true" : "false",
                      classDataItemReader->GetMethodCodeItemOffset());
                return;
            }

            uint16_t firstDvmCode = *((uint16_t*)(begin + classDataItemReader->GetMethodCodeItemOffset() + 16));
            if(firstDvmCode != 0x0012 && firstDvmCode != 0x0016 && firstDvmCode != 0x000e){
                NLOG("this method has code no need to patch");
                return;
            }

            uint32_t dexSize = *((uint32_t*)(begin + 0x20));

            int dexIndex = dexNumber(location);
            auto dexIt = dexMap.find(dexIndex - 1);
            if (dexIt != dexMap.end()) {

                auto dexMemIt = dexMemMap.find(dexIndex);
                if(dexMemIt == dexMemMap.end()){
                    changeDexProtect(begin,location->c_str(),dexSize,dexIndex);
                }


                auto codeItemMap = dexIt->second;
                int methodIdx = classDataItemReader->GetMemberIndex();
                auto codeItemIt = codeItemMap->find(methodIdx);

                if (codeItemIt != codeItemMap->end()) {
                    CodeItem* codeItem = codeItemIt->second;
                    uint8_t  *realCodeItemPtr = (uint8_t*)(begin +
                                                classDataItemReader->GetMethodCodeItemOffset() +
                                                16);

                    memcpy(realCodeItemPtr,codeItem->getInsns(),codeItem->getInsnsSize());
                }
            }
        }
    }
}

(2) 加载dex

其实dex在App启动的时候已经被加载过一次了,但是,我们为什么还要再加载一次?因为系统加载的dex是以只读方式加载的,我们没办法去修改那一部分的内存。而且App的dex加载早于我们Application的启动,这样,我们在代码根本没法感知到,所以我们要重新加载dex。

 private ClassLoader loadDex(Context context){
        String sourcePath = context.getApplicationInfo().sourceDir;
        String nativePath = context.getApplicationInfo().nativeLibraryDir;

        ShellClassLoader shellClassLoader = new ShellClassLoader(sourcePath,nativePath,ClassLoader.getSystemClassLoader());
        return shellClassLoader;
    }

自定义的ClassLoader

public class ShellClassLoader extends PathClassLoader {

    private final String TAG = ShellClassLoader.class.getSimpleName();

    public ShellClassLoader(String dexPath,ClassLoader classLoader) {
        super(dexPath,classLoader);
    }

    public ShellClassLoader(String dexPath, String librarySearchPath,ClassLoader classLoader) {
        super(dexPath, librarySearchPath, classLoader);
    }
}

(3) 替换dexElements

这一步也非常重要,这一步的目的是使ClassLoader从我们新加载的dex文件中加载类。代码如下:

void mergeDexElements(JNIEnv* env,jclass klass,jobject oldClassLoader,jobject newClassLoader){
    jclass BaseDexClassLoaderClass = env->FindClass("dalvik/system/BaseDexClassLoader");
    jfieldID  pathList = env->GetFieldID(BaseDexClassLoaderClass,"pathList","Ldalvik/system/DexPathList;");
    jobject oldDexPathListObj = env->GetObjectField(oldClassLoader,pathList);
    if(env->ExceptionCheck() || nullptr == oldDexPathListObj ){
        env->ExceptionClear();
        DLOGW("mergeDexElements oldDexPathListObj get fail");
        return;
    }
    jobject newDexPathListObj = env->GetObjectField(newClassLoader,pathList);
    if(env->ExceptionCheck() || nullptr == newDexPathListObj){
        env->ExceptionClear();
        DLOGW("mergeDexElements newDexPathListObj get fail");
        return;
    }
    jclass DexPathListClass = env->FindClass("dalvik/system/DexPathList");
    jfieldID  dexElementField = env->GetFieldID(DexPathListClass,"dexElements","[Ldalvik/system/DexPathList$Element;");
    jobjectArray newClassLoaderDexElements = static_cast<jobjectArray>(env->GetObjectField(
            newDexPathListObj, dexElementField));
    if(env->ExceptionCheck() || nullptr == newClassLoaderDexElements){
        env->ExceptionClear();
        DLOGW("mergeDexElements new dexElements get fail");
        return;
    }
    jobjectArray oldClassLoaderDexElements = static_cast<jobjectArray>(env->GetObjectField(
            oldDexPathListObj, dexElementField));
    if(env->ExceptionCheck() || nullptr == oldClassLoaderDexElements){
        env->ExceptionClear();
        DLOGW("mergeDexElements old dexElements get fail");
        return;
    }
    jint oldLen = env->GetArrayLength(oldClassLoaderDexElements);
    jint newLen = env->GetArrayLength(newClassLoaderDexElements);
    DLOGD("mergeDexElements oldlen = %d , newlen = %d",oldLen,newLen);
    jclass ElementClass = env->FindClass("dalvik/system/DexPathList$Element");
    jobjectArray  newElementArray = env->NewObjectArray(oldLen + newLen,ElementClass, nullptr);
    for(int i = 0;i < newLen;i++) {
        jobject elementObj = env->GetObjectArrayElement(newClassLoaderDexElements, i);
        env->SetObjectArrayElement(newElementArray,i,elementObj);
    }
    for(int i = newLen;i < oldLen + newLen;i++) {
        jobject elementObj = env->GetObjectArrayElement(oldClassLoaderDexElements, i - newLen);
        env->SetObjectArrayElement(newElementArray,i,elementObj);
    }
    env->SetObjectField(oldDexPathListObj, dexElementField,newElementArray);
    DLOGD("mergeDexElements success");
}

0x4 总结

做这个壳确实花了不少的时间,其中走过的弯路只有自己知道,不过还好做出来了。dpt未经过大量测试,后续发现问题再慢慢解决。

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