android so壳入口浅析

android so壳入口浅析

转自：.html

前言

  开年来开始接触一些加固样本，基本都对了so进行了处理，拖入ida一看，要么没有 JNI_OnLoad ,要么 JNI_OnLoad 汇编代码羞涩难懂，让人无法下手。 JNI_OnLoad 是真正入口么？

先看看几个文档

1 摘自属性服务一节（《深入理解Android卷1》）

 利用gcc的constructor属性，这个属性指明了一个__libc_prenit函数(这个函数内部就将完成共享内存到本地进程的映射工作)。用法：当bionic libc库被加载时，将自动调用__libc_prenit函数。这样在bionic libc动态库被装载时，系统属性缓冲区地址就被确定了，后续的API调用就能找对位置了。/* We flag the __libc_preinit function as a constructor to ensure * that its address is listed in libc.so's .init_array section. * This ensures that the function is called by the dynamic linker * as soon as the shared library is loaded. */ //constructor属性指示加载器加载该库之后，首先调用__libc_prenit函数。这一点和windows上的动态库的DllMain函数类似
void __attribute__((constructor)) __libc_prenit(void);

从英文说明里面提到到.init_array section，我们可以搜索一下这一节的说明

2 .init_array section

.init_array contains pointers to blocks of code that need to be executed when an application is being initialized (before main() is called). It is used for a number of things, but the primary use is in C++ for running static constructors; a secondary use that is sometimes used is to initialize IO systems in the C library.
If you are not using C++ you may (depending on your C library) be able to live without it entirely; but you’d need to hack your startup code to deal with this.
.init_array probably ends up in ram because its marked read/write — that happens because in a dynamic linking environment the dynamic linker has to fix up all the pointers it contains before it can be used. In a static environment you might be able to get away with forcing it into a read-only section.
来源： <.html>

3 摘自dlopen小结(《程序员的自我修养》)

动态连接器在加载模块时，会执行".init"段的代码,用以完成模块的初始化工作，dlopen的加载过程基本跟动态连接器一致，在完成装载、映射和重定向以后，就会执行".init"段的代码然后返回

看完这个3段资料，我们可以知道在系统加载so，在完成装载、映射和重定向以后，就首先执行.init和.init_array段的代码.

探本溯源，在源码中追踪

我们先从System.loadLibrary ->Runtime.loadLibrary

  public void loadLibrary(String libName) {loadLibrary(libName, CallingClassLoader());}/** Loads and links a library without security checks.*/void loadLibrary(String libraryName, ClassLoader loader) {代码略...String error = nativeLoad(filename, loader);       代码略...}

-> nativeLoad

static void Dalvik_java_lang_Runtime_nativeLoad(const u4* args,JValue* pResult)
{代码略...StringObject* fileNameObj = (StringObject*) args[0];success = dvmLoadNativeCode(fileName, classLoader, &reason);代码略...
}

来源： <.1.2/xref/dalvik/vm/native/java_lang_Runtime.cpp#72>

->dvmLoadNativeCode

bool dvmLoadNativeCode(const char* pathName, Object* classLoader,char** detail)
{代码略...handle = dlopen(pathName, RTLD_LAZY);代码略...vonLoad = dlsym(handle, "JNI_OnLoad");if (vonLoad == NULL) {ALOGD("No JNI_OnLoad found in %s %p, skipping init",pathName, classLoader);} else {/** Call JNI_OnLoad.  We have to override the current class* loader, which will always be "null" since the stuff at the* top of the stack is around Runtime.loadLibrary().  (See* the comments in the JNI FindClass function.)*/OnLoadFunc func = (OnLoadFunc)vonLoad;Object* prevOverride = self->classLoaderOverride;self->classLoaderOverride = classLoader;oldStatus = dvmChangeStatus(self, THREAD_NATIVE);if (gDvm.verboseJni) {ALOGI("[Calling JNI_OnLoad for "%s"]", pathName);}version = (*func)(gDvmJni.jniVm, NULL);dvmChangeStatus(self, oldStatus);self->classLoaderOverride = prevOverride;}代码略...
}

来源： <.1.2/xref/dalvik/vm/Native.cpp#318>

通过dvmLoadNativeCode函数我们知道系统用dlopen加载so完成后，会查看有没有JNI_OnLoad函数，有的话就调用.

我们再到dlopen函数探个究竟:

void *dlopen(const char *filename, int flag)
{soinfo *ret;pthread_mutex_lock(&dl_lock);/*find_library 会判断so是否已经加载，如果没有加载，对so进行加载，完成一些初始化工作,有兴趣的读者可自行分析 */ret = find_library(filename);if (unlikely(ret == NULL)) {set_dlerror(DL_ERR_CANNOT_LOAD_LIBRARY);} else {call_constructors_recursive(ret);ret->refcount++;}pthread_mutex_unlock(&dl_lock);return ret;
}

->call_constructors_recursive

void call_constructors_recursive(soinfo *si)
{if (si->constructors_called)return;// Set this before actually calling the constructors, otherwise it doesn't// protect against recursive constructor calls. One simple example of// constructor recursion is the libc debug malloc, which is implemented in// libc_malloc_debug_leak.so:// 1. The program depends on libc, so libc's constructor is called here.// 2. The libc constructor calls dlopen() to load libc_malloc_debug_leak.so.// 3. dlopen() calls call_constructors_recursive() with the newly created//    soinfo for libc_malloc_debug_leak.so.// 4. The debug so depends on libc, so call_constructors_recursive() is//    called again with the libc soinfo. If it doesn't trigger the early-//    out above, the libc constructor will be called again (recursively!).si->constructors_called = 1;if (si->flags & FLAG_EXE) {TRACE("[ %5d Calling preinit_array @ 0x%08x [%d] for '%s' ]n",pid, (unsigned)si->preinit_array, si->preinit_array_count,si->name);call_array(si->preinit_array, si->preinit_array_count, 0);TRACE("[ %5d Done calling preinit_array for '%s' ]n", pid, si->name);} else {if (si->preinit_array) {DL_ERR("%5d Shared library '%s' has a preinit_array table @ 0x%08x."" This is INVALID.", pid, si->name,(unsigned)si->preinit_array);}}代码略...if (si->init_func) {TRACE("[ %5d Calling init_func @ 0x%08x for '%s' ]n", pid,(unsigned)si->init_func, si->name);si->init_func();TRACE("[ %5d Done calling init_func for '%s' ]n", pid, si->name);}if (si->init_array) {TRACE("[ %5d Calling init_array @ 0x%08x [%d] for '%s' ]n", pid,(unsigned)si->init_array, si->init_array_count, si->name);//遍历函数数组并执行call_array(si->init_array, si->init_array_count, 0);TRACE("[ %5d Done calling init_array for '%s' ]n", pid, si->name);//ps:看到这么多TRACE这么多调试信息，我们把调试开关打开，是不是能拿到诸多信息？
 }
}
来源： <.1.2/xref/bionic/linker/linker.c#1519>

通过可以函数我们知道si->init_func和si->init_array存在的时候，会执行指向的函数
(不知道大家注意到么si->flags & FLAG_EXE时，还有si->preinit_array？ 以后会不会有这方面的东西？)
再找下 si->init_func和si->init_array 的赋值

 case DT_INIT:si->init_func = (void (*)(void))(si->base + *d);DEBUG("%5d %s constructors (init func) found at %pn",pid, si->name, si->init_func);case DT_INIT_ARRAY:si->init_array = (unsigned *)(si->base + *d);DEBUG("%5d %s constructors (init_array) found at %pn",pid, si->name, si->init_array);break;

DEBUG里面说明了constructors (init func)和constructors (init_array)。
  我们再看看一份文档Android Dynamic Linker Design Notes

DT_INITPoints to the address of an initialization functionthat must be called when the file is loaded.
DT_INIT_ARRAYPoints to an array of function addresses that must becalled, in-order, to perform initialization. Some ofthe entries in the array can be 0 or -1, and shouldbe ignored.Note: this is generally stored in a .init_array section

  通过层层分析，我们很清楚知道了系统加载so，在完成装载、映射和重定向以后，就首先执行.init和.init_array段的代码.

前面有一篇文章我已经对so加壳进行简单说明

把源码的dlopen复制出来修改，在把自己so加载起来的时候 ,把自己内存里面某部分地址解密后,用自己的dlopen打开返回一个soinfo结构体 然后把当前soinfo结构体替换原来的soinfo结构体 

小结

  系统加载so，在完成装载、映射和重定向以后，就首先执行.init和.init_array段的代码，之后如果存在JNI_OnLoad 就调用该函数.我们要对一个so进行分析，需要先看看有没有.init_array section和.init section，so加壳一般会在初始化函数进行脱壳操作。

如何在.init和.init_array段添加我们的函数

[1] 共享构造函数，在函数声明时加上"__attribute__((constructor))"属性void __attribute__((constructor)) init_function(void);对应有共享虚构函数，在程序exit()或者dlclose()返回前执行void __attribute__((destructor)) fini_function(void);[2]c++ 静态构造函数

在.init和.init_array下断点

init_array  用ida可以看到，  可以对里面的函数数组下断点
init ida有时没识别出来,可用readelf查看入口点xxx@xx:~$ readelf -a '/home/xxx/桌面/libsecexe.so'0x00000010 (SYMBOLIC)                   0x00x0000000c (INIT)                       0x114010x00000019 (INIT_ARRAY)                 0x28ca40x0000001b (INIT_ARRAYSZ)               8 (bytes)0x0000001a (FINI_ARRAY)                 0x28cac0x0000001c (FINI_ARRAYSZ)               12 (bytes)0x00000004 (HASH)                       0xf4我们看到 INIT 入口为    0x11401
(ps:有时你在0x11401是数据，你需要make code，由于对齐关系，要从0x11401+1开始）
样本：梆梆 爱加密

参考： 
[1] 《深入理解Android卷1》
[2] 《程序员的自我修养-链接、装载与库》
[3] android linker 浅析
[4] Android Dynamic Linker Design Notes