Android so文件的简单加密

这篇文章主要内容是对于so文件进行简单的加密工作，针对Ida等静态分析工具的分析，一旦开始动态调试就应该很容易就可以dump出内存，直接修复了。

主要是两种思路，

• 对文件中指定的section加密，然后在运行时由.initarray进行解密；
• 对指定的函数进行加密，在运行时由.initarray进行解密。

两种不同的方法说到底也就是不同的View而已。

①基于链接视图，对指定的section进行加密工作。原理很简单，就是借助GCC 编译器的__attribute__关键字增加一个自定义的section，然后对增加的section进行加密，最后利用.initarray在so被加载时调用进行解密工作。

②基于执行视图，对指定的函数进行加密工作。同样也要利用的GCC编译器的__attribute__关键字来将解密代码放置于.initarray段，在so被加载时调用。

　　　　　　　　                     　基于链接视图

非常简单的程序，点击Button，然后调用jni层的getString()函数，返回一串字符串

JNIEXPORT jstring JNICALL getString(JNIEnv* env,jobject clazz){    LOGD("getString invoke");    return env->NewStringUTF("Hello_CC");}

 但是函数的声明要注意：

JNIEXPORT jstring JNICALL getString(JNIEnv*,jobject)__attribute__((section(".mytext")));

 将函数放置在”.mytext” section。

 这里Java 层和native层的映射关系是自己手动向虚拟机注册，对于JNI不太了解可以移步这篇博文Android JNI 初体验

static JNINativeMethod g_methods[] = {        {            "getString",            "()Ljava/lang/String;",            (void*)getString        }};

 

jint JNI_OnLoad(JavaVM *vm,void * reserved){    jint result = -1;    JNIEnv *env = NULL;    if(vm->GetEnv((void**)&env,JNI_VERSION_1_4) != JNI_OK)    {        result = -1;    }    //com.example.protectsection    jclass clazz = env->FindClass("com/example/protectsection/MainActivity");    if(clazz == NULL)    {        LOGD("find class failed");        return result;    }    if(env->RegisterNatives(clazz,g_methods,sizeof(g_methods)/sizeof(g_methods[0]))<0)    {        LOGD("register natives failed");        return result;    }    result  = JNI_VERSION_1_4;    return result;}

 然后就开始我们的加密之旅了，下面是加密程序，基于链接视图，找到目标名为”.mytext”的section，然后将该section内的代码进行简单的取反工作，最后写回文件中。

#include <stdio.h>#include <stdlib.h>#include <elf.h>#include <fcntl.h>int main(int argc,char ** argv){    char section_name[] = ".mytext";    Elf32_Ehdr ehdr;    Elf32_Shdr shdr;    char * ptr_shstrtab = NULL;    Elf32_Off    target_section_offset = 0;    Elf32_Word target_section_size = 0;    char * ptr_section_content = NULL;    int page_size = 4096;    int fd;    if(argc < 2)    {        puts("input so file\n");        return -1;    }    fd = open(argv[1],O_RDWR);    if(fd < 0)    {        goto _error;    }    if(read(fd,&ehdr,sizeof(Elf32_Ehdr)) != sizeof(Elf32_Ehdr))    {        puts("read elf header failed\n");        goto _error;    }    lseek(fd,ehdr.e_shoff+sizeof(Elf32_Shdr)*ehdr.e_shstrndx,SEEK_SET);    if(read(fd,&shdr,sizeof(Elf32_Shdr)) != sizeof(Elf32_Shdr))    {        puts("read elf .shstrtab header failed\n");        goto _error;    }    ptr_shstrtab = (char*)malloc(shdr.sh_size);    if(NULL == ptr_shstrtab)    {        puts("apply mem failed\n");        goto _error;    }    //read shstrtab    lseek(fd,shdr.sh_offset,SEEK_SET);    if(read (fd,ptr_shstrtab,shdr.sh_size) != shdr.sh_size)    {        goto _error;    }    lseek(fd,ehdr.e_shoff,SEEK_SET);    int i = 0;    for(i = 0;i < ehdr.e_shnum;i++)    {        if(read(fd,&shdr,sizeof(Elf32_Shdr)) != sizeof(Elf32_Shdr))        {            puts("find target section faile\nd");            goto _error;        }        if( !strcmp(ptr_shstrtab + shdr.sh_name , section_name) )        {            target_section_offset  = shdr.sh_offset;            target_section_size = shdr.sh_size;            break;        }    }    lseek(fd,target_section_offset,SEEK_SET);    ptr_section_content = (char*)malloc(target_section_size);    if(NULL ==ptr_section_content)    {        goto _error;    }    if(read(fd,ptr_section_content,target_section_size) != target_section_size)    {        goto _error;    }    int num_page = target_section_size/page_size + 1;    ehdr.e_entry =  target_section_size;    ehdr.e_shoff =  target_section_offset;    for(i = 0; i<target_section_size;i++)    {        ptr_section_content[i] =~ ptr_section_content[i];    }    lseek(fd,0,SEEK_SET);    if(write(fd,&ehdr,sizeof(Elf32_Ehdr)) != sizeof(Elf32_Ehdr))    {        goto _error;    }    lseek(fd , target_section_offset , SEEK_SET);    if(write(fd , ptr_section_content,target_section_size) != target_section_size)    {        goto _error;    }    puts("completed\n");_error:    if(NULL != ptr_section_content)    {        free(ptr_section_content);    }    if(NULL != ptr_shstrtab)    {        free(ptr_shstrtab);    }    if(NULL != fd)    {        close(fd);    }    return 0;}

 用ida查看的效果图：

 接下来就是运行时的解密工作,通过我们自己的init_getString函数在so初始化时对目标section进行解密:

void init_getString()__attribute__((constructor));  //initarray

 

 完整的init_getString函数如下：

void init_getString(){    //LOGD("Hello init_getString");    unsigned long lib_addr;    Elf32_Ehdr* ptr_ehdr = NULL;    Elf32_Shdr* ptr_shdr = NULL;    unsigned long mytext_addr;    unsigned long mytext_size = 0;    unsigned long page_size = 0x1000;    lib_addr = get_cur_lib_addr();    if(NULL == lib_addr)    {        return ;    }    ptr_ehdr = (Elf32_Ehdr*)lib_addr;    mytext_size = ptr_ehdr->e_entry; //size    mytext_addr = ptr_ehdr->e_shoff + lib_addr;  //offset    unsigned long offset = mytext_addr % page_size;    LOGD("invoke mprotect first");    if(mprotect((const void*)(mytext_addr-offset) , mytext_size,PROT_READ | PROT_WRITE | PROT_EXEC) != 0)    {        LOGD("change the mem failed");        return ;    }    for(int i=0;i<mytext_size;i++)    {        ((char*)mytext_addr)[i] =~ ((char*)mytext_addr)[i];    }    LOGD("invoke mprotect to resume the page");    if(mprotect((const void*)(mytext_addr-offset),mytext_size,PROT_READ | PROT_EXEC)!=0)    {        LOGD("resume mem failed");        return ;    }}

 

 获得自己的so模块在内存中的加载基地址是通过linux的“/proc”文件系统得到的，进程的运行时的状态都会显示在”/proc”文件系统中。例如下图就是某个进程的内存中模块的的信息。

 

unsigned long get_cur_lib_addr(){    unsigned long addr = 0;    char section_name[] = "libprotect_section.so";     pid_t pid = 0;     //<types.h>     char buf[1024] = {0};     FILE* fp = NULL;     char* tmp = NULL;    pid = getpid();   //<unistd.h>    sprintf(buf,"/proc/%d/maps",pid);    //<stdio.h>    fp = fopen(buf,"r");    if(NULL == fp)    {        return 0;    }    while(fgets(buf,sizeof(buf),fp))    {        if(strstr(buf,section_name))        {            tmp = strtok(buf,"-");            addr = strtoul(tmp,NULL,0x10);   //<stdlib.h>            break;        }    }    fclose(fp);    return addr;}

 

 最后的运行结果：

完整代码：https://github.com/ChengChengCC/Android-demo/tree/master/ProtectSection

 

 

 

 　　　　　　　　　　　　　　　　　　　　基于执行视图

就是基于执行视图解释ELF文件格式，如果不太了解请移步这篇博文从dlsym()源码看动态链接过程

 就是通过动态链接的过程，通过.dynamic段定位.symtab ,  .dynstr, .hash，然后找到目标函数的文件偏移和大小，然后将目标函数取反，达到简单的加密，ida查看的效果图如下

 

#include <stdio.h>#include <stdlib.h>#include <stddef.h>#include <elf.h>#include <fcntl.h>#define SYMTAB  0x01#define HASH    0X02#define STRTAB  0x04#define STRSZ   0X08static unsigned elfhash(const char *_name){    const unsigned char *name = (const unsigned char *) _name;    unsigned h = 0, g;    while(*name) {        h = (h << 4) + *name++;        g = h & 0xf0000000;        h ^= g;        h ^= g >> 24;    }    return h;}int main(int argc ,char** argv){    Elf32_Ehdr ehdr;    Elf32_Phdr phdr;    Elf32_Word dyn_size,dyn_strsz;    Elf32_Off  dyn_off;    Elf32_Dyn   dyn;    Elf32_Addr  dyn_sym,dyn_str,dyn_hash;    unsigned   func_hash,nbucket,nchain,func_index;    char * ptr_dynstr = NULL;    char * ptr_func_content = NULL;    Elf32_Sym   func_sym;    int   flag = 0;    int i = 0;    char func_name[] = "getString";    if(argc < 2)    {        printf("input the so file\n");        return 0;    }    int fd = open(argv[1],O_RDWR);    if(fd < 0)    {        printf("open file failed\n");        goto _error;    }    lseek(fd,0,SEEK_SET);    if(read(fd,&ehdr,sizeof(Elf32_Ehdr)) != sizeof(Elf32_Ehdr))    {        printf("read elf header failed\n");        goto _error;    }    lseek(fd,ehdr.e_phoff,SEEK_SET);    for(i=0; i<ehdr.e_phnum;i++)    {        memset(&phdr,0,sizeof(phdr));        if(read(fd,&phdr,sizeof(Elf32_Phdr)) != sizeof(Elf32_Phdr))        {            printf("read segment failed\n");            goto _error;        }        if(phdr.p_type == PT_DYNAMIC)        {            dyn_off = phdr.p_offset;            dyn_size = phdr.p_filesz;            printf("find .dynamic section\n");            break;        }    }    lseek(fd,dyn_off,SEEK_SET);    //for(i = 0;i<dyn_size/sizeof(Elf32_Dyn);i++)    do{        if(read(fd,&dyn,sizeof(Elf32_Dyn)) != sizeof(Elf32_Dyn))        {            printf("read .dynamic failed\n");            goto _error;        }        if(dyn.d_tag == DT_SYMTAB)        {            flag |= SYMTAB;            dyn_sym = dyn.d_un.d_ptr;        }        if(dyn.d_tag == DT_STRTAB)        {            flag |= STRTAB;            dyn_str = dyn.d_un.d_ptr;        }        if(dyn.d_tag == DT_STRSZ)        {            flag |= STRSZ;            dyn_strsz = dyn.d_un.d_val;        }        if(dyn.d_tag == DT_HASH)        {            flag |= HASH;            dyn_hash = dyn.d_un.d_ptr;        }    } while(dyn.d_tag != DT_NULL);    if((flag & 0x0f) != 0x0f)    {        printf("find the needed dynamic section failed\n");        goto _error;    }    ptr_dynstr = (char*)malloc(dyn_strsz);    if(ptr_dynstr == NULL)    {        printf("malloc .dynstr failed\n");        goto _error;    }    lseek(fd,dyn_str,SEEK_SET);    if(read(fd,ptr_dynstr,dyn_strsz) != dyn_strsz)    {        printf("read .dynstr failed\n");        goto _error;    }    func_hash = elfhash(func_name);    lseek(fd,dyn_hash,SEEK_SET);    if(read(fd,&nbucket,4) != 4)    {        printf("read hash nbucket failed\n");        goto _error;    }    if(read(fd,&nchain,4) != 4)    {        printf("read hash nchain failed\n");        goto _error;    }    func_hash = func_hash%nbucket;    lseek(fd,func_hash*4,SEEK_CUR);    if(read(fd,&func_index,4) != 4)//索引是符号表或者chain    {        printf("read funck index failed\n");        goto _error;    }    lseek(fd,dyn_sym+func_index*sizeof(Elf32_Sym),SEEK_SET);    if(read(fd,&func_sym,sizeof(Elf32_Sym)) != sizeof(Elf32_Sym))    {        printf("read func sym entry failed\n'");    }    if(strcmp(ptr_dynstr+func_sym.st_name,func_name) != 0)    {        while(1)  //纯C语言是没有true的        {            lseek(fd,dyn_hash+4*(2+nbucket+func_index),SEEK_SET);            if(read(fd,&func_index,4) != 4)            {                printf("read func index failed\n");                goto _error;            }            lseek(fd,dyn_sym + func_index*sizeof(Elf32_Sym),SEEK_SET);            memset(&func_sym,0,sizeof(Elf32_Sym));            if(read(fd,&func_sym,sizeof(Elf32_Sym)) != sizeof(Elf32_Sym))            {                goto _error;            }            if(strcmp(func_name,dyn_str+func_sym.st_name) == 0)            {                break;            }        }    }    printf("find target func addr: %x,sizeo:%x\n",func_sym.st_value,func_sym.st_size);    ptr_func_content = (char*)malloc(func_sym.st_size);    if(ptr_func_content == NULL)    {        printf("alloc for func failed\n");        goto _error;    }    lseek(fd,func_sym.st_value,SEEK_SET);    if(read(fd,ptr_func_content,func_sym.st_size) != func_sym.st_size)    {        printf("read func content failed\n");        goto _error;    }    for(i=0;i<func_sym.st_size;i++)    {        ptr_func_content[i] = ~ptr_func_content[i];    }    lseek(fd,func_sym.st_value,SEEK_SET);    if(write(fd,ptr_func_content,func_sym.st_size) != func_sym.st_size)    {        printf("write to func failed\n");        goto _error;    }    printf("Complete \n");_error:    if(ptr_dynstr != NULL)    {        free(ptr_dynstr);    }    if(ptr_func_content != NULL)    {        free(ptr_func_content);    }    return 0;}

同样的是在so初始化时进行解密

void init_getString() __attribute__((constructor));

 

void init_getString(){    unsigned long lib_addr = 0;    Elf32_Ehdr*  ptr_ehdr = NULL;    Elf32_Phdr*  ptr_phdr = NULL;    Elf32_Dyn*   ptr_dyn = NULL;;    Elf32_Sym*   ptr_dynsym = NULL;    Elf32_Sym*      sym = NULL;    const char* ptr_hashtab = NULL;    const char* ptr_dynstr = NULL;    int flag = 0;    unsigned long strtab_size = 0;    unsigned func_hash = 0;    const char func_name[] = "getString";    unsigned long func_addr = 0;    unsigned   func_size =0;    unsigned long func = (unsigned long)getString;    size_t nbucket;    size_t nchain;    unsigned* bucket;    unsigned* chain;    const unsigned page_size = 0x1000;    int i =0;    lib_addr = get_cur_lib_addr();    if(0 == lib_addr)    {        LOGD("get_cur_lib_addr failed");        return;    }    ptr_ehdr = (Elf32_Ehdr*)lib_addr;    ptr_phdr = (Elf32_Phdr*)(lib_addr+ptr_ehdr->e_phoff);    for(i=0;i<ptr_ehdr->e_phnum;i++,ptr_phdr++)    {        if(PT_DYNAMIC == ptr_phdr->p_type)        {            ptr_dyn = (Elf32_Dyn*)(lib_addr+ptr_phdr->p_vaddr);            break;        }    }    if(NULL == ptr_dyn)    {        LOGD("find .dynamic failed");        return ;    }    // .dynsym  .dynstr .hash .    for (Elf32_Dyn* d = ptr_dyn; d->d_tag != DT_NULL; ++d)    {        switch(d->d_tag)        {        case DT_SYMTAB:            ptr_dynsym = (Elf32_Sym*)(lib_addr+d->d_un.d_ptr);            flag |= SYMTAB;            break;        case DT_STRTAB:            ptr_dynstr = (const char*)(lib_addr + d->d_un.d_ptr);            flag |= STRTAB;            break;        case DT_STRSZ:            strtab_size = d->d_un.d_val;            flag |= STRSZ;            break;        case DT_HASH:            ptr_hashtab = (const char*)(lib_addr + d->d_un.d_ptr);            flag |= HASH;            break;        }    }    if(flag & 0xf == 0xf)    {        LOGD("all segement get");    }    nbucket = *(unsigned*)ptr_hashtab;    nchain = *(unsigned*)(ptr_hashtab+4);    bucket = (unsigned*)(ptr_hashtab+8);    chain = bucket + nbucket;    func_hash = elfhash(func_name);    for (unsigned n = bucket[func_hash % nbucket]; n != 0; n = chain[n])    {        sym = ptr_dynsym + n;        if (strcmp(ptr_dynstr + sym->st_name, func_name)) continue;        switch(ELF32_ST_BIND(sym->st_info))        {        case STB_GLOBAL:        case STB_WEAK:            if (sym->st_shndx == SHN_UNDEF)            {                  continue;            }        }        func_addr = lib_addr+sym->st_value;        func_size = sym->st_size;        break;    }    unsigned page_off = func_addr % page_size;    if(mprotect((const void*)(func_addr-page_off),func_size+page_off,PROT_WRITE|PROT_READ|PROT_EXEC)!= JNI_OK)    {        int n = errno;        char *msg = strerror(errno);        LOGD("change page protect failed");        LOGD(msg);        return;    }    for(int i =0;i<func_size;i++)    {        ((char*)func_addr)[i] = ~((char*)func_addr)[i];    }    if(mprotect((const void*)(func_addr-page_off),func_size+page_off,PROT_READ|PROT_EXEC) != JNI_OK)    {        int n = errno;        char *msg = strerror(errno);        LOGD("resume page protect failed");        LOGD(msg);        return;    }}

 

运行效果图：

完整代码：https://github.com/ChengChengCC/Android-demo/tree/master/ProtectFunc