前言
对 Windows 程序的加载和运行过程有了基本了解后，手动加载并运行一个PE文件并不成问题。加壳仅仅是在这上面更进一步：把加载程序和被加载的程序合并成一个文件。
这么说可能有点太简单化，大部分的工作其实就在这儿：如何处理被加载的程序？压缩？加密？混淆？加载器（或者叫壳程序）如何反调试？
这里先写一个简单的加壳机，仅仅是把被加载的PE文件作为一个 Section，添加到壳程序里，让壳程序直接从这个 Section 加载并运行。其他花里胡哨的操作都先不整，仅作为证明工作原理的案例。
0x01 壳程序
1.1 思路
和加载一个PE文件不同，既然被加载的程序就在 Section 里，那需要做的只有定位到 Section，然后把 Section 内容当读取进内存的 PE 文件内容处理就好了。
壳程序应该尽量保持轻量，不在原始程序上添加太多东西（加完壳大小翻一倍还多了一堆DLL依赖那谁受得了啊），所以很多标准C库的函数也不能用了，像是memcpy、strcmp 都要自己简单实现一个。
1.2 壳实现
绝大部分内容和之前文章中的 load_PE 一致，入口点修改为 _start，需要注意。
#include <Windows.h>
#include <winnt.h>
void *load_PE(char *PE_data);
void fix_iat(char *p_image_base, IMAGE_NT_HEADERS *p_NT_headers);
void fix_base_reloc(char *p_image_base, IMAGE_NT_HEADERS *p_NT_headers);
int mystrcmp(const char *str1, const char *str2);
void mymemcpy(char *dest, const char *src, size_t length);
int _start(void) {
char *unpacker_VA = (char *)GetModuleHandleA(NULL);
IMAGE_DOS_HEADER *p_DOS_header = (IMAGE_DOS_HEADER *)unpacker_VA;
IMAGE_NT_HEADERS *p_NT_headers = (IMAGE_NT_HEADERS *)(((char *)unpacker_VA) + p_DOS_header->e_lfanew);
IMAGE_SECTION_HEADER *sections = (IMAGE_SECTION_HEADER *)(p_NT_headers + 1);
char *packed = NULL;
char packed_section_name[] = ".packed";
for (int i = 0; i < p_NT_headers->FileHeader.NumberOfSections; i++) {
if (mystrcmp(sections[i].Name, packed_section_name) == 0) {
packed = unpacker_VA + sections[i].VirtualAddress;
break;
}
}
if (packed != NULL) {
void (*entrypoint)(void) = (void (*)(void))load_PE(packed);
entrypoint();
}
return 0;
}
void *load_PE(char *PE_data) {
IMAGE_DOS_HEADER *p_DOS_header = (IMAGE_DOS_HEADER *)PE_data;
IMAGE_NT_HEADERS *p_NT_headers = (IMAGE_NT_HEADERS *)(PE_data + p_DOS_header->e_lfanew);
// extract information from PE header
DWORD size_of_image = p_NT_headers->OptionalHeader.SizeOfImage;
DWORD entry_point_RVA = p_NT_headers->OptionalHeader.AddressOfEntryPoint;
DWORD size_of_headers = p_NT_headers->OptionalHeader.SizeOfHeaders;
// allocate memory
// https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-virtualalloc
char *p_image_base = (char *)VirtualAlloc(NULL, size_of_image, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
if (p_image_base == NULL) {
return NULL;
}
// copy PE headers in memory
mymemcpy(p_image_base, PE_data, size_of_headers);
// Section headers starts right after the IMAGE_NT_HEADERS struct, so we do some pointer arithmetic-fu here.
IMAGE_SECTION_HEADER *sections = (IMAGE_SECTION_HEADER *)(p_NT_headers + 1);
for (int i = 0; i < p_NT_headers->FileHeader.NumberOfSections; i++) {
// calculate the VA we need to copy the content, from the RVA
// section[i].VirtualAddress is a RVA, mind it
char *dest = p_image_base + sections[i].VirtualAddress;
// check if there is Raw data to copy
if (sections[i].SizeOfRawData > 0) {
// We copy SizeOfRaw data bytes, from the offset PointerToRawData in the file
mymemcpy(dest, PE_data + sections[i].PointerToRawData, sections[i].SizeOfRawData);
} else {
for (size_t i = 0; i < sections[i].Misc.VirtualSize; i++) {
dest[i] = 0;
}
}
}
fix_iat(p_image_base, p_NT_headers);
fix_base_reloc(p_image_base, p_NT_headers);
// Set permission for the PE header to read only
DWORD oldProtect;
VirtualProtect(p_image_base, p_NT_headers->OptionalHeader.SizeOfHeaders, PAGE_READONLY, &oldProtect);
for (int i = 0; i < p_NT_headers->FileHeader.NumberOfSections; ++i) {
char *dest = p_image_base + sections[i].VirtualAddress;
DWORD s_perm = sections[i].Characteristics;
DWORD v_perm = 0; // flags are not the same between virtal protect and the section header
if (s_perm & IMAGE_SCN_MEM_EXECUTE) {
v_perm = (s_perm & IMAGE_SCN_MEM_WRITE) ? PAGE_EXECUTE_READWRITE : PAGE_EXECUTE_READ;
} else {
v_perm = (s_perm & IMAGE_SCN_MEM_WRITE) ? PAGE_READWRITE : PAGE_READONLY;
}
VirtualProtect(dest, sections[i].Misc.VirtualSize, v_perm, &oldProtect);
}
return (void *)(p_image_base + entry_point_RVA);
}
void fix_iat(char *p_image_base, IMAGE_NT_HEADERS *p_NT_headers) {
IMAGE_DATA_DIRECTORY *data_directory = p_NT_headers->OptionalHeader.DataDirectory;
// load the address of the import descriptors array
IMAGE_IMPORT_DESCRIPTOR *import_descriptors =
(IMAGE_IMPORT_DESCRIPTOR *)(p_image_base + data_directory[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress);
// this array is null terminated
for (int i = 0; import_descriptors[i].OriginalFirstThunk != 0; ++i) {
// Get the name of the dll, and import it
char *module_name = p_image_base + import_descriptors[i].Name;
HMODULE import_module = LoadLibraryA(module_name);
if (import_module == NULL) {
// panic!
ExitProcess(255);
}
// the lookup table points to function names or ordinals => it is the IDT
IMAGE_THUNK_DATA *lookup_table = (IMAGE_THUNK_DATA *)(p_image_base + import_descriptors[i].OriginalFirstThunk);
// the address table is a copy of the lookup table at first
// but we put the addresses of the loaded function inside => that's the IAT
IMAGE_THUNK_DATA *address_table = (IMAGE_THUNK_DATA *)(p_image_base + import_descriptors[i].FirstThunk);
// null terminated array, again
for (int i = 0; lookup_table[i].u1.AddressOfData != 0; ++i) {
void *function_handle = NULL;
// Check the lookup table for the adresse of the function name to import
DWORD lookup_addr = lookup_table[i].u1.AddressOfData;
if ((lookup_addr & IMAGE_ORDINAL_FLAG) == 0) { // if first bit is not 1
// import by name : get the IMAGE_IMPORT_BY_NAME struct
IMAGE_IMPORT_BY_NAME *image_import = (IMAGE_IMPORT_BY_NAME *)(p_image_base + lookup_addr);
// this struct points to the ASCII function name
char *funct_name = (char *)&(image_import->Name);
// get that function address from it's module and name
function_handle = (void *)GetProcAddress(import_module, funct_name);
} else {
// import by ordinal, directly
function_handle = (void *)GetProcAddress(import_module, (LPSTR)lookup_addr);
}
if (function_handle == NULL) {
ExitProcess(255);
}
// change the IAT, and put the function address inside.
address_table[i].u1.Function = (DWORD)function_handle;
}
}
}
void fix_base_reloc(char *p_image_base, IMAGE_NT_HEADERS *p_NT_headers) {
IMAGE_DATA_DIRECTORY *data_directory = p_NT_headers->OptionalHeader.DataDirectory;
// this is how much we shifted the ImageBase
DWORD delta_VA_reloc = ((DWORD)p_image_base) - p_NT_headers->OptionalHeader.ImageBase;
// if there is a relocation table, and we actually shitfted the ImageBase
if (data_directory[IMAGE_DIRECTORY_ENTRY_BASERELOC].VirtualAddress != 0 && delta_VA_reloc != 0) {
// calculate the relocation table address
IMAGE_BASE_RELOCATION *p_reloc =
(IMAGE_BASE_RELOCATION *)(p_image_base + data_directory[IMAGE_DIRECTORY_ENTRY_BASERELOC].VirtualAddress);
// once again, a null terminated array
while (p_reloc->VirtualAddress != 0) {
// how any relocation in this block
// ie the total size, minus the size of the "header", divided by 2 (those are words, so 2 bytes for each)
DWORD size = (p_reloc->SizeOfBlock - sizeof(IMAGE_BASE_RELOCATION)) / 2;
// the first relocation element in the block, right after the header (using pointer arithmetic again)
WORD *fixups = (WORD *)(p_reloc + 1);
for (size_t i = 0; i < size; ++i) {
// type is the first 4 bits of the relocation word
int type = fixups[i] >> 12;
// offset is the last 12 bits
int offset = fixups[i] & 0x0fff;
// this is the address we are going to change
DWORD *change_addr = (DWORD *)(p_image_base + p_reloc->VirtualAddress + offset);
// there is only one type used that needs to make a change
switch (type) {
case IMAGE_REL_BASED_HIGHLOW:
*change_addr += delta_VA_reloc;
break;
default:
break;
}
}
// switch to the next relocation block, based on the size
p_reloc = (IMAGE_BASE_RELOCATION *)(((DWORD)p_reloc) + p_reloc->SizeOfBlock);
}
}
}
int mystrcmp(const char *str1, const char *str2) {
while (*str1 == *str2 && *str1 != 0) {
str1++;
str2++;
}
if (*str1 == 0 && *str2 == 0) {
return 0;
}
return -1;
}
void mymemcpy(char *dest, const char *src, size_t length) {
for (size_t i = 0; i < length; i++) {
dest[i] = src[i];
}
}
构建参数（CMAKE）
add_executable(loader_2 WIN32 loader_2.c)
target_compile_options(loader_2 PRIVATE /GS-)
target_link_options(loader_2 PRIVATE /NODEFAULTLIB /ENTRY:_start)
参数/GS-是为了避免在/NODEFAULTLIB下出现一些缓存区安全检查代码链接错误。参考文档。
0x02 加壳机
相信已经发现了，上文并没有提到怎么把程序嵌入壳程序里。这是因为加壳并不是在壳程序编译时直接把文件嵌进去=，=虽然理论上来说也可以，但这里不讨论了。仅仅看加壳机加壳的场景吧。
2.1 加壳机原理
加壳机做的事情包括：
- 在 section table 里添加 section
- 根据 section table 和 file_alignment 决定如何分配空间
- 根据 section_alignment 计算 virtual size
- 根据上一个 section 大小和位置计算 virtual address
- 填充 pointer_to_raw_data 和 size_of_raw_data
- 设置合适的 characteristics
- 计算修改 number_of_sections
- 计算修改 size_of_image
- 计算修改 size_of_headers
反正看起来就很麻烦，不过幸好操作 PE 文件的库不少，GitHub 搜一搜就有。这里用 LIEF 这个库，操作蛮简单的。
2.2 源码
#include <Windows.h>
#include <LIEF/LIEF.hpp>
#include <vector>
std::vector<uint8_t> read_file(const std::string &path) {
auto h = CreateFile(path.c_str(), GENERIC_READ, 0, nullptr, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
DWORD readbyte = 0;
auto filesize = GetFileSize(h, nullptr);
auto content = std::vector<uint8_t>();
content.resize(filesize, 0);
if (!ReadFile(h, content.data(), filesize, &readbyte, nullptr)) {
abort();
}
if (readbyte != filesize) {
abort();
}
CloseHandle(h);
return content;
}
int main(int argc, const char *argv[]) {
if (argc < 3) {
printf("loader and program path are required");
return -1;
}
auto loader_path = argv[1];
auto program_path = argv[2];
auto loader_binary = LIEF::PE::Parser::parse(loader_path);
// LIEF 帮我们做了偏移计算之类的工作，这里就只用点逻辑，非常得银杏。
auto program_content = read_file(program_path);
auto packed_section = LIEF::PE::Section(".packed"); // 新建 section
packed_section.content(program_content); // 把被加载程序的内容当成 section 内容
loader_binary->add_section(packed_section, LIEF::PE::PE_SECTION_TYPES::DATA); // 把 section 添加到壳程序里
// 用 lief 实现把修改后的壳程序写入硬盘
auto builder = LIEF::PE::Builder::Builder(loader_binary.get());
builder.build();
builder.write("packed.exe");
return 0;
}
编译指令（CMAKE）参考 LIEF 文档。
# Custom path to the LIEF install directory
set(LIEF_DIR CACHE PATH ${CMAKE_INSTALL_PREFIX})
# Directory to 'FindLIEF.cmake'
list(APPEND CMAKE_MODULE_PATH ${LIEF_DIR}/share/LIEF/cmake)
# include 'FindLIEF.cmake'
include(FindLIEF)
# Find LIEF
find_package(LIEF REQUIRED COMPONENTS STATIC) # COMPONENTS: <SHARED | STATIC> - Default: STATIC
add_executable(packer packer.cpp)
if(MSVC)
target_compile_options(packer PRIVATE /FIiso646.h /MT)
set_property(TARGET packer PROPERTY LINK_FLAGS /NODEFAULTLIB:MSVCRT)
endif()
target_include_directories(packer PRIVATE ${LIEF_INCLUDE_DIRS})
set_property(TARGET packer
PROPERTY CXX_STANDARD 11
PROPERTY CXX_STANDARD_REQUIRED ON)
target_link_libraries(packer PRIVATE ${LIEF_LIBRARIES})
结论
加壳程序反而平平无奇，正印证了那句台下功夫。
这个案例程序依然存在很多问题，比如说不支持64位程序，不支持不支持ASLR的程序，而且加壳后 .packed 内容可以直接看到是个PE，脱壳跟玩一样。
已经到了这一步，我想后续可以做个简单的压缩壳作为实用案例看看。
参考：
加载器部分参考了文章，LIEF部分因为 python 出现莫名其妙的 not supported 错误，于是换成用 C++ 来写了。
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发表于 2021-9-29 22:24
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发表于 2021-10-19 20:15
发表于 2021-10-18 18:04
