1.背景

  最近做驱动开发，开启了系统测试模式，于是桌面的右下角就有一个水印，如下图：

  测试了网上修改注册表方法不起作用，最后找到一款工具Universal Watermark Disabler可以把水印去除掉。于是对其原理有些兴趣，就有了相关的分析及编程实现。

2、相关分析

2.1 相关行为分析

  使用Process Monitor抓取相关操作，发现可用的操作如下：

  2.1.1 创建了一个名为painter_x64.dll的文件

  2.1.2 修改为注册表 HKCR\CLSID\{ab0b37ec-56f6-4a0e-a8fd-7a8bf7c2da96}\InProcServer32的值

  注册表相应的原值现为%SystemRoot%\system32\explorerframe.dll， 而修改后的值为如下

  2.1.3 大致结论

  注册表项的相关文件和桌面进程相关，应该是在桌面进程启动时加载了相应的dll文件，而该工具替换成painter_x64.dll，该dll做了相应的功能来实现水印过滤。结下来应该分析该dll的功能。

2.2 分析painter_x64.dll

  直接上IDA看相关的功能。

  2.2.1 主入口代码如下：

BOOL __stdcall DllEntryPoint(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpReserved)
{HMODULE v4; // rbxHMODULE v5; // raxBOOL (__stdcall *ExtTextOutW)(HDC, int, int, UINT, const RECT *, LPCWSTR, UINT, const INT *); // raxHMODULE v7; // raxint (__stdcall *LoadStringW)(HINSTANCE, UINT, LPWSTR, int); // raxconst CHAR *v9; // rcxHMODULE v10; // raxif ( fdwReason == 1 ){OutputDebugStringA("Loaded");v4 = GetModuleHandleW(L"shell32.dll");if ( v4 ){v5 = GetModuleHandleW(L"gdi32.dll");ExtTextOutW = (BOOL (__stdcall *)(HDC, int, int, UINT, const RECT *, LPCWSTR, UINT, const INT *))GetProcAddress(v5, "ExtTextOutW");if ( ExtTextOutW )sub_180001000("gdi32.dll", (__int64)ExtTextOutW, (__int64)sub_180001120, (__int64)v4);v7 = GetModuleHandleW(L"api-ms-win-core-libraryloader-l1-2-0.dll");LoadStringW = (int (__stdcall *)(HINSTANCE, UINT, LPWSTR, int))GetProcAddress(v7, "LoadStringW");if ( LoadStringW ){v9 = "api-ms-win-core-libraryloader-l1-2-0.dll";
LABEL_9:sub_180001000(v9, (__int64)LoadStringW, (__int64)sub_180001100, (__int64)v4);goto LABEL_10;}v10 = GetModuleHandleW(L"api-ms-win-core-libraryloader-l1-1-1.dll");LoadStringW = (int (__stdcall *)(HINSTANCE, UINT, LPWSTR, int))GetProcAddress(v10, "LoadStringW");if ( LoadStringW ){v9 = "api-ms-win-core-libraryloader-l1-1-1.dll";goto LABEL_9;}}
LABEL_10:DisableThreadLibraryCalls(hinstDLL);}return 1;
}

        2.2.2 sub_180001000功能的代码如下：

__int64 __fastcall sub_180001000(LPCSTR lpString2, __int64 a2, __int64 a3, __int64 a4)
{_DWORD *v8; // rbxunsigned int i; // eax__int64 *v10; // rbxDWORD flOldProtect; // [rsp+40h] [rbp+8h] BYREFif ( !lpString2 || !a2 || !a3 )return 0i64;v8 = (_DWORD *)(a4 + *(unsigned int *)(*(int *)(a4 + 60) + a4 + 144));for ( i = v8[3]; i; v8 += 5 ){if ( !lstrcmpiA((LPCSTR)(a4 + i), lpString2) )break;i = v8[8];}if ( !v8[3] )return 0i64;v10 = (__int64 *)(a4 + (unsigned int)v8[4]);if ( !*v10 )return 0i64;do{if ( *v10 == a2 )break;++v10;}while ( *v10 );if ( !*v10 )return 0i64;VirtualProtect(v10, 8ui64, 0x40u, &flOldProtect);*v10 = a3;VirtualProtect(v10, 8ui64, flOldProtect, &flOldProtect);return 1i64;
}

  2.2.3 sub_180001120代码如下：

BOOL __fastcall sub_180001120(HDC a1, int a2, int a3, UINT a4, const RECT *a5, const WCHAR *a6, UINT a7, const INT *a8)
{BOOL result; // eaxif ( a4 || !a7 )result = ExtTextOutW(a1, a2, a3, a4, a5, a6, a7, a8);elseresult = 1;return result;
}

2.3 结论

  通过分析加猜测（后边也证实了）相关逻辑是启动进程时加载此dll,此dll在加载时进程导入表Hook,对shell32.dll中导入的gdi32.dll的ExtTextOutW函数进行挂钩，然后过滤相关字符串。

3、代码实现

  完整代码如下：

• 头文件 

typedef BOOL(*EXTTEXTOUTW)(HDC hdc, int x, int y, UINT options, RECT* lprect, LPCWSTR lpString, UINT c, INT* lpDx);
typedef int (*GETIAT)(PVOID, BOOLEAN, USHORT, PULONG);
typedef int(__fastcall* LOADSTRINGW)(HINSTANCE, UINT, LPWSTR, int);
extern HMODULE hShell32;
extern HMODULE hGdi32;
extern HMODULE hDbgHelp;
extern EXTTEXTOUTW pExtTextOutW;
extern GETIAT pGetIAT;void InitializeHook();
BOOL HookFunction(LPCSTR szDllName, PVOID pFuncAddress, PVOID pHookFuncAddess, HMODULE hMod);
BOOL __stdcall MyExtTextOutW(HDC hdc, int x, int y, UINT options, RECT* lprect, LPCWSTR lpString, UINT c, INT* lpDx);
int __fastcall MyLoadStringW(HINSTANCE hInstance, UINT uID, LPWSTR lpBuffer, int cchBufferMax);

• cpp文件

#include "pch.h"
#include "Implement.h"
#include 
#include #pragma comment(lib, "dbghelp.lib")
#pragma comment(linker, "/EXPORT:DllCanUnloadNow=explorerframe.DllCanUnloadNow,@1")
#pragma comment(linker, "/EXPORT:DllGetClassObject=explorerframe.DllGetClassObject,@2")HMODULE hShell32 = NULL;
HMODULE hGdi32 = NULL;
HMODULE hDbgHelp = NULL;
EXTTEXTOUTW pExtTextOutW = NULL;
GETIAT pGetIAT = NULL;void InitializeHook()
{do {hShell32 = GetModuleHandleW(L"shell32.dll");if (hShell32 == NULL){break;}hGdi32 = GetModuleHandleW(L"gdi32.dll");if (hGdi32 == NULL){break;}pExtTextOutW = (EXTTEXTOUTW)GetProcAddress(hGdi32, "ExtTextOutW");if (pExtTextOutW == NULL){break;}HMODULE hModuleApiMsWinCoreLibraryloader_l1_2_0 = GetModuleHandleW(L"api-ms-win-core-libraryloader-l1-2-0.dll");LOADSTRINGW pLoadStringW = (LOADSTRINGW)GetProcAddress(hModuleApiMsWinCoreLibraryloader_l1_2_0, "LoadStringW");if (pLoadStringW){HookFunction("api-ms-win-core-libraryloader-l1-2-0.dll", pLoadStringW, MyLoadStringW, hShell32);}else{HMODULE hModuleApiMsWinCoreLibraryloader_l1_1_1 = GetModuleHandleW(L"api-ms-win-core-libraryloader-l1-1-1.dll");pLoadStringW = (LOADSTRINGW)GetProcAddress(hModuleApiMsWinCoreLibraryloader_l1_1_1, "LoadStringW");if (pLoadStringW){HookFunction("api-ms-win-core-libraryloader-l1-1-1.dll", pLoadStringW, MyLoadStringW, hShell32);}}BOOL OK = HookFunction("gdi32.dll", pExtTextOutW, MyExtTextOutW, hShell32);} while (false);
}int __fastcall MyLoadStringW(HINSTANCE hInstance, UINT uID, LPWSTR lpBuffer, int cchBufferMax)
{if (uID - 62000 <= 1){return 0;}else{return LoadStringW(hInstance, uID, lpBuffer, cchBufferMax);}
}BOOL HookFunction(LPCSTR szDllName, PVOID pFuncAddress, PVOID pHookFuncAddess, HMODULE hMod)
{if ((szDllName == NULL) ||(pFuncAddress == NULL) ||(pHookFuncAddess == NULL) ||(hMod == NULL)){return FALSE;}ULONG ulSize = 0;PIMAGE_IMPORT_DESCRIPTOR pImportDescriptor =(PIMAGE_IMPORT_DESCRIPTOR)ImageDirectoryEntryToData(hMod, TRUE,IMAGE_DIRECTORY_ENTRY_IMPORT,&ulSize);CStringA  strModuleName = "";while (pImportDescriptor->Name){PSTR pszModuleName = (PSTR)((PBYTE)hMod + pImportDescriptor->Name);strModuleName = pszModuleName;if (strModuleName.CompareNoCase(szDllName) == 0){break;}pImportDescriptor++;}if (strModuleName.GetLength() > 0){PIMAGE_THUNK_DATA pThunk =(PIMAGE_THUNK_DATA)((PBYTE)hMod + pImportDescriptor->FirstThunk);while (pThunk->u1.Function){PROC* ppfn = (PROC*)&pThunk->u1.Function;BOOL bFound = (*ppfn == pFuncAddress);if (bFound){MEMORY_BASIC_INFORMATION mbi = { 0 };VirtualQuery(ppfn,&mbi,sizeof(MEMORY_BASIC_INFORMATION));VirtualProtect(mbi.BaseAddress,mbi.RegionSize,PAGE_READWRITE,&mbi.Protect);*ppfn = ((PROC)pHookFuncAddess);VirtualProtect(mbi.BaseAddress,mbi.RegionSize,mbi.Protect,&mbi.Protect);return TRUE;}pThunk++;}}return FALSE;
}BOOL __stdcall MyExtTextOutW(HDC hdc, int x, int y, UINT options, RECT* lprect, LPCWSTR lpString, UINT c, INT* lpDx)
{BOOL bResult = FALSE;//CString str = lpString;//if (str.Find(L"测试模式") != -1 ||//     str.Find(L"Windows ") != -1 ||//     str.Find(L"Build ") != -1)//{//	return TRUE;//}if ((options != 0) || (c == 0)){return ExtTextOutW(hdc, x, y, options, lprect, lpString, c, lpDx);}return  TRUE;
}BOOL APIENTRY DllMain(HMODULE hModule,DWORD  ul_reason_for_call,LPVOID lpReserved
)
{switch (ul_reason_for_call){case DLL_PROCESS_ATTACH:{InitializeHook();DisableThreadLibraryCalls(hModule);}break;case DLL_THREAD_ATTACH:case DLL_THREAD_DETACH:case DLL_PROCESS_DETACH:break;}return TRUE;
}

4、PS

  关于自定义ExtTextOutW，用上边代码可以去除一般水印，也可以使用函数中注释的代码去除指定水印。

BOOL __stdcall MyExtTextOutW(HDC hdc, int x, int y, UINT options, RECT* lprect, LPCWSTR lpString, UINT c, INT* lpDx)
{BOOL bResult = FALSE;CString str = lpString;if (str.Find(L"测试模式") != -1) {return TRUE;}return ExtTextOutW(hdc, x, y, options, lprect, lpString, c, lpDx);
}

  上边代码只把测试模式去掉，效果如下

  或者：

BOOL __stdcall MyExtTextOutW(HDC hdc, int x, int y, UINT options, RECT* lprect, LPCWSTR lpString, UINT c, INT* lpDx)
{BOOL bResult = FALSE;CString str = lpString;if (str.Find(L"测试模式") != -1 ||str.Find(L"Windows ") != -1){return TRUE;}return  ExtTextOutW(hdc, x, y, options, lprect, lpString, c, lpDx);;
}

  上边代码把测试模式、Windows都去掉，效果如下：

  或者：

BOOL __stdcall MyExtTextOutW(HDC hdc, int x, int y, UINT options, RECT* lprect, LPCWSTR lpString, UINT c, INT* lpDx)
{BOOL bResult = FALSE;CString str = lpString;if (str.Find(L"测试模式") != -1 ||str.Find(L"Windows ") != -1||str.Find(L"Build ") != -1){return TRUE;}return  ExtTextOutW(hdc, x, y, options, lprect, lpString, c, lpDx);;
}

  以是代码把之前的三行水印都去除，效果如下：

5、特别注意

  完成相关开发后的dll要修改相应的注册表后重启explorer.exe进程起效，注册表修改的路径为HKEY_CLASSES_ROOT\CLSID\{ab0b37ec-56f6-4a0e-a8fd-7a8bf7c2da96}\InProcServer32 默认值。

         在不需要进行去除水印时应将注册表值修改回%SystemRoot%\system32\explorerframe.dll。

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