How to Modify Import Address Table for Run time Loaded DLL

Question

I want to hook functions that are called from a loaded DLL on Run time, i used the class CAPIHook from the book "Windows Via C/C++" (the DLL Injecting done by Install System Wide hook and The hooking by Modify IAT) but this code work only if the DLL name/symbols exist in the IAT in the executable file. (i.e. for Implicit DLL Linking)

this is DLL code:

   CAPIHook::CAPIHook(PSTR pszCalleeModName, PSTR pszFuncName, PROC pfnHook) {

   // Note: the function can be hooked only if the exporting module 
   //       is already loaded. A solution could be to store the function
   //       name as a member; then, in the hooked LoadLibrary* handlers, parse
   //       the list of CAPIHook instances, check if pszCalleeModName
   //       is the name of the loaded module to hook its export table and 
   //       re-hook the import tables of all loaded modules.

   m_pNext  = sm_pHead;    // The next node was at the head
   sm_pHead = this;        // This node is now at the head

   // Save information about this hooked function
   m_pszCalleeModName   = pszCalleeModName;
   m_pszFuncName        = pszFuncName;
   m_pfnHook            = pfnHook;
   m_pfnOrig            =  GetProcAddressRaw(GetModuleHandleA(pszCalleeModName), m_pszFuncName);

   // If function does not exit,... bye bye
   // This happens when the module is not already loaded
   if (m_pfnOrig == NULL)
   {
      wchar_t szPathname[MAX_PATH];
      GetModuleFileNameW(NULL, szPathname, _countof(szPathname));
      wchar_t sz[1024];
      StringCchPrintfW(sz, _countof(sz), 
         TEXT("[%4u - %s] impossible to find %S\r\n"), 
         GetCurrentProcessId(), szPathname, pszFuncName);
      OutputDebugString(sz);
      return;
   }

   // Hook this function in all currently loaded modules
   ReplaceIATEntryInAllMods(m_pszCalleeModName, m_pfnOrig, m_pfnHook);
}

this is the hook functions:

HMODULE WINAPI CAPIHook::LoadLibraryA(PCSTR pszModulePath) {

   HMODULE hmod = ::LoadLibraryA(pszModulePath);
   FixupNewlyLoadedModule(hmod, 0);
   return(hmod);
}

HMODULE WINAPI CAPIHook::LoadLibraryW(PCWSTR pszModulePath) {

   HMODULE hmod = ::LoadLibraryW(pszModulePath);
   FixupNewlyLoadedModule(hmod, 0);
   return(hmod);
}

HMODULE WINAPI CAPIHook::LoadLibraryExA(PCSTR pszModulePath, 
   HANDLE hFile, DWORD dwFlags) {

   HMODULE hmod = ::LoadLibraryExA(pszModulePath, hFile, dwFlags);
   FixupNewlyLoadedModule(hmod, dwFlags);
   return(hmod);
}

HMODULE WINAPI CAPIHook::LoadLibraryExW(PCWSTR pszModulePath, 
   HANDLE hFile, DWORD dwFlags) {

   HMODULE hmod = ::LoadLibraryExW(pszModulePath, hFile, dwFlags);
   FixupNewlyLoadedModule(hmod, dwFlags);
   return(hmod);
}

the method for replacing IAT:

void CAPIHook::ReplaceIATEntryInOneMod(PCSTR pszCalleeModName, 
   PROC pfnCurrent, PROC pfnNew, HMODULE hmodCaller) {

   // Get the address of the module's import section
   ULONG ulSize;

   // An exception was triggered by Explorer (when browsing the content of 
   // a folder) into imagehlp.dll. It looks like one module was unloaded...
   // Maybe some threading problem: the list of modules from Toolhelp might 
   // not be accurate if FreeLibrary is called during the enumeration.
   PIMAGE_IMPORT_DESCRIPTOR pImportDesc = NULL;
   __try {
      pImportDesc = (PIMAGE_IMPORT_DESCRIPTOR) ImageDirectoryEntryToData(
         hmodCaller, TRUE, IMAGE_DIRECTORY_ENTRY_IMPORT, &ulSize);
   } 
   __except (InvalidReadExceptionFilter(GetExceptionInformation())) {
      // Nothing to do in here, thread continues to run normally
      // with NULL for pImportDesc 
   }

   if (pImportDesc == NULL)
      return;  // This module has no import section or is no longer loaded


   // Find the import descriptor containing references to callee's functions
   for (; pImportDesc->Name; pImportDesc++) {
      PSTR pszModName = (PSTR) ((PBYTE) hmodCaller + pImportDesc->Name);
      if (lstrcmpiA(pszModName, pszCalleeModName) == 0) {

         // Get caller's import address table (IAT) for the callee's functions
         PIMAGE_THUNK_DATA pThunk = (PIMAGE_THUNK_DATA) 
            ((PBYTE) hmodCaller + pImportDesc->FirstThunk);

         // Replace current function address with new function address
         for (; pThunk->u1.Function; pThunk++) {

            // Get the address of the function address
            PROC* ppfn = (PROC*) &pThunk->u1.Function;

            // Is this the function we're looking for?
            BOOL bFound = (*ppfn == pfnCurrent);
            if (bFound) {
               if (!WriteProcessMemory(GetCurrentProcess(), ppfn, &pfnNew, 
                    sizeof(pfnNew), NULL) && (ERROR_NOACCESS == GetLastError())) {
                  DWORD dwOldProtect;
                  if (VirtualProtect(ppfn, sizeof(pfnNew), PAGE_WRITECOPY, 
                     &dwOldProtect)) {

                     WriteProcessMemory(GetCurrentProcess(), ppfn, &pfnNew, 
                        sizeof(pfnNew), NULL);
                     VirtualProtect(ppfn, sizeof(pfnNew), dwOldProtect, 
                        &dwOldProtect);
                  }
               }
               return;  // We did it, get out
            }
         }
      }  // Each import section is parsed until the right entry is found and patched
   }
}

the author added comments to add this functionality, but I am not sure how to do it

Note: the function can be hooked only if the exporting module is already loaded. A solution could be to store the function name as a member; then, in the hooked LoadLibrary* handlers, parse the list of CAPIHook instances, check if pszCalleeModName is the name of the loaded module to hook its export table and re-hook the import tables of all loaded modules.

he also write this on the book, but again i don't know what to do

A possible solution is to use the hooked LoadLibrary* functions to detect when a module is exporting an unpatched hooked function and then execute two actions:

Hook again the import table of the module already loaded because it is now possible to call GetProcAddress and get a pointer to the original implementation of the function to hook. Notice that the name of the function needs to be stored as a class member and set in the constructor.

Directly update this hooked function in the Export Address Table of the exporting module as shown by the implementation of the ReplaceEATEntryInOneMod function. That way, all new modules calling the hooked function will call our handler

i try to modify the IAT after loading the DLL, but my hooking function is not called

HMODULE WINAPI CAPIHook::LoadLibraryW(PCWSTR pszModulePath) {

   HMODULE hmod = ::LoadLibraryW(pszModulePath);

   if (StrCmpIW(pszModulePath, myDLLUnicodeName.c_str()) == 0 ) {
        PROC proc =  GetProcAddressRaw(GetModuleHandleA(myDLLName.c_str()), myFunctionName.c_str());

        if ( proc != NULL ) {
            for (CAPIHook* p = sm_pHead; p != NULL; p = p->m_pNext) {
                 if (StrCmpIA(p->m_pszCalleeModName, myDLLName.c_str()) == 0) {
                    MessageBox(NULL, L"This is the New Dynamic DLL", L"Test!", 0);
                    ReplaceIATEntryInAllMods(p->m_pszCalleeModName, proc , p->m_pfnHook);  
                 }
            }
        }
    }

   FixupNewlyLoadedModule(hmod, 0);
   return(hmod);
}

so, how to modify this code to handle dynamic loading case ?

Answer 1

I've done this before. What you want is EAT hooking instead of IAT. Also, hook the ::LoadLibrary API itself so you know when the DLL is loaded, and hook the requested api from the DLL after it is loaded.

There are some examples on the Internet on how to do this. Here is one I found just now: http://board.cheat-project.com/showthread.php?t=10633