Win32, How can i hook functions in compiled programs with C++?

Question

Take for instance this function (viewed in Ollydbg debugger)

The first PUSH EB

Answer 1

First, you will want to inject a dll in the target process. To do so, you can use this code:

Injector.h

#ifndef INJECTOR_H_INCLUDED
#define INJECTOR_H_INCLUDED

#include <Windows.h>
#include <string>

class Injector
{
public:
    /**
     * Loads a DLL into the remote process
     * @Return true on sucess, false on failure
    */
    bool InjectDll(DWORD processId, std::string dllPath);
private:
};

#endif // INJECTOR_H_INCLUDED

Injector.cpp

#include "Injector.h"

bool Injector::InjectDll(DWORD processId, std::string dllPath)
{
    HANDLE hThread, hProcess;
    void*  pLibRemote = 0;  // the address (in the remote process) where szLibPath will be copied to;

    HMODULE hKernel32 = GetModuleHandle("Kernel32");
    HINSTANCE hInst = GetModuleHandle(NULL);

    char DllFullPathName[_MAX_PATH];
    GetFullPathName(dllPath.c_str(), _MAX_PATH, DllFullPathName, NULL);

    // Get process handle
    hProcess = OpenProcess(PROCESS_ALL_ACCESS, FALSE, processId);

    // copy file path in szLibPath
    char szLibPath[_MAX_PATH];
    strcpy_s(szLibPath, DllFullPathName);

    // 1. Allocate memory in the remote process for szLibPath
    pLibRemote = VirtualAllocEx( hProcess, NULL, sizeof(szLibPath), MEM_COMMIT, PAGE_READWRITE );

    if (pLibRemote == NULL)
    {
        // probably because you don't have administrator's right
        return false;
    }

    // 2. Write szLibPath to the allocated memory
    WriteProcessMemory(hProcess, pLibRemote, (void*)szLibPath, sizeof(szLibPath), NULL);

    // 3. Force remote process to load dll
    hThread = CreateRemoteThread(hProcess, NULL, 0, (LPTHREAD_START_ROUTINE) GetProcAddress(hKernel32,"LoadLibraryA"), pLibRemote, 0, NULL);

    if (hThread == NULL)
    {
        return false;
    }

    return true;
}

main.cpp

#include "Injector.h"
int main()
{
    Injector injector;
    DWORD processId = 1653; // change the process id here. 

    if (injector.InjectDll(processId, "injected.dll"))
    {
        printf("Good job, you injected the dll\n");
    }
    else
    {
        printf("Something wrong happened\n");
    }

    while (true);
}

Then you have to make your dll. This is where it gets a little more complicated. First some includes:

injected.dll

#include <Windows.h>
#include <stdio.h>

Then we need to make a function that will detour the right location:

void DetourAddress(void* funcPtr, void* hook, BYTE* mem)
{
    BYTE cmd[5] = { 0xE9, 0x00, 0x00, 0x00, 0x00 }; // jump place holder
    void* RVAaddr = (void*)((DWORD)funcPtr + (DWORD)GetModuleHandle(NULL)); // base + relative address

    // make memory readable/writable
    DWORD dwProtect;
    VirtualProtect(RVAaddr, 5, PAGE_EXECUTE_READWRITE, &dwProtect);

    // read memory
    ReadProcessMemory(GetCurrentProcess(), (LPVOID)RVAaddr, &mem[2], 5, NULL);

    // write jmp in cmd
    DWORD offset = ((DWORD)hook - (DWORD)RVAaddr - 5);  // (dest address) - (source address) - (jmp size)
    memcpy(&cmd[1], &offset, 4); // write address into jmp
    WriteProcessMemory(GetCurrentProcess(), (LPVOID)RVAaddr, cmd, 5, 0); // write jmp

    // write mem
    VirtualProtect(mem, 13, PAGE_EXECUTE_READWRITE, &dwProtect);

    void* returnAdress = (void*)((DWORD)RVAaddr + 5);
    memcpy(&mem[8], &returnAdress, 4); // write return address into mem

    // reprotect
    VirtualProtect(RVAaddr, 5, dwProtect, NULL);
}

If you need to remove your dll at some point, you will need to restore the code:

void PatchAddress(void* funcPtr, BYTE* mem)
{
    void* RVAaddr = (void*)((DWORD)funcPtr + (DWORD)GetModuleHandle(NULL)); // base + relative address

    // make memory readable/writable
    DWORD dwProtect;
    VirtualProtect(funcPtr, 5, PAGE_EXECUTE_READWRITE, &dwProtect);

    WriteProcessMemory(GetCurrentProcess(), (LPVOID)RVAaddr, &mem[2], 5, NULL); // write jmp

    VirtualProtect(RVAaddr, 5, dwProtect, NULL);
}

Next, we need to make a function out of the detoured bytes in order for the program to execute them, so that it isn't affected by our detour. Add this in global space:

// memory (0x5E = pop esi, 0x68 = push DWORD, 0xC3 = RETN)
BYTE detourMem[13] = { 0x5E, 0x5E, 0x0, 0x0, 0x0, 0x0, 0x0, 0x68, 0x00, 0x00, 0x00, 0x00, 0xC3 };

// Convert bytes array to function
typedef void ( * pFunc)();
pFunc funcMem = (pFunc) &detourMem;

// I also added a variable as an example of what you can do with it.
DWORD var = 0;

After that, you need your detouring function:

_declspec(naked) void DetourFunction()
{
    // we need to push all flag and registers on the stack so we don't modify them by accident
    __asm
    {
        PUSHFD
        PUSHAD

        // You can do "whatever" you want here in assembly code
        // ex, put eax value into var:
        mov var, eax
    }

    printf("this code is executed everytime the detoured function is called\n");
    // Do whatever you want in c++ here
    if (var < 7)
    {
        // eax was smaller than 7
    }

    // We pop every flags and registers we first pushed so that the program continue as it was supposed to
    __asm
    {
        // we set everything back to normal
        POPAD
        POPFD
        push esi

        // we call our funcMem
        mov edx, funcMem;
        call edx
    }
}

Finaly, here is how your DLLMain would look like:

BOOL APIENTRY DllMain( HMODULE hModule, DWORD  ul_reason_for_call, LPVOID lpReserved  )
{
    DWORD detouredAddress = 0x689B; // add the RELATIVE ADDRESS of the location you want to detour
    FILE *stream;
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
        // Only add this if you want a console to appears when you inject your dll (don't forget FreeConsole when you remove the dll)
        AllocConsole();
        freopen_s(&stream, "CONOUT$", "w", stdout);

        // If you need to know the base address of the process your injected:   
        printf("base address: 0x%X\n", (DWORD)GetModuleHandle(NULL));

        // Our detour function
        DetourAddress((void*)detouredAddress, (void*)&DetourFunction, detourMem);
        break;
    case DLL_PROCESS_DETACH:
        // We restore the process to have what it was before it was injected
        PatchAddress((void*)detouredAddress, detourMem);

        FreeConsole();
        break;
    }

    return true;
}

I understand this is a lot all at once, so if you have any questions don't hesitate!