How to create a trampoline function for hook

Question

I'm interested in hooking and I decided to see if I could hook some functions. I wasn't interested in using a library like detours because I want to have the experience of doing it on my own. With some sources I found on the internet, I was able to create the code below. It's basic, but it works alright. However when hooking functions that are called by multiple threads it proves to be extremely unstable. If two calls are made at nearly the same time, it'll crash. After some research I think I need to create a trampoline function. After looking for hours all I was not able to find anything other that a general description on what a trampoline was. I could not find anything specifically about writing a trampoline function, or how they really worked. If any one could help me write one, post some sources, or at least point me in the right direction by recommending some articles, sites, books, etc. I would greatly appreciate it.

Below is the code I've written. It's really basic but I hope others might learn from it.

test.cpp

#include "stdafx.h"

Hook hook;

typedef int (WINAPI *tMessageBox)(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType);

DWORD hMessageBox(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType)
{
    hook.removeHook();
    tMessageBox oMessageBox = (tMessageBox)hook.funcPtr; 
    int ret =oMessageBox(hWnd, lpText, "Hooked!", uType);
    hook.applyHook(&hMessageBox);

    return ret;
}

void hookMessageBox()
{
    printf("Hooking MessageBox...\n");
    if(hook.findFunc("User32.dll", "MessageBoxA")) 
    {
        if(hook.applyHook(&hMessageBox))
        {
            printf("hook applied! \n\n");
        } else printf("hook could not be applied\n");
    }   
}

hook.cpp

#include "stdafx.h"

bool Hook::findFunc(char* libName, char* funcName) 
{
    Hook::funcPtr = (void*)GetProcAddress(GetModuleHandleA(libName), funcName); 
    return (Hook::funcPtr != NULL);
}

bool Hook::removeHook() 
{
    DWORD dwProtect;
    if(VirtualProtect(Hook::funcPtr, 6, PAGE_EXECUTE_READWRITE, &dwProtect))
        {
        WriteProcessMemory(GetCurrentProcess(), (LPVOID)Hook::funcPtr, Hook::origData, 6, 0);
        VirtualProtect(Hook::funcPtr, 6, dwProtect, NULL);
        return true;
    } else return false;
}

bool Hook::reapplyHook() 
{
    DWORD dwProtect;
    if(VirtualProtect(funcPtr, 6, PAGE_EXECUTE_READWRITE, &dwProtect)) 
        {
        WriteProcessMemory(GetCurrentProcess(), (LPVOID)funcPtr, Hook::hookData, 6, 0);
        VirtualProtect(funcPtr, 6, dwProtect, NULL);
        return true;
    } else return false;
}

bool Hook::applyHook(void* hook) 
{ 
    return setHookAtAddress(Hook::funcPtr, hook);
}

bool Hook::setHookAtAddress(void* funcPtr, void* hook) 
{
    Hook::funcPtr = funcPtr;
    BYTE jmp[6] = { 0xE9, //jmp
                   0x00, 0x00, 0x00, 0x00,  //address
                   0xC3 //retn 
                 };

    DWORD dwProtect;

    if(VirtualProtect(funcPtr, 6, PAGE_EXECUTE_READWRITE, &dwProtect)) // make memory writable
    {

        ReadProcessMemory(GetCurrentProcess(), (LPVOID)funcPtr, Hook::origData, 6, 0); // save old data
        DWORD offset = ((DWORD)hook - (DWORD)funcPtr - 5);  //((to)-(from)-5)
        memcpy(&jmp[1], &offset, 4); // write address into jmp
        memcpy(Hook::hookData, jmp, 6); // save hook data
        WriteProcessMemory(GetCurrentProcess(), (LPVOID)funcPtr, jmp, 6, 0); // write jmp
        VirtualProtect(funcPtr, 6, dwProtect, NULL); // reprotect

        return true;
    } else return false;
}

Mucin

I was going to post a link to GD, but I just noticed you are a member there too. Have you tried using their search function? It comes up with tons of examples :) – Kaliope

You may check EasyHook's code, I believe it's open-source. There are also plenty of other examples around. If you're planning to use this in a deployed application, I'd recommend using a library (like EasyHook) that can already handle recursion on the hook/trampoline, threading, and some of the fun stuff. – Fontainebleau

@Tom Knapen I searched GD, MPGH and a few other sites before I posted. Searching 'trampoline' on GD returns a few slightly-related posts but not what I'm looking for. – Mucin

Your code is very useful to beginners, thank you – Secure

Answer 1

If you want your hook to be safe when called by multiple threads, you don't want to be constantly unhooking and rehooking the original API.

A trampoline is simply a bit of code you generate that replicates the functionality of the first few bytes of the original API (which you overwrote with your jump), then jumps into the API after the bytes you overwrote.

Rather than unhooking the API, calling it and rehooking it you simply call the trampoline.

This is moderately complicated to do on x86 because you need (a fairly minimal) disassembler to find the instruction boundaries. You also need to check that the code you copy into your trampoline doesn't do anything relative to the instruction pointer (like a jmp, branch or call).

This is sufficient to make calls to the hook thread-safe, but you can't create the hook if multiple threads are using the API. For this, you need to hook the function with a two-byte near jump (which can be written atomically). Windows APIs are frequently preceded by a few NOPs (which can be overwritten with a far jump) to provide a target for this near jump.

Doing this on x64 is much more complicated. You can't simply patch the function with a 64-bit far jump (because there isn't one, and instructions to simulate it are often too long). And, depending on what your trampoline does, you may need to add it to the OS's stack unwind information.

I hope this isn't too general.

Unsustainable

Thanks, but this was pretty much much what I already found and doesn't help me write one. – Mucin

@Stratus: What are you missing? Allocate some executable memory. Copy n bytes from the function prolog to the allocated memory and follow it with a jump to function prolog + n. n is the size of the instructions you need to copy to free-up at least 5 bytes in the function prolog. That's a trampoline. There are some other wrinkles (like don't copy instructions that modify the IP) but that's basicaly it. – Unsustainable

x64 has far jump if you provide it a register eg RAX. to be thread safe you are right, but if you dont need thread safety or dont want to deal with complexities you mention and dont want to use third party library,you can protect regions with critical regions. if you dont write performance critical code or it isnt in hotpath ofc. – Tug

Answer 2

The defacto standard hooking tutorial is from jbremer and available here

Here is a simple x86 detour and trampoline hook based on this tutorial using Direct3D's EndScene() function as a example:

bool Detour32(char* src, char* dst, const intptr_t len)
{
    if (len < 5) return false;

    DWORD  curProtection;
    VirtualProtect(src, len, PAGE_EXECUTE_READWRITE, &curProtection);

    intptr_t  relativeAddress = (intptr_t)(dst - (intptr_t)src) - 5;

    *src = (char)'\xE9';
    *(intptr_t*)((intptr_t)src + 1) = relativeAddress;

    VirtualProtect(src, len, curProtection, &curProtection);
    return true;
}

char* TrampHook32(char* src, char* dst, const intptr_t len)
{
    // Make sure the length is greater than 5
    if (len < 5) return 0;

    // Create the gateway (len + 5 for the overwritten bytes + the jmp)
    void* gateway = VirtualAlloc(0, len + 5, MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);

    //Write the stolen bytes into the gateway
    memcpy(gateway, src, len);

    // Get the gateway to destination addy
    intptr_t  gatewayRelativeAddr = ((intptr_t)src - (intptr_t)gateway) - 5;

    // Add the jmp opcode to the end of the gateway
    *(char*)((intptr_t)gateway + len) = 0xE9;

    // Add the address to the jmp
    *(intptr_t*)((intptr_t)gateway + len + 1) = gatewayRelativeAddr;

    // Perform the detour
    Detour32(src, dst, len);

    return (char*)gateway;
}

typedef HRESULT(APIENTRY* tEndScene)(LPDIRECT3DDEVICE9 pDevice);
tEndScene oEndScene = nullptr;

HRESULT APIENTRY hkEndScene(LPDIRECT3DDEVICE9 pDevice)
{
    //do stuff in here
    return oEndScene(pDevice);
}

//just an example
int main()
{
    oEndScene = (tEndScene)TrampHook32((char*)d3d9Device[42], (char*)hkEndScene, 7);
}

Rakel