与HOOK相关的API.doc

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Introduction
Intercepting Win32 API calls has always been a challenging subject among most of the Windows developers and I have to admit, it's been one of my favorite topics. The term Hooking represents a fundamental technique of getting control over a particular piece of code execution. It provides an straightforward mechanism that can easily alter the operating system's behavior as well as 3rd party products, without having their source code available.
Many modern systems draw the attention to their ability to utilize existing Windows applications by employing spying techniques. A key motivation for hooking, is not only to contribute to advanced functionalities, but also to inject user-supplied code for debugging purposes.
Unlike some relatively "old" operating systems like DOS and Windows 3.xx, the present Windows OS as NT/2K and 9x provide sophisticated mechanisms to separate address spaces of each process. This architecture offers a real memory protection, thus no application is able to corrupt the address space of another process or in the worse case even to crash the operating system itself. This fact makes a lot harder the development of system-aware hooks.
My motivation for writing this article was the need for a really simple hooking framework, that will offer an easy to use interface and ability to capture different APIs. It intends to reveal some of the tricks that can help you to write your own spying system. It suggests a single solution how to build a set for hooking Win32 API functions on NT/2K as well as 98/Me (shortly named in the article 9x) family Windows. For the sake of simplicity I decided not to add a support do UNICODE. However, with some minor modifications of the code you could easily accomplish this task.
Spying of applications provides many advantages:
1. API function's monitoring
The ability to control API function calls is extremely helpful and enables developers to track down specific "invisible" actions that occur during the API call. It contributes to comprehensive validation of parameters as well as reports problems that usually remain overlooked behind the scene. For instance sometimes, it might be very helpful to monitor memory related API functions for catching resource leaks.
2. Debugging and reverse engineering
Besides the standard methods for debugging API hooking has a deserved reputation for being one of the most popular debugging mechanisms. Many developers employ the API hooking technique in order to identify different component implementations and their relationships. API interception is very powerful way of getting information about a binary executable.
3. Peering inside operating system
Often developers are keen to know operating system in dept and are inspired by the role of being a "debugger". Hooking is also quite useful technique for decoding undocumented or poorly documented APIs.
4. Extending originally offered functionalities by embedding custom modules into external Windows applications Re-routing the normal code execution by injecting hooks can provide an easy way to change and extend existing module functionalities. For example many 3rd party products sometimes don't meet specific security requirements and have to be adjusted to your specific needs. Spying of applications allows developers to add sophisticated pre- and post-processing around the original API functions. This ability is an extremely useful for altering the behavior of the already compiled code.
Functional requirements of a hooking system
There are few important decisions that have to be made, before you start implementing any kind of API hooking system. First of all, you should determine whether to hook a single application or to install a system-aware engine. For instance if you would like to monitor just one application, you don't need to install a system-wide hook but if your job is to track down all calls to TerminateProcess() or WriteProcessMemory() the only way to do so is to have a system-aware hook. What approach you will choose depends on the particular situation and addresses specific problems.
General design of an API spying framework
Usually a Hook system is composed of at least two parts - a Hook Server and a Driver. The Hook Server is responsible for injecting the Driver into targeted processes at the appropriate moment. It also administers the driver and optionally can receive information from the Driver about its activities whereas the Driver module that performs the actual interception.  
This design is rough and beyond doubt doesn't cover all possible implementations. However it outlines the boundaries of a hook framework.

Once you have the requirement specification of a hook framework, there are few design points you should take into account:
· What applications do you need to hook
· How to inject the DLL into targeted processes or which implanting technique to follow
· Which interception mechanism to use
I hope next the few sections will provide answers to those issues.
Injecting techniques
1. Registry
In order to inject a DLL into processes that link with USER32.DLL, you simply can add the DLL name to the value of the following registry key:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs
Its value contains a single DLL name or group of DLLs separated either by comma or spaces. According to MSDN documentation [7], all DLLs specified by the value of that key are loaded by each Windows-based application running within the current logon session. It is interesting that the actual loading of these DLLs occurs as a part of USER32's initialization. USER32 reads the value of mentioned registry key and calls LoadLibrary() for these DLLs in its DllMain code. However this trick applies only to applications that use USER32.DLL. Another restriction is that this built-in mechanism is supported only by NT and 2K operating systems. Although it is a harmless way to inject a DLL into a Windows processes there are few shortcomings:
o In order to activate/deactivate the injection process you have to reboot Windows.
o The DLL you want to inject will be mapped only into these processes that use USER32.DLL, thus you cannot expect to get your hook injected into console applications, since they usually don't import functions from USER32.DLL.
o On the other hand you don't have any control over the injection process. It means that it is implanted into every single GUI application, regardless you want it or not. It is a redundant overhead especially if you intend to hook few applications only. For more details see [2] "Injecting a DLL Using the Registry"

2. System-wide Windows Hooks
Certainly a very popular technique for injecting DLL into a targeted process relies on provided by Windows Hooks. As pointed out in MSDN a hook is a trap in the system message-handling mechanism. An application can install a custom filter function to monitor the message traffic in the system and process certain types of messages before they reach the target window procedure.
A hook is normally implemented in a DLL in order to meet the basic requirement for system-wide hooks. The basic concept of that sort of hooks is that the hook callback procedure is executed in the address spaces of each hooked up process in the system. To install a hook you call SetWindowsHookEx() with the appropriate parameters. Once the application installs a system-wide hook, the operating system maps the DLL into the address space in each of its client processes. Therefore global variables within the DLL will be "per-process" and cannot be shared among the processes that have loaded the hook DLL. All variables that contain shared data must be placed in a shared data section. The diagram bellow shows an example of a hook registered by Hook Server and injected into the address spaces named "Application one" and "Application two".
Figure 1

A system-wide hook is registered just ones when SetWindowsHookEx() is executed. If no error occurs a handle to the hook is returned. The returned value is required at the end of the custom hook function when a call to CallNextHookEx() has to be made. After a successful call to SetWindowsHookEx() , the operating system injects the DLL automatically (but not necessary immediately) into all processes that meet the requirements for this particular hook filter. Let's have a closer look at the following dummy WH_GETMESSAGE filter function:
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//---------------------------------------------------------------------------

// GetMsgProc

//

// Filter function for the WH_GETMESSAGE - it's just a dummy function

//---------------------------------------------------------------------------

LRESULT CALLBACK GetMsgProc(
int code, // hook code

WPARAM wParam, // removal option

LPARAM lParam // message

)
{
// We must pass the all messages on to CallNextHookEx.

return ::CallNextHookEx(sg_hGetMsgHook, code, wParam, lParam);
}
A system-wide hook is loaded by multiple processes that don't share the same address space.
For instance hook handle sg_hGetMsgHook, that is obtained by SetWindowsHookEx() and is used as parameter in CallNextHookEx() must be used virtually in all address spaces. It means that its value must be shared among hooked processes as well as the Hook Server application. In order to make this variable "visible" to all processes we should store it in the shared data section.
The following is an example of employing #pragma data_seg(). Here I would like to mention that the data within the shared section must be initialized, otherwise the variables will be assigned to the default data segment and #pragma data_seg() will have no effect.
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//---------------------------------------------------------------------------

// Shared by all processes variables

//---------------------------------------------------------------------------

#pragma data_seg(".HKT")
HHOOK sg_hGetMsgHook = NULL;
BOOL sg_bHookInstalled = FALSE;
// We get this from the application who calls SetWindowsHookEx()'s wrapper

HWND sg_hwndServer = NULL;
#pragma data_seg()
You should add a SECTIONS statement to the DLL's DEF file as well
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SECTIONS
.HKT Read Write Shared
or use
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#pragma comment(linker, "/section:.HKT, rws")
Once a hook DLL is loaded into the address space of the targeted process, there is no way to unload it unless the Hook Server calls UnhookWindowsHookEx() or the hooked application shuts down. When the Hook Server calls UnhookWindowsHookEx() the operating system loops through an internal list with all processes which have been forced to load the hook DLL. The operating system decrements the DLL's lock count and when it becomes 0, the DLL is automatically unmapped from the process's address space.
Here are some of the advantages of this approach:
o This mechanism is supported by NT/2K and 9x Windows family and hopefully will be maintained by future Windows versions as well.
o Unlike the registry mechanism of injecting DLLs this method allows DLL to be unloaded when Hook Server decides that DLL is no longer needed and makes a call to UnhookWindowsHookEx()
Although I consider Windows Hooks as very handy injection technique, it comes with its own disadvantages:
o Windows Hooks can degrade significantly the entire performance of the system, because they increase the amount of processing the system must perform for each message.
o It requires lot of efforts to debug system-wide Windows Hooks. However if you use more than one instance of VC++ running in the same time, it would simplify the debugging process for more complex scenarios.
o Last but not least, this kind of hooks affect the processing of the whole system and under certain circumstances (say a bug) you must reboot your machine in order to recover it.
3. Injecting DLL by using CreateRemoteThread() API function
Well, this is my favorite one. Unfortunately it is supported only by NT and Windows 2K operating systems. It is bizarre, that you are allowed to call (link with) this API on Win 9x as well, but it just returns NULL without doing anything.
Injecting DLLs by remote threads is Jeffrey Ritcher's idea and is well documented in his article [9] "Load Your 32-bit DLL into Another Process's Address Space Using INJLIB".
The basic concept is quite simple, but very elegant. Any process can load a DLL dynamically using LoadLibrary() API. The issue is how do we force an external process to call LoadLibrary() on our behalf, if we don't have any access to process's threads? Well, there is a function, called CreateRemoteThread() that addresses creating a remote thread. Here comes the trick - have a look at the signature of thread function, whose pointer is passed as parameter (i.e. LPTHREAD_START_ROUTINE) to the CreateRemoteThread():
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DWORD WINAPI ThreadProc(LPVOID lpParameter);
And here is the prototype of LoadLibrary API
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HMODULE WINAPI LoadLibrary(LPCTSTR lpFileName);
Yes, they do have "identical" pattern. They use the same calling convention WINAPI, they both accept one parameter and the size of returned value is the same. This match gives us a hint that we can use LoadLibrary() as thread function, which will be executed after the remote thread has been created. Let's have a look at the following sample code:
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hThread = ::CreateRemoteThread(
hProcessForHooking,
NULL,
0,
pfnLoadLibrary,
"C:\\HookTool.dll",
0,
NULL);

By using GetProcAddress() API we get the address of the LoadLibrary() API. The dodgy thing here is that Kernel32.DLL is mapped always to the same address space of each process, thus the address of LoadLibrary() function has the same value in address space of any running process. This ensures that we pass a valid pointer (i.e. pfnLoadLibrary) as parameter of CreateRemoteThread().
As parameter of the thread function we use the full path name of the DLL, casting it to LPVOID. When the remote thread is resumed, it passes the name of the DLL to the ThreadFunction (i.e. LoadLibrary). That's the whole trick with regard to using remote threads for injection purposes.
There is an important thing we should consider, if implanting through CreateRemoteThread() API. Every time before the injector application operate on the virtual memory of the targeted process and makes a call to CreateRemoteThread(), it first opens the process using OpenProcess() API and passes PROCESS_ALL_ACCESS flag as parameter. This flag is used when we want to get maximum access rights to this process. In this scenario OpenProcess() will return NULL for some of the processes with low ID number. This error (although we use a valid process ID) is caused by not running under security context that has enough permissions. If you think for a moment about it, you will realize that it makes perfect sense. All those restricted proces