Things to watch out for if you hook functions on Windows Vista

There are a couple of things that I have ran into that you should keep in mind if you are hooking functions and are planning to run under Windows Vista.

First, watch out for things being moved around in memory. For example, in Windows Vista, the VirtualProtect function in kernel32 and the CreateProcessA function in kernel32 are now on the same page, for the x86 build [NOTE: this is subject to rapid change with hotfixes, and may not still be the case on RTM]. If you have some code that works conceptually like so:

DWORD  OldProt;
PVOID  MyCreateProcessA;
PUCHAR _CreateProcessA;
static ULONG MyHook;

MyHook = (ULONG)&MyCreateProcessA;

VirtualProtect(_CreateProcessA, 6,

// [...] Disassembly and stub saving
//       code goes here...

// jmp dword ptr [MyHook]

_CreateProcessA[0] = 0xFF:
_CreateProcessA[1] = 0x25;
*(PULONG)(&_CreateProcessA[2]) = &MyHook;

VirtualProtect(_CreateProcessA, 6,
	OldProt, &OldProt);

… you’ll run into some strange crashes in Vista, because you might end up making the pages backing VirtualProtect’s implementation non-executable by accident. (Remember that memory protections only have page granularity.)

The solution? Use PAGE_EXECUTE_READWRITE for your “intermediate” states when hooking things.

Secondly, watch out for AcLayers.dll and ShimEng.dll. These two DLLs are the core of Microsoft’s Application Compatibility Layer, which is the engine used to apply compatibility fixes at runtime to broken programs that would otherwise fail to work on Windows Vista. (This engine is also used if you select a particular compatibility layer in the property sheet for a shortcut to an executable or an executable.)

The thing to watch out for here is that AcLayers likes to do import table hooking on various kernel32 APIs. In particular, AcLayers tends to hook GetProcAddress and then occasionally redirect returned function pointers to point into AcLayers.dll and not kernel32.dll. If you have a program that assumes that any pointer that it retrieves from kernel32.dll via GetProcAddress will remain at the same address for any other process in the same session, this can result in some unpleasant surprises.

For instance, consider the classic case of wanting to inject some code to run before the main process entrypoint of a child process. You might do something like inject some code that calls kernel32!LoadLibraryA on some DLL your application surprise, and then kernel32!GetProcAddress to get the address of a function in that DLL. Then the patch code might invoke a function in your DLL and return to the initial program entrypoint of the child process. This is actually a fairly common paradigm if you need to modify some sort of behavior of a child process. Unfortunately, it can easily break if the parent process is under the influence of the dreaded application compatibility layer.

The main problem here is that when you, say, find the address of LoadLibraryA or GetProcAddress in kernel32, AcLayers.dll steps in and actually hands you the address of a stub function inside AcLayers.dll which filters requests to load DLLs or get function pointers. This is all well and fine with the parent process; AcLayers.dll is there and can do whatever it’s work is whenever you call GetProcAddress or LoadLibraryA.

The catch is what happens when you try to make a child process call LoadLibraryA on a DLL before it runs the main program entrypoint. In this case, instead of passing a pointer into kernel32 (which is guaranteed to be present and at the same base address in every Win32 process in the same session), you are passing a pointer into AcLayers.dll to the child process. The problem case is when AcLayers.dll is not loaded immediately into the child process. Here, your patch code in the newly created child process might try to call LoadLibraryA to get your custom DLL unloaded. However, it actually tries to call an internal AcLayers.dll function – but AcLayers.dll isn’t actually loaded into the address space of the child process (or might have even been rebased), so your child process mysteriously crashes instantly. This typically manifests itself as nothing happening when you try to launch a child program, depending on computer configuration.

There is unfortunately no particularly elegant way to work around this particular problem that I have found. The best advice I have to offer here is to try and bypass any possibility that any function pointer you pass to another process (in kernel32.dll) is never intercepted by AcLayers.dll. Perhaps the most fool-proof way to do this is to manually walk the export table of kernel32.dll and locate the address of the export that you are interested in, although this is not a particularly easy task.

2 Responses to “Things to watch out for if you hook functions on Windows Vista”

  1. Marc Sherman says:

    Thanks for the heads up. We do some function hooking and I’ll be sure to watch out for this on Vista.


  2. Rob says:

    Wow I’m glad you wrote this article. I thought I was going insane.

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