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Created: Tuesday, July 18 2006 00:51.37 CDT  
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PaiMei Hooking Library
Author: pedram # Views: 5945

For those of you who are interested, Ero and I are giving a 2-day course in Vegas at Blackhat on Reverse Engineering on Windows. While the class will have a malware centric focus, the main purpose of the course is to glean general reversing knowledge and techniques. In the process PaiMei will certainly be covered, explored and experimented with. With that shameless self plug said and done, onto the main reason behind this posting.

I was talking with Gera about an older project of his where he combined the IDA debugger with some OpenGL code to create a real-time plot of heap manipulations. We decided that the functionality should be easy to port to PaiMei, but first a convenient interface to API hooking was needed. As such, I wrote utils.hook_container. I haven't updated the PaiMei release to include this just yet, but if you want to try it out simply copy the file over to your utils directory and modify utils/__init__.py to include the proper references.

Now you can easily hook arbitrary API calls. To start, instantiate a container that will house the various hooks:

    hooks = utils.hook_container()

Next resolve and add hooks for your target functions. In our case, we will need to hook RtlAllocateHeap, RtlFreeHeap and RtlReAllocateHeap. All located within NTDLL.DLL:

    a = dbg.func_resolve("ntdll", "RtlAllocateHeap")
    f = dbg.func_resolve("ntdll", "RtlFreeHeap")
    r = dbg.func_resolve("ntdll", "RtlReAllocateHeap")
    hooks.add(dbg, a, 3, None, RtlAllocateHeap)
    hooks.add(dbg, f, 3, None, RtlFreeHeap)
    hooks.add(dbg, r, 4, None, RtlReAllocateHeap)

The first argument to the hook container object is an instance of PyDbg, the second is the address of the API to hook, followed by the number of arguments the API supports, a callback function for when the API is entered and finally a callback function for when the API exits. The entry-point callback provides you with the argument list, allowing you to instrument the arguments prior to passing control back to the API. The exit-point callback provides you with the argument list and return value, allowing you to instrument the return value prior to passing control back to the caller.

With that out of the way it's fairly trivial to generate and dynamically maintain a pgraph structure as well as display the results in real time through uDraw. Whenever RtlAllocateHeap is called, we'll create an orange node containing the address of the calling instruction, a blue node containing the allocation size and we'll connect the two nodes together. This is sufficient for a demo, but as we are hooking the lowest user-mode level heap manipulation routines the calling instruction address will likely lie within a Windows DLL and is not all that interesting. To improve this we could examine dbg.stack_unwind() and utilize the first address that lies within a non Microsoft DLL. Whenever RtlFreeHeap is called we will examine the arguments and remove the buffer address from the graph. Finally, whenever RtlReAllocateHeap is called, we'll resize the target buffer and paint the node yellow. We can then easily tie it to uDraw through the udraw_connector. All said and done, here is a flash excerpt from the code in action:


You can grab the code behind this application from heap_trace.py. As an experiment I tossed in some to disk rendering once the graph node count reaches 1000.

It's all pretty simple. One of the nice things about this class is that it (I think / hope) transparently takes care of various thread-related race conditions that make pairing arguments and return values more tricky than trivial.

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