This post could really be extended and divided into a few posts, but I decided to try and keep it small as much as I can. If I see it draws serious attention I might elaborate on the topic.
Signature matching for finding functions is a very old technique, but I haven’t found anyone who talks about it with juicy details or at all, and decided to show you a real life example. It is related to the last post about finding service functions in the kernel. The problem is that sometimes inside the kernel you want to use internal functions, which are not exported. Don’t start with “this is not documented story”, I don’t care, sometimes we need to get things done no matter what. Sometimes there is no documented way to do what you want. Even in legitimate code, it doesn’t have to be a rootkit, alright? I can say, however, that when you wanna add new functionality to an existing and working system, in whatever level it might be, you would better depend as much as you can on the existing functionality that was written by the original programmers of that system. So yes, it requires lot of good reversing, before injecting more code and mess up with the process.
The example of a signature I’m going to talk about is again about getting the function ZwProtectVirtualMemory address in the kernel. See the old post here to remember what’s going on. Obviously the solution in the older post is almost 100% reliable, because we have anchors to rely upon. But sometimes with signature matching the only anchors you have are binary pieces of:
* immediate operand values
* disassembled instructions
* a call graph to walk on
and the list gets longer and can really get crazy and it does, but that’s another story.
I don’t wanna convert this post into a guideline of how to write a good signature, though I have lots of experience with it, even for various archs, though I will just say that you never wanna put binary code as part of your signature, only in extreme cases (I am talking about the actual opcode bytes), simply because you usually don’t know what the compiler is going to do with the source code, how it’s going to look in assembly, etc. The idea of a good signature is that it will be as generic as possible so it will survive (hopefully) the updates of the target binary you’re searching in. This is probably the most important rule about binary signatures. Unfortunately we can never guarantee a signature is to be future compatible with new updates. But always test that the signature matches on a few versions of the binary file. Suppose it’s a .DLL, then try to get as many versions of that DLL file as possible and make a script to try it out on all of them, a must. The more DLLs the signature is able to work on successfully, the better the signature is! Usually the goal is to write a single signature that covers all versions at once.
The reason you can’t rely on opcodes in your binary signature is because they get changed many times, almost in every compilation of the code in a different version, the compiler will allocate new registers for the instructions and thus change the instructions. Or since code might get compiled to many variations which effectively do the same thing, I.E: MOV EAX, 0 and XOR EAX, EAX.
One more note, a good signature is one that you can find FAST. We don’t really wanna disassemble the whole file and run on the listing it generated. Anyway, caching is always a good idea and if you have many passes to do for many signatures to find different things, you can always cache lots of stuff, and save precious loading time. So think well before you write a signature and be sure you’re using a good algorithm. Finding an XREF for a relative branch takes lots of time, try to avoid that, that should be cached, in one pass of scanning the whole code section of the file, into a dictionary of “target:source” pairs, with false positives (another long story) that can be looked up for a range of addresses…
I almost forgot to mention, I used such a binary signature inside the patch I wrote as a member of ZERT, for closing a vulnerability in Internet Explorer, I needed to find the weak function and patch it in memory, so you can both grab the source code and see for yourself. Though the example does use opcodes (and lots of them) as part of the signature, but there’s special reason for it. Long story made short: The signature won’t match once the function will get officially patched by MS (recall that we published that solution before MS acted), and then this way the patcher will know that it didn’t find the signature and probably the function was already patched well, so we don’t need to patch it on top of the new patch.. confusing shit.
The reason I find signatures amazing is because only reversers can do them well and it takes lots of skills to generate good ones,
And surprisingly I found the following link which is interesting: http://wiki.amxmodx.org/Signature_Scanning
So let’s delve into my example, at last.
Here’s a real example of a signature for ZwProtectVirtualMemory in a Kernel driver.
Signature Source Code
From my tests this signature worked well on many versions…though always expect it might be broken.