Articles tagged windows-internals

  1. Finding the Base of the Windows Kernel


    Recently-ish (~2020), Microsoft changed the way the kernel image is mapped and also some implementation details of hal.dll. The kernel changes have caused existing methods of finding the base of the kernel via shellcode or a leak and arbitrary read to crash. This obviously isn't great, so I decided to figure out a way around the issue to support some code I've been writing in my free time (maybe more on that later).

    Our discussion is going to start at Windows 10 1903 and then move up through Windows 10 21H2. These changes are also still present in Windows 11.

    What's the point(er)?

    Finding the base of the kernel is important for kernel exploits and kernel shellcode. If you can find the base of the kernel you can look up functions inside of it via the export table in its PE header. Various functions inside of the kernel allow you to allocate memory, start threads, and resolve other kernel module bases via the PsLoadedModuleList. Without being able to utilize kernel routines and symbols, you're pretty limited in what you can do if you're executing in kernel. Hopefully this clarifies why this post is even necessary.

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  2. Windows 10 KVAS and Software SMEP


    Kernel Virtual Address Shadow (KVAS) is the Windows implementation of Kernel Page Table Isolation (KPTI). It was introduced to mitigate the Meltdown vulnerability, which allowed an attacker that could execute code in user mode to leak out data from the kernel by abusing a side channel. While there are plenty of papers and blog posts on Meltdown and KVAS, there isn't much info on an interesting feature that KVAS enables: software SMEP. Unfortunately or fortunately, depending on your interest level in this post and Windows internals, understanding how software SMEP works requires knowledge of x86_64 paging, regular SMEP, and KVAS, so I'll be getting into those topics enough to give you an understanding of the underlying technology. Near the end I'll be running some experiments to show the internals of what I covered in the technical sections prior.

    x64 Paging on Windows

    First, I'm going to dive into a short introduction to x86_64 (4-level) paging, the structures involved, and WinDbg commands to interact with the page hierarchy, just so the experiments later on are more understandable; plus a lot of this information is almost never presented together, so I think collecting it in a here's what you need to know format is useful. If you want more info consult the Intel manuals or check out Connor McGarr's blog. Connor does a great job of explaining the basics, so you may want to read his post over before continuing here if you don't already have at least a vague understanding of multi-level paging.

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