Big RAM and the kernel debugger data blockPublish date: Apr 28, 2017
Tags: memory volatility rekall windows
I recently was provided with a 64GB raw memory image created by the free DumpIt tool from
a Windows 2012 R2 x64 server. To my dismay, neither Volatility nor Rekall could make heads
or tails of it. After debugging a little further, I discovered that neither tool was able
to find the kernel debugger data block (
After a lot of Googling and consultation of the handy bible on this subject matter, The Art of Memory Forensics, I think I finally understand what went wrong (but I’m eagerly seeking clarification/correction from the DFIR/infosec community if I’m mistaken!).
When Volatility attempts to make sense of a Windows 64-bit RAM dump, they use the global
PsActiveProcessHead, which is a pointer to the kernel’s list of
structures (and other global variables such as
PsLoadedModuleList, which is a pointer
to the kernel’s list of drivers). Since these variables (symbols) are not exported by the
kernel, they can’t simply be looked up in
ntoskrnl. “Profiles” in tools such as
Volatility contain hard-coded addresses where it can expect to find such variables in
specific versions of Windows with specific patches/hotfixes applied (and so it knows which
symbols, data structures, and algorithms to use when parsing).
Within the Kernel Processor Control Region (KPCR) data structure there is a field that
contains a pointer to a
_DBGKD_GET_VERSION64 structure which contains a linked list of
_KDDEBUGGER_DATA64 structures (kernel debugger data blocks). According to The Art of Memory Forensics,
“The debugger data structure is typically located inside the NT kernel module (
nt!KdDebuggerDataBlock). It contains a build string such as
3790.srv03_sp2_rtm.070216-1710, numerical values that indicate the major and minor build numbers, and the service pack level for the target operating system…. [T]he debugger data block also contains pointers to the start of the active process and loaded module lists…. Volatility scans for the
_KDDEBUGGER_DATA64by looking for constant values embedded in the structure, including the hard-coded 4-byte signature of KDBG…. Additionally, in some cases there may be more than one
_KDDEBUGGER_DATA64structure resident in physical memory. This can happen if the target system hasn’t rebooted since applying a hot patch that updated some kernel files, or if the machine rebooted so quickly that the entire contents of RAM was not flushed…. [M]any Volatility plugins rely on finding the debugger data block and then walking the active process and loaded module lists” (pp. 62–65).
The kernel debugger data block is used by the kernel debugger so that it can find active processes, handles to objects, loaded modules, etc. when figuring out the cause of a crash. The data in this structure is relatively stable so that Windows (and tools such as WinDbg) can find them, so the memory addresses of many kernel variables can be readily identified using this structure. This makes it very handy for memory forensics.
However, the kernel debugger data block is encoded on some 64-bit versions of Windows Vista and later with large amounts of RAM and some tools do not properly decode it when creating a raw memory image (such as DumpIt). DumpIt only properly decodes this structure when a crashdump is taken (rather than a raw image). Takahiro Haruyama explains this in detail in a blog post he wrote in 2014 entitled “64bit Big Sized RAM Image Acquisition Problem” (he used the term “encrypted” rather than “encoded”, but the latter term is used by Volatility in a blog post referencing Haruyama’s post so I’ve elected to echo that language).
As a result, if using DumpIt, you must take a crashdump rather than a raw image. As of the time that Haruyama wrote his post in early 2014, this was the only supported tool that he tested. However, I was able to successfully acquire and analyze the RAM in AFF4 format using winpmem 2.1.post4 and Rekall 1.6.0 (Gotthard), respectively (your mileage may vary). This is because Rekall no longer relies on the kernel debugger data block to identify the necessary global variables/symbols. According to the Rekall documentation,
“Rekall no longer uses the Kernel Debugger Block for analysis — instead accurate global symbol information are fetched from Microsoft PDB files containing debugging symbols.”
It was a good learning experience and now I know to be wary of certain collection tools when collecting large amounts of RAM from Windows 64-bit Vista+ systems! I figured I’d share the lesson. I must also give credit to Brian Moran as his blog post on this issue was my first stop when Googling this issue and was very helpful as I researched this (he also ran into this issue in 2014).
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