On Mon, Aug 12, 2013 at 7:46 AM, George M. Garner Jr. <ggarner_online@gmgsystemsinc.com> wrote:

This post is not specifically about Volatility; however, I know that the topic will be of interest to many list members. Reliable memory analysis depends on reliable memory acquisition. No matter how good an analysis tool is, it can only analyze what is in the memory "dump." If the memory acquisition tool produces an incomplete or erroneous memory dump the analysis will also fail or be misleading. It is thus with pleasure that I note the progress that is being made toward developing a critical framework for evaluating and testing memory acquisition tools. The latest contribution is by Stefan Vömel and Johannes Stüttgen whose paper, "An evaluation platform for forensic memory acquisition software" was presented at the recent DFRWS conference and may be downloaded from the following link: http://www.dfrws.org/2013/proceedings/DFRWS2013-11.pdf.

The authors adopt an approach which they call "white-box testing" whereby the authors modify the source code for various open source tools (win32dd, mdd, winpmem) to insert hypercalls at various locations in the acquisition process. The hypercalls inform the test platform of various important system events and operations and inspect the state of the subject system at the moment of the hypercall. The state as recorded by the hypercalls is then used as a metric to evaluate the reliability (i.e. "correctness") of the tool which is under test.

The approach has a number of significant limitations which the authors acknowledge, the most significant of which is that they require the source code which must be modified in order to perform the test. Most commercial tool vendors do not provide the source code to their memory acquisition tools. Even one of the open source tools tested by Vömel and Stüttgen, win32dd, is an old version of the current closed-source Moonsols tool which contains many bug fixes which are not in the open source precursor. As far as I am aware the MDD tool is no longer supported or under active development. Michael Cohen's winpmem is the only currently supported tool that the authors were able to test.

Another significant limitation is that the test platform is tied to a highly customized version of the Bochs x86 PC emulator. The test platform is restricted to 32-bit versions of Windows with not more than 2 GiB of memory. The acquisition of physical memory from systems equipped with more than 4 GiB of memory and from 64-bit versions of Microsoft Windows are areas where memory acquisition tool vendors have stumbled in the past. Possibly all contemporary memory acquisition tools handle 64-bit systems and systems with more than 4 GiB of memory correctly; however, we would like to be able to test this and not rely solely on faith.

One limitation which the authors do not discuss is the impact of restricting the test platform to a particular VM (i.e. Bochs). In our experience VMs provide a much more highly predictable and stable environment than real hardware and may not be a good indication of how a memory acquisition tool will perform on real hardware. In addition, as was mentioned previously on this list, different VM manufacturers have chosen to emulate very different PC designs. How a tool performs on VMWare may not be a good indicator of how the same tool will perform on Microsoft Hyper-V or VirtualBox or KVM.

Also, the authors do not acknowledge the possibility that memory acquisition tools may perform differently on different versions of the Microsoft operating system. Each new version of the Microsoft operating system has brought changes to the Windows memory manager, in some cases significant changes. Currently, we are (out of necessity) using acquisition methods that differ in varying degrees on Microsoft Windows XP, Windows 2003 SP1 and later, Windows Vista, Windows 7 and Windows 8. For Windows 8.1 Preview we have found it necessary to invent new methods that are different from all of the methods which we previously employed.

Finally, the authors do not articulate the theoretical framework for forensic volatile memory acquisition which serves as the basis for their notion of "correctness." Historically, computer forensic experts have evaluated the acquisition of volatile memory as an "imaging" process. Most computer forensic experts were familiar with the imaging of "dead" hard drives. It was natural to assume memory acquisition was doing much the same thing. The problem is that a running computer system is by nature a stochastic process (more precisely, it is a "continuous time stochastic process") which cannot be "imaged." It can only be sampled. Sampling is a common technique in forensic science, other than in computer forensics. Computer forensics is somewhat unique among forensic "sciences" in its almost exclusive reliance upon "imaging" as a standard of reliability. It is difficult not to note the severe tension which exists between this theoretic framework and the reality of modern computer design. Not only is "live" imaging of hard drives now commonplace for practical reasons (which equally aren't true images) but also, as we now know, a hard drive is an embedded computer system which may alter the physical layout of data on the disks any time power is applied to the drive.

The theoretical approach which we have advocated (see, e.g. Garner, Maut Design Document (unpublished manuscript, 2011), is to view volatile memory acquisition as a process of sampling the state of a continuous-time stochastic process. We further propose to use the structural elements created by the operating system and the hardware as a metric to evaluate the reliability of the sampling process. We believe that these structural elements may be used for this purpose because they possess the Markhov property. Their future state is predictable, depending entirely on their present state and is conditionally independent of the past. A sample is said to be reliable when it is "representative" of the entire population. In other words, a sample is reliable with respect to a specific issue of material fact if an inference drawn upon the basis of the sample will arrive at the same conclusion as if the inference were drawn upon the basis of the entire population.

The authors appear to assume the tradition "imaging" framework for volatile computer memory acquisition, however, this assumption should be stated since the entire rest of the paper depends on it.

In conclusion, this paper makes an important contribution to a topic that is important for the future of computer forensics. However, the authors need to better articulate their assumptions. Development of a professional memory forensic tool testing platform will require the development of a test enviroment which overcomes the current limitations.

Regards,

George M. Garner Jr.
President
GMG Systems, Inc.
_______________________________________________
Vol-users mailing list
Vol-users@volatilityfoundation.org
http://lists.volatilityfoundation.org/mailman/listinfo/vol-users