Monday, August 21, 2006

“Memory forensics” related debugger extension DLLs for Microsoft Debuggers

One of the biggest problem with Windows “memory forensics” related tools is that such tools have to be updated concurrently because of new version of operating system or service pack. I’m thinking about offsets to fields inside various internal kernel structures which can vary. It is obvious that sooner or later you will have to use description of some internal structures to find digital evidence. If you write your own script or tool to parse a physical memory image you will have to take into consideration this problem. Even if you prepare signatures to grep some objects you will need information about offsets. Now just think about generic solution which is based on using symbols which can be download automatically or manually from Microsoft servers. Instead of hard coding information about Windows internal kernel structures and offsets for various versions of operating systems you just write one code for all of them. Firstly, your code is smaller (you can avoid many mistakes, too). Secondly, you can save a lot of time. So if you are lazy this solutions will be exactly for you :).

I decided to take a look closer at Microsoft Debugging Tools for Windows and debugger extensions which can be used by MS Debuggers and allow to use new debugger commands.

As we should know physical memory device objects (\\.\PhysicalMemory and \\.\DebugMemory) in Windows operating systems represent a raw data. It is impossible to load an image of such device object into WinDbg or KD because this file will be not recognized by Debuggers. Fortunately, you can convert raw data to recognizable format (crashdump format). A part of dump header format is described by Andreas Schuster at [1].


The next step is to download Debugging Tools for Windows and Symbols from [2]. If you have a direct access to the Internet you will not have to download Symbols because the debugger tool downloads Symbols for you automatically.

Debugger extension commands are exposed by DLLs. A short description about how to write new debugger commands can be found in the debugger.chm file which is installed with Debugging Tools for Windows (You have to use custom installation to install examples).

An environment described above can be used to perform offline analysis of physical memory dumps. In the other hand debugger extension DLLs can be used to verify system integrity on a live system. You have to use the livekd tool from [3] to load and execute commands exported by dll extensions.

A few useful functions which allow you to resolve Symbols or find out the offset are described below.
  1. GetOffsetByName(Symbol, Address), where the Symbol is the name of symbol like PsInitialSystemProcess or PsLoadedModuleList. The address of requested symbol is returned by the Address parameter.
  2. GetSymbolTypeId(Symbol, TypeId, Module), where the TypeId is an index within PDB file which is associated with the Module.
After that you can call other functions which take the TypeId and Module as parameters:
  • GetTypeSize(Module, TypeId, Size) to receive size of requested internal structure,
  • GetFieldName(Module, TypeId, Iteration, Name, MAX_PATH, NULL) to receive the Name of field pointed by a number = the Iteration,
  • or GetFieldOffset(Module, TypeId, Name, Offset) to receive the offset of requested field defined by the Name.

I wrote the function “offset(structname, fieldname)” which receives the offset to requested field of requested structure.

ULONG offset(CHAR *structname, CHAR *fieldname)
{

ULONG64 Module;
ULONG i1, TypeId;
CHAR Name[MAX_PATH];

g_ExtSymbols->GetSymbolTypeId(structname, &TypeId, &Module);

for (i1=0; ;i1++) {
HRESULT Hr1;
ULONG Offset=0;

Hr1 = g_ExtSymbols->GetFieldName(Module, TypeId, i1, Name, MAX_PATH, NULL);
if (Hr1 == S_OK) {
g_ExtSymbols->GetFieldOffset(Module, TypeId, Name, &Offset);
if (strcmp(Name,fieldname) == 0) {
return Offset;
}
}
else
if (Hr1 == E_INVALIDARG) {
break;
}
else {
dprintf("GetFieldName Failed %lx\n", Hr1);
break;
}
}
return 0;
}

You can call this function in the following way:

ULONG OffsetAPL = offset("_EPROCESS","ActiveProcessLinks");

Of course it is enough to use “dt _EPROCESS” command to receive the same result so now something more useful. At my website http://forensic.seccure.net [4] you can find the extension dll called hidden.dll which allows to detect all hidden processes – even hidden by the DKOM method. The command to call proper function is “!"full path to directory with hidden.dll file"hidden. allprocesses”. For example: “kd>!c:\temp\hidden.allprocesses”

You can record in external file all executed commands and results by using the command “.logappend “c:\forensics.log””.

As I mention above dll extensions can be executed at any version of Windows operating system. On a live system you have to use the livekd tool [3].

Useful links:
[1] http://computer.forensikblog.de/en/2006/03/dmp_file_structure.html
[2] http://www.microsoft.com/whdc/devtools/debugging/default.mspx
[3] http://www.sysinternals.com/Utilities/LiveKd.html
[4] http://forensic.seccure.net/tools/hidden.zip

2 comments:

Sektör said...
This comment has been removed by a blog administrator.
Alana said...

I recently came across your blog and have been reading along. I thought I would leave my first comment. I dont know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.


Maria

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