Category Archives: Windows

WinDbg: Custom workspace and color scheme

WinDbg is a great debugging tool but it comes with very simple and minimalist workspace and color scheme.

This paper outlines main steps and techniques used to customize WinDbg. I also provide a configuration file of my debugging environment.

Workspaces Customization

WinDbg stores theme settings in a registry key located under HKCU\Software\Microsoft\Windbg\Workspaces. Importing a new theme is as simple as overwriting this registry key using an appropriate .reg file. Deleting this key will clear all of your workspace data.

The themes subdirectory where the WinDbg is installed contains several themes stored as .reg files. There is a also a themes.doc file that describes loading workspaces as well as providing screenshots of the provided workspaces.

Steps required to customize a workspace:

– Backup HKCU\Software\Microsoft\Windbg\Workspaces key in case you need to revert your changes.

– Clear all of your workspace data by deleting the previous registry key or import a theme (such themes/standard.reg) to start setting things up.

– Start WinDbg with no command-line parameters. This will open the default workspace.

– Tweak your theme:

  • Add all the windows you wish to see (Command, Calls, Watch, Locals, Registers …) and adjust their positioning.
  • To change the font, choose Font… from the View menu.
  • To change the syntax colors, choose Options… from the View menu. Then, in the Colors area, select a syntax element and click the Change button to change the color.
  • Optionally set paths (symbol path, source path, executable image path …) to make sure that WinDbg can locate all of the files that it needs to debug effectively.

– Open File menu then select Save Workspace then exit WinDbg.  Answer yes to “save information for workspace ?”. The newly created workspace will now be remembered as your default workspace.

– Export HKCU\Software\Microsoft\Windbg\Workspaces to a .reg file in order to share or backup your workspaces.

My settings

Below is my WinDbg debugging environment layout (light grey theme):

 

WinDbg_GreyTheme

You can download my registry key for this settings from WinDbg_GreyTheme_reg (rename to WinDbg_GreyTheme.reg after download). This registry key contains some path information that is specific to my machine (for instance, the placeholder window for the Source window) that will need to be updated once installed on your machine.

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WinDbg: Some debugging commands

This article describes some WinDbg commands that can be used on daily basis.

Getting Help

– .help : Display a list of all meta-commands.
– .hh command : Open help file index for the specified command.
– ! [ExtensionDLL.]help : Display help text that describes the extension commands exported from ExtensionDLL or from the extension dll at the top of the chain.

Sources

– .srcpath : Display the source search path.
– .srcpath+ path : Append path to the current source file search path.

– .srcnoisy : Display the current status of noisy source loading.
– .srcnoisy {0 | 1 | 2 | 3} : control the verbosity level for source file loading.

– .cls : Clear the Debugger Command window display.

– lsf FileName : Load the source file FileName.
– lsf- : Unload the source file FileName.

– ls first, count : Display count lines starting from first line from the current source file.
– lsp Leading Trailing : Specify Leading and Trailing lines to show before and after the current line.

– lsc : Display the current source file name and line number.
– lse : Open an editor for the current source file.

– l+l / l-l : Show/turn of source line numbers at the command prompt.
– l+s / l-s : Display/hide source lines and source line numbers at the command prompt.
– l+t / l-t : Start source mode / set the debugger is in assembly mode.

– .lines [-e | -d | -t] : Toggle source-line information support [enable | disable | toggle].

Symbols

– x *! : List all modules.
– x module!* : List all symbols loaded from module.
– x /t /v module!* : List all symbol in module with data type, symbol type and size.
– x module!pattern* : show me all the symbols loaded from module that begin with pattern.
– x module!*pattern* : Finds all of the symbols in module that contain the string pattern.

– .sympath+ path : Add symbols located in path.
– !sym noisy : Activate noisy symbol loading.

– .reload /f driver.sys : Force the debugger to immediately reload symbols for driver.sys.
– .reload /u driver.sys : Unload driver.sys and all its symbols.

Loaded modules

– lm : Display all loaded and unloaded modules.
– lm f : List loaded modules with full path.
– lm t : List loaded modules with last modified timestamp.
– lm v m module : Display all possible information for module.

– !dlls -v -c ntdll : Display table entries and version information for ntdll.dll, including Loadcount.

– !lmi driver.sys : Display detailed information about driver.sys, including symbol information.
– !dh ndis : Display file headers for ndis.sys.

Processes

– |* : Print status of all processes being debugged.
– |. : Print status of the current process.

– .tlist : Lists all processes running on the system.
– !peb : Display formatted view of the process’s environment block (PEB).

– !process 0 0 : List all active processes with basic details.
– !process 0 7 process : Display full details for process.

Threads

– ~ : Display status for all threads in the current process.

– ~* k : Call stack for all threads.
– ~number n : Suspend thread whose ordinal is number.
– ~number m : Resume thread whose ordinal is number.
– ~number f : Freeze thread whose ordinal is number.
– ~number u : Unfreeze thread whose ordinal is number.
– ~number s : Switch to thread whose ordinal is number.

– ~# f : Freeze the thread causing the current exception.

– !thread : Display current thread on the target system.
– !thread -t ThreadID : Dump thread which ID is ThreadID.
– !ready : Display summary information about each thread in the system in a READY state.

– !analyse -v : Display verbose information about the current exception or bug check.
– !analyze -show BugCheckCode : Display information about BugCheckCode bug check code.

Breakpoints

– bl – List existing breakpoints. Each breakpoint listed has a number in the list.
– bc * : Clear all breakpoints.
– bc number : Clear breakpoint identified by number.
– be number : Enable breakpoint identified by number.
– bd number : Disable breakpoint identified by number.

– bp `module!source.c:20` : Set breakpoint at source.c line 20 in module.
– bm module!pattern* : Set a breakpoint on symbols starting with pattern in module.
– bu module!function : Set a breakpoint on function as soon as module is loaded.

– ba r4 variable : Set a breakpoint for read access on 4 bytes of variable.
– ba w4 address : Set a breakpoint for write access on 4 bytes at address.
– bp @@(class::method) : Break on method defined in class. Useful if the same method is overloaded and thus present on several addresses.

– bp module!function /1 : Trigger only once a breakpoint at function in module.
– bp module!function k : Hit breakpoint at function in module after k-1 passes.

– ba w4 address “k;g” : Display call stack every write access on 4 bytes at address.
– bu module!function “.dump C:\Dump.dmp; g” : Create a dump in C:\Dump.dmp every time breakpoint at function in module is hit.

– bp /t thread : Set a kernel mode breakpoint that only triggers when hit in the context of the associated thread.
– bp /p process : Set a kernel mode breakpoint that only triggers when hit in the context of the associated process.

-.logopen FilePath; .bpcmds; .logclose : Save breakpoints to FilePath.
– $<FilePath : Reload breakpoints from FilePath.

Tracing and stepping

– g : Start executing the given process or thread.
– g `:number`; ? poi(variable); g : Executes the current program to source line number, print the value of variable then resume execution.
– gc : Resume execution from a conditional breakpoint.
– gu : Execute until the current function is complete.
– gh : Go with Exception Handled.
– gn,: Go with Exception Not Handled.
– p : Step over.
– t : Step in.
– pr : Toggle displaying of registers.
– p count “kb” : Step through count source lines then execute “kb”.
– pc : Step to next CALL instruction.
– pt – Steps through until the next return instruction.
– pa address : Step until address is reached.
– wt : Trace and watch the execution flow of a function and its sub-functions.
– wt -l Depth -oR : Trace sub-functions to Depth and display their return values.

Call stack

– k : Display call stack.
– kn : Display call stack with frame numbers.
– kb : Display call stack with first three parameters passed to each function.
– kb FrameCount : Display first FrameCount frames only.
– kp : Display all of the parameters for each function that is called in the stack trace.
– kn : Display frame numbers.

– !findstack symbol 2 : Display all stacks that contain symbol.

– .frame : Show current frame.
– .frame FrameNumber : Set frame FrameNumber for the local context.
– .frame /r FrameNumber : Display registers in frame FrameNumber.

– !running -ti : Dump the stacks of each thread that is running on all processors.
– !stacks : Give a brief summary of the state of every thread.

Registers

– rm ? : Show possible Mask bits.
– rm 1 : Enable integer registers only.
– r : Display the integer registers.
– r eax, edx : Display only eax and edx.
– r eax=5, edx=6 : Assign new values to eax and edx.
– r eax:1ub : Display only the first byte from eax.
– rF : Display the floating-point register.

Variables

– dv /t /i /V : Dump local variables with type information, addresses and EBP offsets and classify them into categories.
– dt module!pattern* -v -s Length : List with verbose output all variables that begin with pattern in module that have Length bytes size.

– dt ntdll!_PEB : Dump _PEB structure.
– dt module!struct : Show fields of the structure struct defined in module with their offsets and types.
– dt module!struct -rCount : Dump fields of the structure struct defined in module recursively for Count levels.

– dt module!struct var. : Dump var defined in strcut in module and expand its subfields.
– dt module!struct var.. : Expand subfields of var defined in strcut in module for 2 levels.

Memory

– dd address : Display double-words at address.
– dd address LLength: Display Length double-words at address.
– du address : Display unicode chars at address.
– du address LLength : Display Length unicode chars at address.
– !mapped_file address : Display name of file that contains address.
– !address : Show all memory regions of our process.
– !address address : Retreive inforamation about a region of memory at address.

– eb address value : Set byte at address to value.
– ew address value : Set word at address to value.
– ed address value : Set double-word at address to value.

– ds /c width address : Display width chars at address.
– dS /c width address : Display width unicode chars at address.

– c address1 LLength address2 : Compare Length bytes at address1 with address2.
– m address1 LLength address2 : Move Length bytes at address1 to address2.
– f address LLength ‘A’ ‘B’ ‘C’ – Fill memory location from address to address + Length – 1 with the pattern “ABC”, repeated as many times as necessary.

– s -a address LLengthpattern” : Search memory location from address to address + Length – 1 for pattern.
– s -wa address LLength “pattern” : Search only writable memory from address to address + Length – 1 for pattern.

– !poolused : Display memory use summaries, based on the tag used for each pool allocation.
– !vm : Display summary information about virtual memory use statistics on the target system.

– u address : Unassemble code at address.

Heap

– dt ntdll!_HEAP : Dump _HEAP structure.
– !heap : List all heaps with index and Heap address.
– !heap -h : List all of the current process heap with start and end addresses.
– !heap -h HeapIndex : Display detailed heap information for heap with index HeapIndex.
– !heap -s 0 : Display summary for all heaps including reserved and committed memory …
– !heap -flt s 0x50 : Display all of the allocations of size 0x50.
– !heap -stat -h address : Display heap usage statistics for HeapHandle is equal to address.
– !heap -b alloc tag HeapIndex : Breakpoint in heap with index HeapIndex on HeapAlloc calls with TAG equal to tag.
– !heap -p -all : Display details of all allocations in all heaps in the process.
– !heap -l : Make the debugger detect leaked heap blocks.

Memory dump

– .dump FileName : Dump small memory image into FileName.
– .dump /ma FileName : Dump complete memory image into FileName.

Locks

– !locks : Display all kernel mode locks held on resources by threads.
– !qlocks : Display the state of all queued spin locks.

Extension DLLs

– .load ExtensionDLL : Load the extension DLL ExtensionDLL into the debugger.
– .unload ExtensionDLL : Unload the extension DLL ExtensionDLL.

– .chain : List all extensions that the debugger has loaded.
– .unloadall : Unload all extension DLLs from the debugger.
– .setdll ExtensionDLL : Change the default extension DLL to ExtensionDLL for the debugger

Application Verifier

– !avrf : Display a variety of output produced by Application Verifier. If a Stop has occurred, reveal the its nature and what caused it.
– !verifier 0xf : Display the status of Driver Verifier and its actions.
– !verifier 0x80 address : Display log associated with the specified address within the kernel pool Allocate and Free operations.
– !verifier 0x100 address : Display log associated with the IRP at address.

Signing Windows 7 x64 drivers

For security reasons, 64-bit versions of Windows Vista and newer require the drivers to be digitally signed to load. This strict driver signing policy leads to one of the following:

  • The driver must contain an embedded driver signature.
  • The driver must be distributed together with an INF file and a signed catalog file.

For development and test purposes, the signing requirement can be bypassed using several methods. This tutorial focus on using a test certificate and the Test Mode to install and load self-signed drivers.

Prerequisites

  • Windows 7 x64.
  • WDK version 7.1.0 (free download from the Microsoft Download Center).

Once you are familiar with driver signing tools and requirement, you can update the previous configuration to target other platforms.

Download the WDK’s ISO file, extract it to your hard drive (I recommend using 7-Zip) then run KitSetup.exe to install it into C:\WinDDK\7600.16385.1\ (default folder).

My working folder is C:\Briolidz\MyDrivers.

Windows Test Mode

The Test Mode allow loading any type of test-signed kernel-mode code by adding the TESTSIGNING boot configuration option.

To activate Test Mode you need to start a command prompt with administrator rights then execute the following command (use off switch to disable it):

C:\Windows\system32> bcdedit /set testsigning on

Restart the computer for the change to take effect.

Upon reboot you will notice that the Test Mode watermark is present on the desktop. If you can’t stand seeing this watermark you can use Remove Watermarks to remove it. I highly recommend backing up the original user32.dll.mui file on your system before it gets patched.

Create certificate

– Open with administrator rights the WDK command prompt : Start -> Windows Driver Kits -> WDK 7600.16385.1 -> Build Environments -> Windows 7 -> x64 Free Build Environment. Then move to the working folder:

C:\WinDDK\7600.16385.1> cd C:\Briolidz\MyDrivers

– Create a certificate and install it into the Trusted Root Certification Authorities store:

C:\Briolidz\MyDrivers> makecert.exe -$ individual -r -pe -ss "Briolidz Certificate Store" -n CN="Briolidz Certificate" "Briolidz Certificate.cer"

C:\Briolidz\MyDrivers> certmgr.exe /add "Briolidz Certificate.cer" /s /r localMachine root

C:\Briolidz\MyDrivers> certmgr.exe /add "Briolidz Certificate.cer" /s /r localMachine trustedpublisher

– Verify that the certificate is correctly installed in “Briolidz Certificate Store” -> Certificates:

C:\Briolidz\MyDrivers> %WINDIR%\system32\certmgr.msc

Driver without an INF file

– Sign the driver (for instance MyDriverNoInf.sys) with the certificate:

C:\Briolidz\MyDrivers> signtool.exe sign /v /s "Briolidz Certificate Store" /n "Briolidz Certificate" /t http://timestamp.verisign.com/scripts/timestamp.dll MyDriverNoInf.sys

– Verify the signed driver validity:

C:\Briolidz\MyDrivers> signtool.exe verify /pa /v MyDriverNoInf.sys

– Install the driver.

Driver with INF file(s)

– Move to the working folder and open your INF file (for instance MyDriverWithInf.inf) in a text editor:

Ensure that under the [version] section you have an entry specifying a .cat file (see “CatalogFile” key).  Also make sure that the date format is correct. Below a sample:

[version]
Signature   = "$Windows NT$"
Class       = NetService
ClassGUID = {4D36E974-E325-11CE-BFC1-08002BE10318}
Provider  = %Msft%
DriverVer = 01/01/2012,7.0.5019.0
CatalogFile = MyDriverWithInf.cat

– Create .cat (catalog) file:

CAT files are catalogue files that contain the information about the driver package. These need to be generated from the INF files to allow the signing to be done.

C:\Briolidz\MyDrivers> Inf2cat /driver:"C:\Briolidz\MyDrivers" /os:7_x64

Running this successfully will generate MyDriverWithInf.cat (Note that the CAT file specify Windows 7 x64 for installation). Update “/os” switch to target other operating systems.

– Sign the catalog file:

C:\Briolidz\MyDrivers> signtool.exe sign /v /s "Briolidz Certificate Store" /n "Briolidz Certificate" /t http://timestamp.verisign.com/scripts/timestamp.dll MyDriverWithInf.cat

– Some drivers have several INF files. They can be located in driver’s folder or in its subfolders. In this case, you MUST REPEAT PREVIOUS COMMANDS for each INF file.

– Install the driver.

VBScript: Recursively Search and Replace in Windows

Recently I needed a command-line tool to perform recursive search and replacement inside text files located in a directory and its subfolders under Windows platforms. Several Open Source and proprietary tools exist but I ended by writing my own VBScript for simplicity, customization and portability reasons.

Command-line:


{CScript [/nologo]|WScript} /Path/to/SearchReplaceInFiles.vbs {Folder Path} {String} {ReplaceStringWith} {*.Extension}

Where:

Folder Path: Folder and its subfolders to find files
String: The string to be searched
ReplaceStringWith: The replacement string
*.Extension: File(s) extension. It can be *.*, *.txt …

Below a sample command-line:


C:\> CScript /nologo C:\Briolidz\Dev\Tools\SearchReplaceInFiles.vbs C:\Briolidz\Dev "My Word 1" "My Word 2" *.txt

SearchReplaceInFiles.vbs forces to run in CScript to prevent a separate popup for each WScript.Echo line.

SearchReplaceInFiles.vbs source code:

'
' Recursive search and replacement inside text files located in a directory and
' its subfolders under Windows platforms.
'
' {CScript [/nologo]|WScript} /Path/to/SearchReplaceInFiles.vbs {Folder Path} {String} {ReplaceStringWith} {*.Extension}
'

' Force script to run in CScript to avoid WScript.Echo messages pop-up box
ForceCScript()

Dim Args, FolderPath, FindString, ReplaceStringWith, FSO

' Check arguments
Args = WScript.Arguments.Count
If Args <> 4 then
 WScript.Echo
 WScript.Echo "Usage : {CScript [/nologo]|WScript} /Path/to/SearchReplaceInFiles.vbs {Folder Path} {String} {ReplaceStringWith} {*.Extension}"
 WScript.Echo
 WScript.Echo " Folder Path: Folder and its subfolders to find files"
 WScript.Echo " String: The string to be searched"
 WScript.Echo " ReplaceStringWith: The replacement string"
 WScript.Echo " *.Extension: file(s) extension. Can be *.*, *.txt ..."
 WScript.Echo
 WScript.Quit(1)
end If

' Get arguments
FolderPath = WScript.Arguments(0)
FindString = WScript.Arguments(1)
ReplaceStringWith = WScript.Arguments(2)
Extension = WScript.Arguments(3)

' Creating File System Object
Set FSO = CreateObject("Scripting.FileSystemObject")

' Check extension format
If Not Left(Extension, 2) = "*." Then
 WScript.Echo("Invalid extension " & Extension)
 WScript.Quit(1)
End If

' Check destionation folder
If Not FSO.FolderExists(FolderPath) Then
 WScript.Echo("Folder " & FolderPath & " does not exist !")
 WScript.Quit(1)
End If

' Files extension
Extension = LCase(Mid(Extension, 3))

' Call the GetFiles function to get all files
WScript.Echo
WScript.Echo("Searching for *." & Extension & " files in " & FolderPath & " folder ...")
GetFiles FolderPath, Extension
WScript.Echo("Completed")

Set FSO = Nothing

Function ForceCScript()
 Dim StrArg, Str

 If Not LCase(Right(WScript.FullName, 12)) = "\cscript.exe" Then
  For Each StrArg In WScript.Arguments
   If InStr(StrArg, " ") Then StrArg = """" & StrArg & """"
   Str = Str & " " & StrArg
  Next

  StrCmd = "CScript //nologo """ & WScript.ScriptFullName & """" & Str
  Set WshShell = CreateObject("WScript.Shell")

  ' Rerun the script in CScript
  Set ObjExec = WshShell.Exec(StrCmd)

  ' Wait until the script exits
  Do While ObjExec.Status = 0
   WScript.Sleep 100
  Loop

  ' Exit with CScript's return code
  WScript.Quit ObjExec.ExitCode

  Set WshShell = Nothing
  Set ObjExec = Nothing
 End If
End Function

Function GetFiles(FolderPath, Extension)
 On Error Resume Next

 Dim ObjFolder, ObjSubFolders
 Dim ObjFiles, ObjFile

 Set ObjFolder = FSO.GetFolder(FolderPath)
 Set ObjFiles = ObjFolder.Files

 ' Write all files to output files
 For Each ObjFile In ObjFiles
  StrExtension = LCase(FSO.GetExtensionName(ObjFile))

  If Extension = "*" Or StrExtension = Extension Then
   ' Read source text file
   FileContents = GetFile(ObjFile)

   ' Replace all string In the source file
   NewFileContents = Replace(FileContents, FindString, ReplaceStringWith, 1, -1, 1)
   If NewFileContents <> FileContents Then
    ' Write result If different
    WScript.Echo(" Updating File: " & ObjFile.Path)
    WriteFile ObjFile, NewFileContents
   End If
  End If
 Next

 ' Getting all subfolders
 Set ObjSubFolders = ObjFolder.SubFolders

 ' Getting all Files from subfolder
 For Each ObjFolder In ObjSubFolders
  GetFiles ObjFolder.Path, Extension
 Next
End Function

' Read text file
function GetFile(FileName)
 If FileName <> "" Then
  Dim FS, FileStream

  Set FS = CreateObject("Scripting.FileSystemObject")

  on error resume Next
  Set FileStream = FS.OpenTextFile(FileName)
  GetFile = FileStream.ReadAll
 End If
End Function

' Write string As a text file
function WriteFile(FileName, Contents)
 Dim OutStream, FS

 on error resume Next
 Set FS = CreateObject("Scripting.FileSystemObject")
 Set OutStream = FS.OpenTextFile(FileName, 2, True)
 OutStream.Write Contents
End Function

Windows driver debugging with WinDbg and VMWare

Virtualization Software such as VMware Workstation enables driver and kernel-mode code developers to speed up development, debugging and testing resulting in faster time to deployment. Snapshots provide a fast and easy way to revert the virtual machine to a previous state. This feature greatly simplifies recreation of specific states or conditions to troubleshoot problems and system crashes.

WinDbg is a debugging tool from Microsoft for user and kernel mode debugging. WinDbg is a GUI interface and a console interface along with some debugging extensions. Using virtual machines, WinDbg can be used to debug kernel code without the need for two physical computers.

This tutorial shows how to debug a simple Windows driver running inside a VMware virtual machine with WinDbg using a single physical machine. The following typical debugging scenarios are covered:

    • Debugging host’s virtual machine: WinDbg runs inside a physical computer to debug a virtual machine.
    • Debugging between two virtual machines: WinDbg runs inside a virtual machine to debug the second one.

Basic familiarity with device driver development and kernel debugging is assumed.

Prerequisites

The working folder is C:\Briolidz\Dev\BriolidzSampDrv.

I assume that your host and guests root system partition is C:\. The kernel debugging setup will use a virtual serial port on COM2 at 115200 bps. The custom named pipe is called BriolidzDbgPipe.

These parameters can be changed to fit your own setup.

VMware machine configuration

In the first debugging scenario, you will debug a virtual machine directly over a named pipe from the host operating system. In the second scenario, you will connect two virtual machines to the same named pipe by creating a virtual null-modem cable. The following configuration steps can be easily adapted for other virtualization software such as VirtualBox and Virtual PC.

In both scenarios, you need to setup the guest virtual machine to debug as follows:

1. Powered off the virtual machine.
2. Open VM then select Settings in VMware Workstation menu.
3. In the VMware Machine Settings dialog box, click Add.
4. In the Add Hardware Wizard dialog box, select Serial Port and click Next.
5. On next page, select Output to named pipe and click Next.
6. Set Name pipe to \\.\pipe\BriolidzDbgPipe. Make sure you select This end is the server and The other end is virtual machine. Check Connect at power on then click Finish.
7. After clicking Finish, select the newly created serial port and check Yield CPU on poll.

You should have something similar to the following screenshot. Note that new serial port has number 2 which corresponds to COM2. If you get assigned another number then make sure to replace COM2 with COM<your assigned number> in the Virtual machine configuration section.

Skip this step, if you are going to use WinDbg on your host (physical computer).

So you decided to use a second virtual machine as a development and debugging machine to replace the host physical machine. In this case, do:

1. Powered off the virtual machine.
2. Open VM then select Settings in VMware Workstation menu.
3. In the VMware Machine Settings dialog box, click Add.
4. In the Add Hardware Wizard dialog box, select Serial Port and click Next.
5. On next page, select Output to named pipe and click Next.
6. Set Name pipe to \\.\pipe\BriolidzDbgPipe. Make sure you select This end is the client and The other end is virtual machine. Check Connect at power on then click Finish.
7. After clicking Finish, select the newly created serial port and check Yield CPU on poll.

You should have something similar to the following screenshot. Note that the new serial port has number 2 which corresponds to COM2. As stated earlier, if you get assigned another number then replace COM2 with COM<your number> in the Attaching WinDBG Debugger section.

Virtual machine configuration

To enable kernel debugging, you need to change Windows boot parameters. Your virtual machine will also be configured to use serial debugging on COM2 at 115200 bps. Power on your virtual machine, log in then apply the following modifications according to your operating system version.

1. Windows XP

Windows XP uses a configuration file called boot.ini on the root of the system partition (generally the C:\ drive) to control how the operating system is booted and any startup options.

The boot.ini file has the Hidden, System, and Read-Only attributes set by default. Open a command prompt, and change them:

attrib -s -h -r C:\boot.ini

The boot.ini file is a standard ASCII text editor. Double click it to edit and duplicate the matching default entry defined in boot loader section, change its title by adding [DEBUG] for instance, add /Debug, /debugport=com2 and /baudrate=115200 switches. Below, a modified sample for Windows XP professional:

[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional [DEBUG]" /noexecute=optin /fastdetect /Debug /debugport=com2 /baudrate=115200
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect

You can reduce the timeout value in order to speed up booting.

Restore the Read-Only, Hidden, and System attributes of the boot.ini file, type the following at a command prompt:

attrib +h +r +s C:\boot.ini

2. Windows Vista and 7

Windows Vista and 7 use the bcdedit command line tool to configure the boot menu. Start a command prompt with administrator privileges and run the following commands:

bcdedit /set {current} debug yes
bcdedit /set {current} debugtype serial
bcdedit /set {current} debugport 2
bcdedit /set {current} baudrate 115200

Set the operating system selection menu to be displayed when booting and loading the operating system.

bcdedit /set {bootmgr} displaybootmenu yes

You can reduce boot timeout as follows:

bcdedit /timeout 10

To view the current configuration, run:

bcdedit

Sample driver code

Depending on your debugging scenario, log in to your development machine: the host (the physical computer) or the virtual machine. You will write the necessary files to create a do-nothing sample driver called BriolidzSampDrv.sys. This driver has minimal DriverEntry and DriverUnload routines and will write some debug message.

In the working folder, create a file called BriolidzSampDrv.c containing the following source code:

#include <wdm.h>
 
void DriverUnload(PDRIVER_OBJECT pDriverObject)
{
    DbgPrint("BriolidzSampDrv: Driver unloading.\n");
}

NTSTATUS DriverEntry(PDRIVER_OBJECT DriverObject, PUNICODE_STRING RegistryPath)
{
    DriverObject->DriverUnload = DriverUnload;
	
    DbgPrint("BriolidzSampDrv: DriverEntry.\n");
	
    return STATUS_SUCCESS;
}

There are two additional files required to build a device driver: sources and makefile.

Create sources file containing the following lines:

TARGETNAME=BriolidzSampDrv
TARGETTYPE=DRIVER
SOURCES=BriolidzSampDrv.c

The makefile file only needs to contain this line:

!INCLUDE $(NTMAKEENV)\makefile.def

To install the sample driver, you will use a basic method based on a registry file. Create InstallSampDrv.reg file containing the following lines:

Windows Registry Editor Version 5.00
  
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\BriolidzSampDrv]
"Type"=dword:00000001
"Start"=dword:00000000
"ErrorControl"=dword:00000001
"Group"="Base"
"ImagePath"="\\SystemRoot\\System32\\Drivers\\BriolidzSampDrv.sys"
"Description"="Briolidz - Sample Driver"
"DisplayName"="BriolidzSampDrv"

Building the sample driver

Download the Windows Driver Kit. Run the installer and make sure to select at least Build Environments and Debugging Tools for Windows. This will install WinDbg and the necessary environments to build drivers.

When writing this tutorial, I used WDK version is 7.1.0 which include WinDbg version 6.12.2.633. Under Windows 7 professional 64-bit, the default installation folder is C:\WinDDK\7600.16385.1 and the WDk’s start menu group is called Windows Driver Kits. WinDbg is installed in C:\WinDDK\7600.16385.1\Debuggers folder. In case you are using a different WDK version, you will just have to update paths in the following sections.

From the Windows Driver Kits start menu group, launch the command-line build environment for the desired target platform and architecture. Since The BriolidzSampDrv sample driver does not use platform specific code, you can safely use the x86 Checked Build Environment and x64 Checked Build Environment under Winows 7 submenu.

For both target platforms, open the command-line build environment, change to the working directory containing the sample driver source code and enter the command:

C:\Briolidz\Dev\BriolidzSampDrv> build

The 64-bit driver is built in C:\Briolidz\Dev\BriolidzSampDrv\objchk_win7_amd64\amd64\BriolidzSampDrv.sys and the 32-bit driver is built in C:\Briolidz\Dev\BriolidzSampDrv\objchk_win7_x86\i386\BriolidzSampDrv.sys.

Sample driver installation

Reboot the guest virtual machine to debug. When the boot menu is displayed, always choose the menu item containing [debugger enabled].

If you are planning to debug the x64 driver version on Windows Vista/7 (necessarily x64), then press F8 key and select Disable Driver Signature Enforcement from the menu. This operation must be done at every boot because 64-bit Windows systems will not allow drivers to be loaded unless they have a valid digital signature.

If you cannot digitally sign your driver, you can test-sign it and enable the TESTSIGNING mode using the bcdedit command to avoid manually disabling driver signature procedure at every boot time.

After logging in a session with administrator privileges, copy the BriolidzSampDrv.sys (from your working folder) to C:\Windows\System32\drivers. Also copy the InstallSampDrv.reg file to a location of your choice then double-click it to install the driver.

Changes will take effect after the next reboot.

Attaching WinDBG Debugger

At this point your debugging platform is ready to be used. These are the main steps to follow:

1. The WinDbg command line gives you the ability to use environment variables in order to create workspaces that contain your custom debugging settings.

In the following, if you are debugging the 32-bit driver then replace C:\Briolidz\Dev\BriolidzSampDrv\objchk_win7_amd64\amd64 with C:\Briolidz\Dev\BriolidzSampDrv\objchk_win7_x86\i386.

Inside the development machine (host or virtual machine), open a command prompt and type:

set _NT_EXECUTABLE_IMAGE_PATH= C:\Briolidz\Dev\BriolidzSampDrv\objchk_win7_amd64\amd64
set _NT_SOURCE_PATH=C:\Briolidz\Dev\BriolidzSampDrv

If you chose to debug a virtual machine from your host (physical computer):

"C:\WinDDK\7600.16385.1\Debuggers\windbg.exe" -b -k com:pipe,port=\\.\pipe\BriolidzDbgPipe,resets=0,reconnect

Otherwise:

"C:\WinDDK\7600.16385.1\Debuggers\windbg.exe" -b -k com:port=com2,resets=0,reconnect

WinDbg GUI will popup and display Waiting to reconnect message.

2. Start the target virtual machine.

3. While booting, WinDbg will halt the target system. In the WinDbg command pane, set a breakpoint in DriverEntry routine as follows:

bu BriolidzSampDrv!DriverEntry

By default, DbgPrint messages do not appear in WinDbg when the driver is running on Windows Vista/7 due to filtering reasons. You can clear this filtering using this simple call:

ed nt!Kd_DEFAULT_Mask 0x8

Press g or F5 key to continue.

After few seconds (or more depending on your computer speed), WinDbg will halt again. You should see something similar to this screenshot.

At this level, you can debug the driver by setting other breakpoints, stepping in the code, monitoring variables and expressions.

Network traffic filtering technologies for Windows

Network traffic Filtering techniques for Windows, either in user-mode or kernel-mode, falls into one of two categories: stream and packet methods. This document presents useful techniques to build robust security software products such as personal firewalls and VPN clients for Windows 2000 or higher.

Before going further with this article, I would personally recommend WPF for Vista and higher, and TDI filters + NDIS Hook for earlier versions to build a combined stream and packet filtering solutions.

Winsock Layered Service Provider

A Winsock Layered Service Provider (LSP) is a DLL that operates on the Winsock functions to inspect, modify and intercept the inbound and outbound Internet traffic as streams and not as packets. LSP also runs in the workspace of the process it intercepts making easy to filter streams based on caller PID, short name or full path.

LSP can be chained and are useful tool for data-monitoring, content filtering, stream based sniffers, Quality of Service (QoS), authentication, encryption … LSP technology is often exploited by spyware and adware programs to bombard users with advertisements and email spam.

There is one known limitation and one common issue with LSPs. On some Windows versions, LSP can be bypassed by calling TCP/IP stack directly via TDI making useless, for instance, Trojan or virus protections at this level. A bogus LSP or improper LSP removal/unregistration operation may break the whole TCP/IP stack or leave the machine without working network connection.

Windows 2000/XP Filter Hook Driver

A Filter Hook driver is supported on Windows 2000/XP only and is implemented as a kernel mode driver. It operates by registering a callback with the IP Filter Driver that gets called when sending a receiving a packet. Filtering rules are limited to pass, drop or forward decision based on IP addresses and ports information.

The callback registration process uses an IRP with IOCTL_PF_SET_EXTENSION_POINTER as an IO control code and a PF_SET_EXTENSION_HOOK_INFO structure filled with a pointer to the callback routine.

A Filter Hook driver is simple to implement but has three serious limitations. Only one callback routine can be installed each time on the system. It is not possible to filter Ethernet frames. Outgoing packets cannot be modified.

Windows 2000/XP Firewall Hook Driver

A Firewall Hook driver is very similar to a Filter-Hook driver but installs a callback in the IP driver. The callback registration process uses an IRP with IOCTL_IP_SET_FIREWALL_HOOK as an IO control code and an IP_SET_FIREWALL_HOOK_INFO structure filled with a pointer to the callback routine.

Although it is not well documented, writing a Firewall Hook driver requires few lines of code. The main limitation is the support of Windows 2000 and XP only.

NDIS Hook Driver

There are two approved techniques to write an NDIS Hook driver. The first one is based on interception of some NDIS wrapper functions at runtime by writing a kernel mode driver that patches NDIS.sys in memory to replace the addresses of NdisRegisterProtocol, NdisDeregisterProtocol, NdisOpenAdapter and NdisCloseAdapter functions with internal ones.

The second one is based on registering a fake NDIS Protocol driver just to get a pointer to an internal NDIS structure NDIS_PROTOCOL_BLOCK.

At this level, both methods have enough information to substitute all protocols and adapters handlers to getting full control over all network traffic.

Although these approaches use sophisticated hacking techniques and require good understanding of different NDIS versions internals, an NDIS Hook driver is easy to install and able to filter, inject or modify packets. Several security software products including personal firewalls and VPN clients use these techniques.

This approach is discouraged for Windows Vista and higher.

NDIS Intermediate Driver

An NDIS intermediate driver, also called NDIS IM driver, is inserted just above miniport drivers and just below transport protocols in the overall networking protocol stack allowing incoming and outgoing packets filtering, inspection or modification. An NDIS Intermediate driver is a documented alternative to NDIS Hook drivers and offers the same functionalities.

NDIS intermediate drivers should be digitally signed at Microsoft to allow silent installations. This technology is replaced by NDIS Lightweight Filter drivers on Vista and higher.

NDIS Lightweight Filter Driver

NDIS Lightweight Filter drivers (LWF drivers) are introduced in Windows Vista and higher to replace NDIS Intermediate driver technology. They offer the same packets filtering, inspection or modification capabilities.

NDIS Lightweight Filter drivers are easier to implement and are designed to improve overall performances.

TDI Filter Driver

The Transport Driver Interface (TDI) defines a kernel mode network interface that is exposed at the upper edge of all transport protocol stacks. TDI also provides standard methods for protocol addressing, sending and receiving datagrams, writing and reading streams, initiating connections, detecting disconnects making it the only socket interface in the kernel.

TDI Filter drivers sit between TDI clients (such as AFD.sys, NETBT.sys) and TDI transports (such as TCPIP.sys) and intercept the communication between them. In case of TCP/IP filtering, the technique consists in writing a kernel-mode driver that layers itself over devices created by TCPIP.sys driver (\Device\RawIp, \Device\Udp, \Device\Tcp, \Device\Ip and \Device\MULTICAST) using IoAttachDevice routine. A good understanding of how to handle and interact with IRPs is required.

It is recommended to stop using TDI filters and move to Windows Filtering Platform (WFP) on Vista and later platforms. Windows makes it possible for TDI filters to see TCP/IP traffic is just for compatibility reasons and it does not yield good performance.

Windows Filtering Platform

Windows Filtering Platform (WPF) is a new architecture available in Windows Vista and higher that was built to replace all existing packet filtering technologies such as Winsock LSP, TDI filter and NDIS Intermediate driver and to provide better performance and less development complexities. Callout drivers, Filter Engine, Base Filtering Engine and Shims are components of the WPF architecture.

The WFP API consists of a user-mode API and a kernel-mode API that interacts with the packet processing that takes place at several layers in the networking stack. With WFP, incoming and outgoing packets can be filtered and modified before they reach their destinations, making this architecture ideal for implementing various filtering applications or solutions (such as personal firewalls, intrusion detection systems, antivirus programs, network monitoring tools, and parental controls). WFP arbitration rules also minimize the risk that software components get affected by any future Service Pack release.

WPF is highly recommended for developing security related solutions on Vista and higher.

NDIS driver types

Windows network driver developers interested in network interface card (NIC) drivers, Ethernet/IP packet interception, filtering and modification must have a good understanding of the Network Driver Interface Specification (NDIS).
In this document, I mainly focus on NDIS 5.0 or higher and on the most common network drivers (old Windows versions prior to 2000 are no longer widely used).

Introduction

NDIS is originally developed to hide the underlying complexity of the NIC hardware and to allow for multiple network adapters and higher level protocol drivers (such as TCP/IP, NetBEUI …) to coexist in a single computer. Early versions of NDIS were jointly developed by Microsoft and the 3Com Corporation. Versions starting from 2.0 are Microsoft proprietary specifications. The open source ndiswrapper project also allows many NDIS-compliant NICs to be used with Linux.

NDIS is implemented as kernel mode driver called NDIS.sys sometimes referred to as the NDIS wrapper. The later is a small piece of code surrounding all types of the NDIS device drivers. It aims to hide platform dependencies and to maintain state information and parameters for network drivers.
Next sections will describe most common NDIS driver types as illustrated in the following simplified diagram:

NDIS Protocol Driver

The NDIS Protocol is the highest driver in the NDIS hierarchy of drivers. At it upper edge, it exports ProtocolXxx functions to the lower edge of the transport protocol stack (such as a TCP/IP stack). At its lower edge, it interfaces with NDIS Intermediate drivers and NDIS miniport drivers.

Protocol drivers always use NDIS-provided functions to communicate with underlying NDIS drivers to send and receive packets. The NDIS wrapper also calls the ProtocolXxx functions for its own purposes or on behalf of lower-level drivers to indicate up received packets, indicate the status of lower-level drivers …
NDIS protocols driver are often used to inject or capture packets on the network.

NDIS Miniport Driver

A miniport driver is a driver that connects hardware devices to higher-level drivers (protocol drivers, Intermediate drivers and filter drivers) and implements sending and receiving data on the network adapter. The most common miniport drivers are:

    • Connectionless miniport drivers
    • Connection-oriented miniport drivers
    • NDISWAN miniport drivers
    • Non-NDIS Lower Interface miniport drivers

Connectionless miniport drivers control NICs for connectionless network media, such as Ethernet, FDDI, and Token Ring. Connectionless miniport drivers can be serialized or deserialized. Serialized drivers rely on NDIS to sequence calls to miniport functions and to manage send queues. Deserialized miniport drivers internally queue all incoming send packets rather than using NDIS. This can result in a better full duplex performance.

Connection-oriented miniport drivers control NICs for connection-oriented network media, such as ISDN. Connection-oriented miniport drivers are always deserialized and a connection must be established between two points before data can be exchanged.

NDISWAN miniport contains the necessary code to operate the dial-up equipment. They are used with ISDN, Frame Relay and X.25.

A Non-NDIS lower interface driver is a connection-oriented miniport driver that exposes a standard NDIS miniport driver interface on the top, but on the bottom can interface to devices, such as USB, IEEE 1394, and serial devices by sending I/O request packets (IRPs).

NDIS Virtual Miniport Driver

A virtual miniport driver is a miniport driver that does not interact directly with any physical network adapter. A virtual miniport driver adds a virtual adapter that shows up in the network connections and ipconfig result.

NDIS virtual miniport drivers are used in several VPN clients and virtualization softwares.

NDIS Intermediate Driver

An NDIS intermediate driver, also called NDIS IM driver, looks like a protocol driver to an underlying miniport driver and looks like a miniport driver to an overlying protocol driver. An intermediate driver is inserted just above miniport drivers and just below transport protocols in the overall networking protocol stack allowing incoming and outgoing packets filtering, inspection or modification. There are two types of NDIS intermediate driver: the LAN emulation intermediate driver and the Filter driver.

The LAN emulation intermediate driver translates packets from the overlying connectionless transport’s LAN format to the connection-oriented format (such as ATM) allow them to be sent over a separate and a different medium.

Filter drivers perform special operations (such as compression, encryption and tracing) on packets being transported through them. Various services utilize this type driver, such as the packet scheduler in Quality of Service (QoS) and Network Load Balancing.

NDIS Lightweight Filter Driver

NDIS Lightweight Filter drivers (LWF drivers) are introduced in NDIS 6.0 to replace NDIS intermediate drivers. They are typically layered between miniport adapters and protocol bindings and offer the same packets filtering, inspection or modification capabilities. NDIS Lightweight Filter drivers always use NDIS-provided functions, are easier to implement and are designed to improve overall performances.