WinDbg-kd, Windows Kernel Debugging

To debug a Windows kernel, here is what is needed:

  • WinDbg (for example with VisualStudio Express Edition)

  • A kernel booted in debug mode. For local debugging, the boot can be configured with these commands (on Windows<=7, the second one fails but WinDbg still supports local kernel debugging):

    bcdedit /debug on
    
    # Since Windows 8, otherwise debugging is enabled on serial port COM2
    bcdedit /dbgsettings local
    

(bcdedit configures the Boot Configuration Database)

It is then possible to run windbg -kl as administrator to start a Local Kernel debugging session.

To verify whether local kernel debugging is enabled:

cd C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\
kdbgctrl -c

In order to configure kernel debugging on a virtual machine, it is possible to use network debugging, with a key which consists in 3 words separated with dots:

bcdedit /dbgsettings NET HOSTIP:172.16.0.1 PORT:50000 KEY:a.b.c.d

# On the debugger host (breaking happens in nt!DbgBreakPointWithStatus)
WinDbg -k net:port=52000,key:a.b.c.d,target:DESKTOP-AB01CDE

To load kernel symbols, it is possible to download a Windows Symbol Package from MSDN, but it is simpler to make WinDbg download them automatically. This can be done either with an environment variable (cf. https://support.microsoft.com/en-us/kb/311503):

_NT_SYMBOL_PATH=SRV*C:\DebugSymbols*http://msdl.microsoft.com/download/symbols

# To make it persistent:
setx _NT_SYMBOL_PATH srv*C:\DebugSymbols*http://msdl.microsoft.com/download/symbols

or with WinDbg commands:

.sympath SRV*C:\DebugSymbols*http://msdl.microsoft.com/download/symbols
.reload /f

Usual commands

  • Get help about a command: .hh x (examine), .hh uu (unassemble)

  • Examine symbols starting with NtCreate in the kernel: x nt!NtCreate*.

  • Display values at an address, with optional argument L size:

    • db: display hexadecimal bytes (128 by default)

    • dc: display DWORD values and their ASCII equivalents (32 by default)

    • dd: display DWORD values (32 by default)

    • dp: display ULONG_PTR values (128 bytes by default)

    • dps: display pointers and symbols (dp with symbols)

    • dpp: like dps, with an extra pointer dereference

    • dq: display ULONG64_PTR values (128 bytes by default)

    • du: display UNICODE characters values (16 2-byte characters by default)

    • dw: display WORD values (64 by default)

    • ds /c width addr: display width characters at addr

    • dS /c width addr: display width unicode characters at addr

  • Disassemble the beginning of a function (use L2a to show 42 instructions):

    lkd> u nt!KiSystemCall64
    nt!KiSystemCall64:
    fffff800`29f9d040 0f01f8          swapgs
    fffff800`29f9d043 654889242510000000 mov   qword ptr gs:[10h],rsp
    fffff800`29f9d04c 65488b2425a8010000 mov   rsp,qword ptr gs:[1A8h]
    fffff800`29f9d055 6a2b            push    2Bh
    fffff800`29f9d057 65ff342510000000 push    qword ptr gs:[10h]
    fffff800`29f9d05f 4153            push    r11
    fffff800`29f9d061 6a33            push    33h
    fffff800`29f9d063 51              push    rcx
    
  • Display the types of fields of a structure: dt nt!_KUSER_SHARED_DATA

x86-specific commands

  • Dump the Interrupt Descriptor Table, which contains Interrupt Service Routines:

    lkd> !idt
    
    Dumping IDT: fffff8002b8e4080
    
    00: fffff80029f9aa00 nt!KiDivideErrorFault
    01: fffff80029f9ab00 nt!KiDebugTrapOrFault
    02: fffff80029f9acc0 nt!KiNmiInterrupt  Stack = 0xFFFFF8002B8FF000
    03: fffff80029f9b040 nt!KiBreakpointTrap
    04: fffff80029f9b140 nt!KiOverflowTrap
    05: fffff80029f9b240 nt!KiBoundFault
    06: fffff80029f9b340 nt!KiInvalidOpcodeFault
    07: fffff80029f9b580 nt!KiNpxNotAvailableFault
    08: fffff80029f9b640 nt!KiDoubleFaultAbort  Stack = 0xFFFFF8002B8FD000
    09: fffff80029f9b700 nt!KiNpxSegmentOverrunAbort
    0a: fffff80029f9b7c0 nt!KiInvalidTssFault
    0b: fffff80029f9b880 nt!KiSegmentNotPresentFault
    0c: fffff80029f9b9c0 nt!KiStackFault
    0d: fffff80029f9bb00 nt!KiGeneralProtectionFault
    0e: fffff80029f9bc00 nt!KiPageFault
    10: fffff80029f9bfc0 nt!KiFloatingErrorFault
    11: fffff80029f9c140 nt!KiAlignmentFault
    12: fffff80029f9c240 nt!KiMcheckAbort   Stack = 0xFFFFF8002B901000
    13: fffff80029f9c8c0 nt!KiXmmException
    1f: fffff80029f964a0 nt!KiApcInterrupt
    20: fffff80029f99fc0 nt!KiSwInterrupt
    29: fffff80029f9ca80 nt!KiRaiseSecurityCheckFailure
    2c: fffff80029f9cb80 nt!KiRaiseAssertion
    2d: fffff80029f9cc80 nt!KiDebugServiceTrap
    2f: fffff80029f96770 nt!KiDpcInterrupt
    30: fffff80029f969a0 nt!KiHvInterrupt
    31: fffff80029f96d10 nt!KiVmbusInterrupt0
    32: fffff80029f97070 nt!KiVmbusInterrupt1
    33: fffff80029f973d0 nt!KiVmbusInterrupt2
    34: fffff80029f97730 nt!KiVmbusInterrupt3
    35: fffff80029e53090 hal!HalpInterruptCmciService (KINTERRUPT fffff80029e53000)
    50: ffffd0017e709bd0 ataport!IdePortInterrupt (KINTERRUPT ffffd0017e709b40)
    60: ffffd0017e709d10 ataport!IdePortInterrupt (KINTERRUPT ffffd0017e709c80)
    80: ffffd0017e709810 i8042prt!I8042MouseInterruptService (KINTERRUPT ffffd0017e709780)
    81: ffffd0017e709590 ndis!ndisMiniportIsr (KINTERRUPT ffffd0017e709500)
    90: ffffd0017e7096d0 i8042prt!I8042KeyboardInterruptService (KINTERRUPT ffffd0017e709640)
    91: ffffd0017e709310 USBPORT!USBPORT_InterruptService (KINTERRUPT ffffd0017e709280)
    a0: ffffd0017e7091d0 serial!SerialCIsrSw (KINTERRUPT ffffd0017e709140)
    b0: ffffd0017e709e50 ACPI!ACPIInterruptServiceRoutine (KINTERRUPT ffffd0017e709dc0)
    b1: ffffd0017e709a90 storport!RaidpAdapterInterruptRoutine (KINTERRUPT ffffd0017e709a00)
                         HDAudBus!HdaController::Isr (KINTERRUPT ffffd0017e7093c0)
    ce: fffff80029e53a90 hal!HalpIommuInterruptRoutine (KINTERRUPT fffff80029e53a00)
    d1: fffff80029e53890 hal!HalpTimerClockInterrupt (KINTERRUPT fffff80029e53800)
    d2: fffff80029e53790 hal!HalpTimerClockIpiRoutine (KINTERRUPT fffff80029e53700)
    d7: fffff80029e53590 hal!HalpInterruptRebootService (KINTERRUPT fffff80029e53500)
    d8: fffff80029e53390 hal!HalpInterruptStubService (KINTERRUPT fffff80029e53300)
    df: fffff80029e53290 hal!HalpInterruptSpuriousService (KINTERRUPT fffff80029e53200)
    e1: fffff80029f97aa0 nt!KiIpiInterrupt
    e2: fffff80029e53490 hal!HalpInterruptLocalErrorService (KINTERRUPT fffff80029e53400)
    e3: fffff80029e53190 hal!HalpInterruptDeferredRecoveryService (KINTERRUPT fffff80029e53100)
    fd: fffff80029e53990 hal!HalpTimerProfileInterrupt (KINTERRUPT fffff80029e53900)
    fe: fffff80029e53690 hal!HalpPerfInterrupt (KINTERRUPT fffff80029e53600)
    
  • Read a MSR register, like STAR (syscall target):

    lkd> rdmsr c0000082
    msr[c0000082] = fffff800`29f9d040
    kd> u fffff800`29f9d040 L1
    nt!KiSystemCall64:
    fffff800`29f9d040 0f01f8          swapgs
    
  • To get the Kernel Processor Control Region (KPCR), use fs_base MSR (0xc0000100) on x86-32 and gs_base (0xc0000101) on x86-64:

    lkd> rdmsr c0000101
    msr[c0000101] = ffffd001`aef40000
    lkd> dt nt!_KPCR ffffd001aef40000
    
  • The GDT (Global Descriptor Table) can be read by a remote debugger with r gdtr and the detail of each descriptors can be seen with commands such as dp @cs. When using a local debugger, it is nevertheless possible to compile a userland program which shows the result of sgdt instruction, which is not privileged, and then use the pointer in WinDBG with nt!_KGDTENTRY (or nt!_KGDTENTRY64) structure.

NatVis and JavaScript (since Windows 10)

Since Windows 10, WinDbg supports NatVis (Native Visualization, which was added to Visual Studio 2013).

This allows such a syntax to dump a structure such as an EPROCESS:

lkd> dx *(nt!_EPROCESS**)&nt!PsInitialSystemProcess
*(nt!_EPROCESS**)&nt!PsInitialSystemProcess                 : 0xffff9c8708c83040 [Type: _EPROCESS *]
    [+0x000] Pcb              [Type: _KPROCESS]
    [+0x2e0] ProcessLock      [Type: _EX_PUSH_LOCK]
    [+0x2e8] UniqueProcessId  : 0x4 [Type: void *]
    [+0x2f0] ActiveProcessLinks [Type: _LIST_ENTRY]
    [+0x300] RundownProtect   [Type: _EX_RUNDOWN_REF]
...

NatVis files are in C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\Visualizers (for example there is stl.natvis in order to represent types such as std::string, std::list, std::map, etc.)

NatVis commands:

# Define variables
dx @$myVar = "My sttring"
dx @$myVar
dx @$myVar.Length

# Show methods
dx -v @$myVar

# Show a variable with 2 levels of recursion
dx -r2 @$myVar

# Create an array from memory
dx @$testArray = (int(*)[10])0x7ffe0010
dx @$testPointersArray = (int*(**)[10])0x7ffe0010

# Copy a variable into some memory
dx *(TYPE*)0x10000 = *@$myNewContent

# Show the processor blocks
dx (nt!_KPRCB**)&nt!KiProcessorBlock, [*(int*)&nt!KeNumberProcessors]

# Enumerate all variables
dx @$vars

# Remove a variable
@$vars.Remove("myVar")

# Create an anonymous structure
dx @$myobject = new { Type = 5, Name = "Process Object" }

# Create a new function (with anonymous function syntax)
dx @$mul = (num1, num2) => num * num2

# Load a NatVis file
.nvload C:\path\to\myfile.natvis

In WinDbg, there are 3 default NatVis variables: @$curprocess, @$curthread and @$cursession:

lkd> dx @$cursession
@$cursession                 : Local KD
    Processes
    Devices

lkd> dx @$cursession.Processes
@$cursession.Processes
    [0x0]            : <Unknown Image>
    [0x4]            : <Unknown Image>
    [0x38]           : <Unknown Image>
    [0x68]           : <Unknown Image>
    [0x1bc]          : smss.exe

With LINQ (Language INtegrated Query) it is possible to process results using SQL-like functions:

# Find processes named smss.exe
dx @$cursession.Processes.Where(p=>p.Name == "smss.exe")

# Get the name of the process
dx @$cursession.Processes.Where(p=>p.Name == "csrss.exe").First()
    .KernelObject.SeAuditProcessCreationInfo.ImageFileName->Name

# List processes with their protection levels
dx -g @$cursession.Processes.Where(p=>p.KernelObject.Protection.Level != 0)
    .Select(p => new{name=p.Name, Level=p.KernelObject.Protection.Level})

# Get the names of handles opened by csrss
dx -r2 @$cursession.Processes.Where(p=>p.Name=="csrss.exe").Select(
    p => p.Io.Handles.Select(h => h.ObjectName))

In order to build complex structures from data, there are some helpers:

dx -r0 @$processList = *(nt!_LIST_ENTRY*)&nt!PsActiveProcessHead
dx -r0 @$processes = Debugger.Utility.Collections.FromListEntry(
    @$processList, "nt!_EPROCESS", "ActiveProcessLinks")

dx Debugger.Utility.Control.ExecuteCommand("!filetime 0").First()

Using JavaScript provider:

.load jsprovider.dll

# call initializeScript()
.scriptload C:\path\to\MyScriptName.js

# like .scriptload and call invokeScript()
.scriptrun C:\path\to\MyScriptName.js

dx Debugger.State.Scripts.MyScriptName.Contents.my_function(args)
dx @$scripts.MyScriptName.Contents.my_function(args)
dx @$scriptContents.my_function(args)

# call uninitializeScript() and unload
.scriptunload MyScriptName.js

In JavaScript:

  • host is a COM object for the Degugger

  • host.memory represents the target memory (method readMemoryValues(ptr, size) returns an ArrayBuffer with memory)

  • host.diagnostics.debugLog("...") prints logs

  • host.typeSystem.marshalAs(variable, "nt", "SOME_TYPE") to cast

  • new host.metadata.valueWithMetadata(myInteger, {PreferredRadix:10}) to print an integer as decimal

  • host.evaluateExpression(expr) or host.namespace.Debugger.Utility.Control.ExecuteCommand to run a debugger command (like .printf %y to convert a pointer to a symbol using MASM syntax)

To add new aliases in Javascript:

function initializeScript()
{
    return [new host.functionAlias(my_function, "myfct") /*, ... */];
}
// In WinDBG:
//   !myfct arg1, arg2
//   dx @$myfct(arg1, arg2)