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My Kernel Debugger Won't Connect

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Hello ntdebugging readers, the Debug Ninja is back again with a quick blog this holiday season.  I recently encountered a situation where the kernel debugger could not connect to a Windows Server 2008 R2 system running in a Hyper-V virtual machine.  The configuration appeared correct; however, the debugger would not connect to the VM.

 

In windbg you can use Ctrl+Alt+D to view the debugger’s internal information flow.  In KD use Ctrl+D followed by ENTER to toggle the output.  Enabling this output I could see that the debugger was unable to read from the debug port, and that it was getting timeouts.  The error "SYNCTARGET: Timeout." is a clear indication that the debug host cannot communicate with the debug target, especially when this error appears after a “Send Break in” message.

SYNCTARGET: Timeout

 

Because I was using a named pipe on a Hyper-V VM I knew that I didn't have a bad cable, although this is a common cause of kernel debug failures.  I also knew that the configuration of the VM was correct, and I could use the debugger for other VMs on this server.  The problem was most likely with the OS running in the VM.

 

By checking Device Manager I was able to confirm that there was a problem with the configuration of the OS running in the VM.  The bcdedit settings were configured to use COM1, and this should make COM1 unavailable in the OS, however, COM1 was present in device manager.  For some reason the debugger was not capturing COM1 on boot as it was configured to.

Device Manager

 

Examining the bcd configuration of this server I found that the bcd configuration was not correct.  In the bcd store of normal Windows 7 or Windows Server 2008 R2 OS, the Windows Boot Loader sections of bcdedit have an inherit setting.  You can view this information on your system from an elevated command prompt using the command ‘bcdedit /enum all’.  Ordinarily the Windows Boot Loader inherits the {bootloadersettings}, the {bootloadersettings} inherit the {globalsettings}, and the {globalsettings} inherit the {dbgsettings}.  Without the inherit settings, the debugger configuration will not be read by the boot loader.

 

Below are the bcd settings from the broken VM.  You can see that all of the normal inherited settings are missing.

C:\Windows\system32>bcdedit /enum all

 

Windows Boot Manager

--------------------

identifier              {bootmgr}

device                  partition=C:

path                    \bootmgr

description             Windows Boot Manager

locale                  en-US

default                 {current}

displayorder            {current}

timeout                 30

 

Windows Boot Loader

-------------------

identifier              {current}

device                  partition=C:

path                    \Windows\system32\winload.exe

description             Windows Server 2008 R2 Standard (recovered)

locale                  en-US

osdevice                partition=C:

systemroot              \Windows

resumeobject            {2ec5363f-2a92-11e1-bbe4-806e6f6e6963}

usefirmwarepcisettings  No

debug                   Yes

 

Resume from Hibernate

---------------------

identifier              {2ec5363f-2a92-11e1-bbe4-806e6f6e6963}

device                  partition=C:

path                    \Windows\system32\winresume.exe

description             Windows Server 2008 R2 Standard (recovered)

locale                  en-US

inherit                 {resumeloadersettings}

filedevice              partition=C:

filepath                \hiberfil.sys

debugoptionenabled      Yes

 

Windows Memory Tester

---------------------

identifier              {memdiag}

device                  partition=C:

path                    \boot\memtest.exe

description             Windows Memory Diagnostic

locale                  en-US

 

Debugger Settings

-----------------

identifier              {dbgsettings}

debugtype               Serial

debugport               1

baudrate                115200

 

Because my only interest in this VM was to get the debugger working, I did not add all of the missing settings to the bcd store.  I was able to force the debugger configuration to be read on boot using this command:

bcdedit /set inherit {dbgsettings}

 

I hope this helps the next time you are trying to configure a debugger and it does not work.  Remember that we don't just need the debugger to be turned on and be configured; we need the settings to be inherited as well.

Comments

  • Anonymous
    December 29, 2011
    And to think today is my birthday, as well.  Thanks guys! Happy birthday to me.  Happy birthday to me!  This blog post solved a problem I was having, happy birthday to me!! JamesNT [Glad to hear it helped.  Happy Birthday James.]

  • Anonymous
    April 25, 2012
    Thanks for sharing your solution, I ran into a similar issue and this fixed it!

  • Anonymous
    February 11, 2015
    Oh my, just spent about 3 hours trying to figure this out! Thank you so much.