Sunday, June 7, 2015

Reverse Engineering Windbg Commands for Profit

In this article, I will introduce benefit of reverse engineering Windbg for understanding the Windows kernel with looking at an undocumented command, fixing an issue in it and re-implementing the same functionality on a device driver.

Windbg is a powerful resource not only because you can see thorough run-time information even if you do not know how to manually do that but also you can learn how Windbg does that with reverse engineering it. Implementation of the !timer command is, for example, where you should examine if you want to know how to enumerate all scheduled timer callbacks.

But, what if you cannot find the command that works with internals you are interested in? In my case, I was looking for a way to list items inserted into the work queues with ExQueueWorkItem(), and documents of Windbg did not tell what command I could use.

Finding an Undocumented Command

There are many undocumented commands in Windbg. By using strings.exe against DLL files under the Windbg folder, you will get hints about them and/or where to look at. Here is a result of search for "workqueue":

Debuggers\x64>strings -n 5 -s *.dll | findstr /i workqueue
Debuggers\x64\winxp\kdexts.dll: **** NUMA Node %i RealTime WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** NUMA Node %i HyperCritical WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** NUMA Node %i SuperCritical WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** NUMA Node %i Critical WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** NUMA Node %i Delayed WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** NUMA Node %i Normal WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** NUMA Node %i Background WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** Critical WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** Delayed WorkQueue
Debuggers\x64\winxp\kdexts.dll: **** HyperCritical WorkQueue
Debuggers\x64\winxp\kdexts.dll: ExWorkQueue

With firing up IDA, I easily found that there was an undocumented (*1) command named !exqueue and it was exactly what I wanted.

kd> !
exqueue [flags]              - Dump the ExWorkerQueues

Fixing an Issue

The issue was that this commend did not work against Windows 8.1 and 10 targets.

kd> !exqueue
GetGlobalValue: unable to get NT!KeNumberNodes type size

So, I started to debug and reverse engineer kdexts.dll a bit more, then noticed that Windbg was failing to read the nt!KeNumberNodes, which is always 1 unless you use NUMA, and a solution was just patching code to set 1 to eax.

.text:00000001801161DA   lea rcx, aNtKenumbernode ; "NT!KeNumberNodes"
.text:00000001801161E1   call read_global_variable
.text:00000001801161E1   mov eax, 1
.text:00000001801161E6   mov rbx, cs:qword_1801A8500
.text:00000001801161ED   mov rcx, rbx
.text:00000001801161F0   mov rbp, rax

Here is an output:

kd> !exqueue

**** NUMA Node 0 - ( Threads: 7/4096 ) ****

 -> Priority 12 - ( Concurrency: 0/2 )
    ExWorkItem (ffffe00084a91160) 
      Routine ListWorkItems!<lambda_bae...> (fffff800746e5290) 
      Parameter (fffff800746e7220)
    ExWorkItem (ffffe0008105ff10) 
      Routine dxgkrnl!DxgkpProcessTerminationListThread (...) 
      Parameter (ffffe0008105fbb0)
    WdfWorkItem (ffffe00082b035a0) 
      Routine cdrom!IoctlWorkItemRoutine (fffff80072c6e900)
    ExWorkItem (fffff8006b8ff0c0) 
      Routine nt!CmpDelayDerefKCBWorker (fffff8006ba5d008) 
      Parameter (0000000000000000)
 -> Priority 31 - ( Concurrency: 0/2 )

 -> Associated Threads

THREAD ffffe000828fe040  Cid 0004.0170  Teb: 0000000000000000 Win32Thread: 0000000000000000 WAIT

Re-implementing the Command

My goal was, however, not to use the command; the goal was to know how it works, so I reverse engineered !exqueue more to learn the details of the work queues and how to enumerate items in them by hand.

Analyzing Windbg commands is often a lot easier than analyzing the kernel file because it contains a lot of strings that help you know what structures are dealt with.
Image1: Strings in code of !enqueue
This time was not exception. Basically, for Windows 8 and later, the command gets a NUMA node (nt!_ENODE) structure containing a reference to the work queues with looking at nt!KeNumberNodes and nt!KeNodeBlock first. Here, I follow the same procedure as !exqueue using basic commands.

There is only one NUMA node block as I do not configure NUMA.

kd> dw nt!KeNumberNodes
fffff803`f5774008  0001

kd> dps nt!KeNodeBlock
fffff803`f576c800  fffff803`f56c3240 nt!ExNode0
fffff803`f576c808  00000000`00000000
fffff803`f576c810  00000000`00000000

Then, the command refers to the _ENODE.ExWorkQueue.WorkPriQueue.EntryListHead field, which is an array of the prioritized work queues.

kd> dt nt!_ENODE ExWorkQueue.WorkPriQueue. fffff803`f56c3240
   +0x0c0 ExWorkQueues              : [8]
   +0x100 ExWorkQueue               :
      +0x000 WorkPriQueue              :
         +0x000 Header                    : _DISPATCHER_HEADER
         +0x018 EntryListHead             : [32] _LIST_ENTRY [
                                               0xfffff803`f56c3358 -
                                               0xfffff803`f56c3358 ]
         +0x218 CurrentCount              : [32] 0n0
         +0x298 MaximumCount              : 4
         +0x2a0 ThreadListHead            : _LIST_ENTRY [
                                               0xffffe000`eb78f248 -
                                               0xffffe000`ecd89a88 ]

Each element of the EntryListHead is a list of work items (nt!_WORK_QUEUE_ITEM), and the array represents priorities of each list; in other words, there are 32 work queues and each manages items with priority 0 (the lowest) to 31 (the highest) respectively.

kd> dt nt!_ENODE -a ExWorkQueue.WorkPriQueue.EntryListHead fffff803`f56c3240
          [12] _LIST_ENTRY [ 0xffffe000`846f62f0 - 0xffffe000`82bea2c0 ]

kd> dt nt!_WORK_QUEUE_ITEM 0xffffe000`846f62f0
   +0x000 List             : _LIST_ENTRY [ 0xffffe000`8305d130 -
                                           0xfffff800`6b8ca418 ]
   +0x010 WorkerRoutine    : 0xfffff800`746dd290     
                             void  ListWorkItems!<lambda_7d382b...>+0
   +0x018 Parameter        : 0xfffff800`746df220 Void

The !exqueue command shows the contents of the lists as well as associated worker threads referred by the ThreadListHead field.

All you can do, I can do :) I wrote a driver that dumps all items in each work queue to confirm that the above analysis was correct. Note that because this driver is just PoC, it works only on the 64 bit version of Win8.1 and Win10.


Analyzing Windbg commands often gives you good understanding of the Windows kernel with a less effort than analyzing the kernel file, and even if you do not find a helpful command at a glance, there may be an undocumented command which you can reverse engineer to unveil the Windows internals.

Side Notes

  1. It seems that !exqueue used to be documented and then abandoned for some reasons. I found a description about it on a help file came with the Windbg version
  • The ExWorkQueues field in the _ENODE also holds pointers to the WorkPriQueue structures. It is likely that the structures are allocated for each processor but only one associated with the processor 0 is really used. 
  • Apparently, an item does not go to the queue if any of worker threads is in a wait state (ie, waiting for an item) when the item is being queued. I guess the item is associated with a thread without being stored in the queue for performance in this case. For this reason, the command does not show the first item when multiple items are queued at once.