Reversing in Reverse, Part 2: More Linked-List Pool Corruption

Reversing in Reverse, Part 2: More Linked-List Pool Corruption

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Hello - It's Ryan again with the second installment of my list corruption walkthrough. The previous blog post is here - Reversing in Reverse: Linked-List Pool Corruption, a Complete Walkthrough (Part 1) 

In part one we walked through the analysis of a memory.dmp collected during a bugcheck caused by pool corruption. The post also discussed doubly linked lists and demonstrated an unconventional order of debugging steps in which we did not begin our examination with the backtrace of the bad pointer value.

Today’s continuation will consist of another crash dump 'debugging walkthrough' explaining a lot of the typical commands used for debugging. This one involves pool corruption affecting a linked list which led us to a kernel-mode crash.  I will also discuss the removal of an entry from the head of a linked list.  As in the previous post, I shall provide demonstrations of the topics within the debugger in an attempt to relate the information to a real-world problem. Despite the title, we won't be working in reverse today.

 As is typical, I began this windbg session with a quick  !analyze -v to get a feel for what went wrong.

IRQL_NOT_LESS_OR_EQUAL (a)   An attempt was made to access a pageable (or completely invalid) address at an interrupt request level (IRQL) that is too high.  This is usually caused by drivers using improper addresses. If a kernel debugger is available get the stack backtrace.
Arg1: 00000004, memory referenced  <--
Looks like a BAD pointer
Arg2: d0000002, IRQL
Arg3: 00000001, bitfield :
                        bit 0 : value 0 = read operation, 1 = write operation
                        bit 3 : value 0 = not an execute operation, 1 = execute operation (only on chips which support this level of status)
Arg4: 808436e8, address which referenced memory
TRAP_FRAME:  f7906cdc -- (.trap 0xfffffffff7906cdc)

The output from this command lets us know that the code trapped accessing the invalid address 0x00000004. When we crash attempting to access low addresses like this, it is almost always due to dereferencing a null pointer. Such invalid address values are often obtained by adding some value to the pointer in an attempt to access fields in the structure that the pointer should have been referencing.  Recall in part one of this blog, the invalid address was also close to zero. The invalid address in the previous blog’s memory.dmp was obtained by subtracting negative values from a null pointer.   For example, we may find in our analysis today that we attempted to access a structure field that was located at offset 0x4 within the structure.  However, as I mentioned in part one, there are other ways that we could have ended up this value besides a NULL pointer. Some examples would include a hardware problem, following a corrupted pointer to a location that had some bad value, or some code improperly writing some value over a pointer.  Let’s start our analysis and identify what happened.

Dumping the stack without setting the trap frame shows the code executing within the PageFault Trap Handler

0: kd> kC

nt!_KiTrap0E   ß We were in the handler for Trap 0xE






0: kd> .formats 0xE 

Evaluate expression:

  Hex:     0000000e

  Decimal: 14  ßTrap number 14 which per the x86 Intel Manuals is a PageFault Trap


0: kd> rcr2    ßPageFault trap results in the address that triggered the trap being stored in the CR2 register 

Last set context:

cr2=00000004  ß Here is the invalid address


Next, we’ll set the trap frame 

0: kd> .trap 0xfffffffff7906cdc

After setting the trap frame, I looked at the stack again just to get a quick overview of what calls were made in this thread:

0: kd> kC

nt!MiRemoveUnusedSegments  <-- This is the function executing when the trap occurred.  Take notice of the function name.





Next, I went straight to the assembly to explore what happened. I’ll begin by examining the faulting instruction.

0: kd> r

Last set context:

eax=00000000 ebx=80a5e540 ecx=808ab4a8 edx=00000000 esi=86f4c658 edi=80a5e4d0

eip=808436e8 esp=f7906d50 ebp=f7906d90 iopl=0         nv up ei ng nz ac po cy

cs=0008  ss=0010  ds=0023  es=0023  fs=0030  gs=0000             efl=00010293


808436e8 c74004a8b48a80  mov     dword ptr [eax+4],offset nt!MmUnusedSegmentList (808ab4a8) ds:0023:00000004=????????


As you can see, the trap was triggered by attempting to add the value four to the null value in EAX. Also, I would like to point out that parameter number three of the bugcheck provides supporting information which is sometimes important in developing a clear understanding of what occurred: 

0: kd> .bugcheck

Bugcheck code 0000000A

Arguments 00000004 d0000002 00000001 808436e8


This is illustrated by the output of !analyze –v


Arg1: 00000004, memory referenced

Arg2: d0000002, IRQL

Arg3: 00000001, bitfield :

      bit 0 : value 0 = read operation, 1 = write operation

      bit 3 : value 0 = not an execute operation, 1 = execute operation (only on chips which support this level of status)

Arg4: 808436e8, address which referenced memory


0: kd> .formats 0x1

Evaluate expression:

  Binary:  00000000 00000000 00000000 00000001   ßWrite Operation, Not an execute operation.


If you do not need to see the entire output of !analyze –v, you can get a very abbreviated version of the !analyze–v output by using !analyze –a as displayed below.

0: kd> !analyze -a


Bugcheck Analysis                              


Use !analyze -v to get detailed debugging information.

BugCheck A, {4, d0000002, 1, 808436e8}

Probably caused by : memory_corruption ( nt!MiRemoveUnusedSegments+716 )

Followup: MachineOwner


You can get further information about this bugcheck by typing .hh bug check 0xA into the debugger

windbg> .hh bug check 0xA

This will bring up the following screen which displays helpful information.

 Help File

Now that we understand that this crash was due to a NULL value present in EAX, the next goal of this postmortem memory.dmp debug session becomes identification of the source of this NULL value. This task can often prove extremely challenging due to the fact that it sometimes involves backtracing through several highly optimized functions full of jumps without symbols. Other times, we may be at the beginning of a very simple function for which we have private symbols. Let's see how we luck out today.

 I’ll proceed by disassembling the code around the point of the trap. Here is my favorite command for accomplishing this. 

0: kd> ub @$ip L4;u . L3;r$ip


808436dc 8bf0            mov     esi,eax       

 808436de 8b06            mov     eax,dword ptr [esi]    

The first  two instructions (highlighted)  together accomplish the following “Overwrite the value in EAX with the data that EAX is presently pointing to”

808436e0 a3a8b48a80      mov     dword ptr [nt!MmUnusedSegmentList (808ab4a8)],eax

808436e5 83c6fc          add     esi,0FFFFFFFCh


808436e8 c74004a8b48a80  mov     dword ptr [eax+4],offset nt!MmUnusedSegmentList (808ab4a8)    ßHowever EAX was unexpectedly NULL here.

808436ef ff0d8cb48a80    dec     dword ptr [nt!MmUnusedSegmentCount (808ab48c)]

808436f5 33c9            xor     ecx,ecx

 Last set context:



0: kd> reax

Last set context:



In reviewing the code above, I observed that we had just moved this value from EAX to nt!MmUnusedSegmentList two instructions ago.  To summarize, the code moved a value (that obviously wasn’t expected to be NULL)  to nt!MmUnusedSegmentList and then dereferenced this value (plus four) which of course caused us to crash because 0x4 is not a valid address.

 Let's take a quick look at nt!MmUnusedSegmentList

0: kd> x nt!MmUnusedSegmentList

   808ab4a8 nt!MmUnusedSegmentList = <no type information>


0: kd> dt 808ab4a8 nt!_LIST_ENTRY

     [ 0x0 - 0x86f05234 ]

       +0x000 Flink            : (null)

            +0x004 Blink            : 0x86f05234 _LIST_ENTRY [ 0x808ab4a8 - 0x8724392c ] 


MmUnusedSegmentList is the head of a doubly-linked list. You can see that the value that we just moved to this list head's Flink(forward link) entry is NULL.  As covered in part one of this blog, a Flink is a pointer to the next _LIST_ENTRY in the list. A Blink (backwards link) is an entry to the previous _LIST_ENTRY in the list . In this case we moved a NULL Flink from EAX into the list head (MmUnusedSegmentList->Flink) and crashed trying to dereference offset 4 from this same NULL value. The flink of the list head should contain a pointer to the first _LIST_ENTRY. The flink and blink should never contain NULL values. If the list is empty, both entries will be pointing to the list head ListHead->Flink and ListHead->Blink will both contain a pointer to the ListHead itself.

The symbols have provided us with some clues to what the assembly was doing. Based on the name of this list head MmUnusedSegmentList, the fact that the code seems to be decrementing MmUnusedSegmentCount, and the function name MiRemoveUnusedSegments, it appears obvious that we trying to remove an entry from a list of unused segments. Also, I can tell that we are in a memory manager function by the Mm function prefix. You can find a list of Commonly Used Prefixes in chapter two of Mark Russinovich's book "Windows Internals 4th Edition".

 It is a common operation when working with a linked list to remove an entry from the beginning of a doubly linked list of LIST_ENTRY structures.  I’ll explain this by providing a simplified fictitious example. Let's pretend that we have a doubly linked list that is used in keeping track of unused Widgets.  Let’s also pretend that we have a function that code can call as follows:

pWidget = giveMeAWidget ()

This function first finds the head for the list of unused widgets which we are going to stored in a global variable called UnusedWidgetListHead. It then finds an available widget to return by following UnusedWidgetListHead->Flink. Recall from part one of this blog, that unless the Linked List was @ offset 0x0 from the start of the WIDGET structure, the code would have to subtract the offset of the LIST_ENTRY to reach the base of the actual WIDGET structure.  Before returning a pointer to the widget back to the calling code, it will be necessary to remove this widget from the doubly linked list of unused widgets since this widget will now be in use.  Ignoring possible synchronization requirements, the process of removing this first entry from the list would typically involve code that sets ListHead->Flink to point to the second entry in the list instead of the first entry. It would also be necessary to update the second widget’s blink member to point to the ListHead instead of the first entry that it pointed to before. This would now make entry number two the first widget therefore removing entry number one from the list.



While  reviewing the code, I noticed that a pattern in the assembly that matched the type of operation that I described above where we are removing the first entry from the list.  Let's review the assembly again, this time in more detail. We will break down the instructions to identify what transpired.  You can use . or @$ip to represent the current instruction pointer. In this case, we I’ll use the pseudo-register @$ip.


0: kd> ub @$ip L4;u @$ip L1


808436dc 8bf0            mov     esi,eax               This is copying the ListHead->Flink to esi. The code uses the ListHead flink to find ListMember1->Flink (Not an actual name, I am simply referring to the _LIST_ENTRY as ListMember# just to demonstrate what is taking place). Why does it need ListMember1->Flink? Because it points to ListMember2 which it needs to convert to the new ListMember1 in the manner described above. This is very important for our debugging since it may be able to obtain the value of Member1 from esi.

808436de 8b06            mov     eax,dword ptr [esi]    follow this flink that it just moved by dereferencing it to obtain ListMember1->Flink and place it in eax. This should be a pointer to ListMember2, however it was null.

808436e0 a3a8b48a80      mov     dword ptr [nt!MmUnusedSegmentList (808ab4a8)],eax  move this null value to ListHead->Flink. This operation should be setting the list head to point to Member2, therefore converting it to Member1. However since Member1->Flink was null, it now contains a NULL value.

808436e5 83c6fc          add     esi,0FFFFFFFCh    Earlier I mentioned that the code might be able to obtain the address of Member1 from ESI since esi had the ListHead->flink pointing to it, however ESI was modified here. This means that it can't rely on the value of esi. The code added 0x0ffffffC here;  however,  this was really just a compiler optimized (fancy) way of subtracting four.   It doesn't care why that it's subtracting four from this value since the code crashed before ever using address-4, so I won't be investigating that today. However, I suspect we were subtracting the offset of the following field.

0: kd> dt nt!_CONTROL_AREA DereferenceList

   +0x004 DereferenceList : _LIST_ENTRY

Instead of digging into that, we simply want to identify the value of the list member. To obtain our value here, we will perform the reverse of the addition operation and simply subtract 0x0FFFFFFFC as a fancy way to add four :                


0: kd> resi

Last set context:



We effectively added 4 to get the original value of esi


0: kd> ? 86f4c658-0x0FFFFFFFC

Evaluate expression: -2030778788 = 86f4c65c


So this address should contain a NULL flink


0: kd> dd 86f4c65c L2        

86f4c65c  00000000 808ab4a8

And there it is (the underlined value above) ....

And as expected in the output above, the flink was null. Also note that the blink (highlighted) was in fact pointing to 808ab4a8, which is the list head. So this does appear to be the address of the original member1. If you can't recall the address of the list head, don't scroll up in the debugger text (or this blog text), we can prove this quickly as follows;

0: kd> dd nt!MmUnusedSegmentList L1

808ab4a8  00000000


Let’s dump the address out:

0: kd> !address 86f4c65c 

  83041000 - 07fbf000                           

          Usage       KernelSpaceUsageNonPagedPool

0: kd> !pool 86f4c65c 

Pool page 86f4c65c region is Nonpaged pool

 86f4c000 size:   98 previous size:    0  (Allocated)  File (Protected)

86f4c098 is not a valid large pool allocation, checking large session pool...

bf7f4000: Unable to get contents of pool block


So we are dealing with pool corruption. Now that we have followed the NULL value in the corrupt pool and loaded EAX with zero, lets proceed with the inspection of the next instruction which caused the trap leading to the bugcheck:


nt!MiRemoveUnusedSegments+0x716 808436e8

c74004a8b48a80  mov     dword ptr [eax+4],offset nt!MmUnusedSegmentList (808ab4a8)    This instruction was attempting to place the address of the List Head into eax+4.  Why? Well, EAX has the new Member1. If we add four to this value, this would bring us to Member1->Blink which should be pointing to the list head. Had the new Member1 actually been Member2 instead of NULL, then we would be overwriting the pointer to Member1 with a pointer to the list head. However, the pointer had been zeroed out and that brought us to address 4. Next, we trapped and the system bugchecked. As discussed earlier, we died on a write operation attempting to write a value to the address referenced by [EAX+4]

For completeness, let’s review what the next two instructions would have executed had we not trapped

08436ef ff0d8cb48a80    dec     dword ptr [nt!MmUnusedSegmentCount (808ab48c)] If the instruction above would not have trapped, then we would have decremented the count. However we did not make it this far. If you remove an item from a linked list, you should decrement the value of any variable tracking the number of items in such list. This instruction would have accomplished this.

808436f5 33c9            xor     ecx,ecx          This would have simply zeroed out ecx.

So this crash was in fact due to a null pointer. More specifically, it was caused by a null flink.  Let's dump the linked list located at nt!MmUnusedSegmentCount  using the dlb just to see what happens. We won't be able to go forward since the flink is null; however, we should be able to dump the linked list going backwards.  If the list loops back on itself or if a null pointer is encountered, the dl command will stop traversing the list.

First, I would like to know how large this list is since the dl command accepts a value that limits its length.

The following global should tell us how many member are in this list.

0: kd> x nt!*UnusedSegmentCount*

808ab48c nt!MmUnusedSegmentCount = <no type information>


0: kd> dd 808ab48c L1

808ab48c  0000e7ce 


0: kd> .formats 0xe7ce

Evaluate expression:

  Hex:     0000e7ce

  Decimal: 59342


This is a huge list, so let’s see if we can traverse it. Based on the size above, I dumped out the list using ffff for the limit.

0: kd> dlb nt!MmUnusedSegmentList ffff 2

808ab4a8  00000000 86f05234

86f05234  808ab4a8 8724392c

8724392c  86f05234 8867fd94

8867fd94  8724392c 877d600c

877d600c  8867fd94 87849944



(omitting lengthy unneeded output, all links were valid)



86ecbaac  86f04354 86f95664

86f95664  86ecbaac 86f4c28c

86f4c28c  00000000 00000000  <-- After over a minute of output, we have a null pointer, but it's at a different address.


First Address we found to be corrupt:         86f4c65c 

Second Address we found to be corrupt:    86f4c28c 

Now that we have located the address above which appear to be incorrectly zeroed out. Let’s dump out these areas in an attempt to get more information.   It would be great if we could verify if the pool that we are using is corrupt. It would be even better if we  some pointers, text, or other clues that may lead us closer to the problem.  Let's dump the two addresses out in various ways. I’ll start by examining the pool for corruption.

 The !pool extension as used below displays information about a specific pool allocation


0: kd> !pool 86f4c65c;!pool 86f4c28c

Pool page 86f4c65c region is Nonpaged pool

 86f4c000 size:   98 previous size:    0  (Allocated)  File (Protected)

86f4c098 is not a valid large pool allocation, checking large session pool...

bf7f4000: Unable to get contents of pool block

Pool page 86f4c28c region is Nonpaged pool

 86f4c000 size:   98 previous size:    0  (Allocated)  File (Protected)

86f4c098 is not a valid large pool allocation, checking large session pool...

bf7f4000: Unable to get contents of pool block


The !poolval extension analyzes the headers for a pool page and diagnoses any possible corruption.


0: kd> !poolval 86f4c65c;!poolval 86f4c28c

Pool page 86f4c65c region is Nonpaged pool

Validating Pool headers for pool page: 86f4c65c

Pool page [ 86f4c000 ] is __inVALID.

Analyzing linked list...

[ 86f4c000 --> 86f4c6c0 (size = 0x6c0 bytes)]: Corrupt region

Scanning for single bit errors...

None found

Pool page 86f4c28c region is Nonpaged pool

Validating Pool headers for pool page: 86f4c28c

Pool page [ 86f4c000 ] is __inVALID.

Analyzing linked list...

[ 86f4c000 --> 86f4c6c0 (size = 0x6c0 bytes)]: Corrupt region

Scanning for single bit errors...

None found


We can also dump out the memory around the addresses in question using various commands.  As discussed previously, we are seeking clues in text format, pointers, etc.  The output from the following command shows that the memory is mostly zeroed.  One value appears to be present. Perhaps the entire region was overwritten and then this value was updated. Dd dumps the raw data as dwords. Dc will dump the data as type char.


0: kd> dd 86f4c65c-100 86f4c65c+100;dd 86f4c28c-100 86f4c28c+100

….<omitting zeros>

86f4c63c  00000000 00000000 00000000 00000000

86f4c64c  00000000 00000000 00000000 00000000

86f4c65c  00000000 808ab4a8 00000000 00000000

86f4c66c  00000000 00000000 00000000 00000000

86f4c67c  00000000 00000000 00000000 00000000

86f4c68c  00000000 00000000 00000000

….<omitting zeros>

86f4c26c  00000000 00000000 00000000 00000000

86f4c27c  00000000 00000000 00000000 00000000

86f4c28c  00000000 00000000 00000000 00000000

86f4c29c  00000000 00000000 00000000 00000000

….<omitting zeros>


      0: kd> dc 86f4c28c-1000 86f4c65c


86f4c62c  00000000 00000000 00000000 00000000  ................

86f4c63c  00000000 00000000 00000000 00000000  ................

86f4c64c  00000000 00000000 00000000 00000000  ................

86f4c65c  00000000                            ....

0: kd> dc

86f4c660  808ab4a8


We can see from the command below that we are in fact dealing with a NonPagedPool address range.

0: kd> !address 86f4c65c;!address 86f4c28c

  83041000 - 07fbf000                          

          Usage       KernelSpaceUsageNonPagedPool

  83041000 - 07fbf000                           

          Usage       KernelSpaceUsageNonPagedPool


0: kd> dps 86f4c65c-8 86f4c65c+8

86f4c654  00000000

86f4c658  00000000

86f4c65c  00000000

86f4c660  808ab4a8 nt!MmUnusedSegmentList

86f4c664  00000000 


Just as in the previous dump, in order to identify the source of the pool corruption we need to use Special Pool.  Special Pool will use guard pages to catch a buffer overrun or underrun and should provide us with a dump that shows the code that causes the corruption.  You can find more information on Special Pool in KB188831.  Also, in some situations memory corruption can be caused by a driver overflowing on a DMA transfer causing corruption to physical pages rather than virtual pages as we see in typical pool. For more information on linked lists and list heads refer to the following MSDN article:

Singly and Doubly Linked Lists-

For an example of a function that removes the first item of a linked list:

*RemoveHeadList()  -  The RemoveHeadList routine removes an entry from the beginning of a doubly linked list of LIST_ENTRY structures.

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  • Nice Articles, Ryan.

    I want to know what the final resolution was, you kinda left me in the middle of a detective novel :)

  • Hi there.

    Really enjoyed this post. Could someone explain this command a bit deeper:

    0: kd> ub @$ip L4;u . L3;r$ip

    I've not seen the dot "." used with a "u" command before. What does this mean? Also what does the "L4" and "L3" syntax mean?

  • I follow your logic right up to this point:

    "0: kd> x nt!MmUnusedSegmentList

      808ab4a8 nt!MmUnusedSegmentList = <no type information>"

    This says we don't know what the type of nt!MmUnusedSegmentList is, yet the next line you type:

    "0: kd> dt 808ab4a8 nt!_LIST_ENTRY

        [ 0x0 - 0x86f05234 ]

          +0x000 Flink            : (null)

               +0x004 Blink            : 0x86f05234 _LIST_ENTRY [ 0x808ab4a8 - 0x8724392c ]"

    How did you make the leap to get the variable type?


    ["Thanks for the great question.


    The name MmUnusedSegmentList suggested to me that this might be the head of a linked list. However I wasn't sure since it didn't have the word Head in the variable name.  I needed a way to test if it was a singly (_SINGLE_LIST_ENTRY) or doubly (_LIST_ENTRY) linked list.  Since I usually have three or four windbg instances open on my desktop at any given time, I entered the following commands into another windbg session:


    1: kd> dd nt!MmUnusedSegmentList L2
    80560d68  ffaf121c ffb4cc2c


    1: kd> dd ffaf121c L2
    ffaf121c  ff8b5d24 80560d68

    1: kd> dl nt!MmUnusedSegmentList 0xffff 2
    80560d68  ffaf121c ffb4cc2c
    ffaf121c  ff8b5d24 80560d68
    ff8b5d24  ff98afa4 ffaf121c

    <ommitting lenghty output>...

    ffae99d4  ff9e9c6c ff941a6c
    ff9e9c6c  ffb4cc2c ffae99d4
    ffb4cc2c  80560d68 ff9e9c6c


    The confirmation of the flinks/blinks above led me to suspect that this was a doubly linked list of the following type:

    1: kd> dt ntkrpamp!_LIST_ENTRY
       +0x000 Flink            : Ptr32 _LIST_ENTRY
       +0x004 Blink            : Ptr32 _LIST_ENTRY

    Now that i knew the type information, I entered the command


    "0: kd> dt 808ab4a8 nt!_LIST_ENTRY

        [ 0x0 - 0x86f05234 ]

          +0x000 Flink            : (null)

               +0x004 Blink            : 0x86f05234 _LIST_ENTRY [ 0x808ab4a8 - 0x8724392c ]"]

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