How to use the special pool feature to isolate pool damage

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

322756 How to back up and restore the registry in Windows

The special Pool utility allocates two pages of virtual memory for every pool allocation that is requested through ExAllocatePoolWithTag that matches the following criteria:

• The allocation request must be for a size smaller than the maximum allocation that will fit in a pool page.
• The request must match the PoolTag specification in the registry.

For pool overrun detection, the first page is used to contain the allocation at the end of the page. The second page is a guard page. For pool underrun detection, the first page is the guard page. It is followed by a page that contains the allocation at the beginning of the page.

Overrun detection is probably the most used. For overrun detection, the requested allocation is positioned at the end of the first page by backing up the request size from the end of the page. The allocation size is rounded up to an 8-byte boundary. A pattern key, the size, and the pool tag information are written to the header at the first eight bytes of the first page. The pattern is also propagated throughout the page. Because the allocation is positioned on the nearest 8-byte boundary, there may be as many as seven slop bytes following the allocation. The pattern is also written to the slop bytes following the allocation.

The second page is the guard page. The guard page consists of a special page table entry (PTE) that is marked with no-access protection. By marking this second page with no-access protection, any code trying to read or write beyond the end of the first page immediately causes an access violation that results in a Stop 0x0000000A or Stop 0x0000001E error. This enables someone debugging the system to find the exact instruction that causes pool damage.

As a backup check to catch violators that write beyond the end of the allocation but not beyond the end of the page, the slop bytes at the end of the allocation are validated during the free pool request (ExFreePoolWithTag). The slop bytes are compared to the pattern in the allocation header to determine whether anything is overwritten in the slop byte area. If the verification check does not work, a Stop 0x00000001A error occurs.

This check does not necessarily find the exact block of code that causes pool damage. However, it may help identify the component that causes the damage.

To enable the Special Pool utility, add the following keys and values to the registry: The pool tag mask is the pool tag ID that contains masking characters of the pool in which you want to put in the special pool. This mask must be specified in hexadecimal in reverse order. This mask may also contain "?" to mask a single character or "*" to mask from here to the end of the tag. For example, to monitor all pools with a pool tag that begins with "Nt", specify "2A744E" (without the quotation marks), which represents "*tN".

The following table lists more examples.

   Pool to monitor   Character representation   Pool tag mask
   ----------------------------------------------------------               
   All pools         "*"                        0x2A
   N??s              "s??N"                     0x733F3F4E
 

Allocation size mask specification places all pool allocations of a specified size into the special pool. This is also specified in hexadecimal. For example, if all allocations of 32 bytes are positioned in the special pool, specify 0x20.

When zero (0x0) is specified, the Special Pool utility is not initialized. In addition, the Special Pool utility is not initialized if the PoolTag registry value is not defined in the registry. 1 indicates that pool allocation overruns are detected for the tag specified. The allocation is located at the end of the page and the guard page follows.

0 indicates that pool allocation underruns are detected for the tag. The allocation is located at the beginning of the page, and the guard page comes before the page that contains the allocation.

Common use examples to create these two registry keys would be as follows:

Note For Windows NT Terminal Server 4.0, you must disable KStackPool when you use special pool. To do this, add the following registry value: If you do not make this change on Terminal Server computers that have 256 MB or more memory, you may receive "STOP 0x00000078 (PHASE0_EXCEPTION)" error messages.


Note The Special Pool feature may not work in Windows Server 2003 if there are insufficient special page table entries (PTEs). This issue typically occurs on single-user terminal servers that contain more than 1 GB of RAM and that have the memory management tuning values of the computer set to default values. This issue does not occur in 64-bit versions of Windows Server 2003.

A memory dump file analysis of the issue may contain the following information: You can verify the issue by the size of the Special Pool that is allocated from the analysis. In this case, the issue occurs because the following special pool PTE values are equal:

• MiSpecialPoolFirstPte
• MiSpecialPoolLastPte
  

This issue may also occur if the value of the MmSpecialPoolRejected registry entry is three or larger.

To work around this issue, configure the value of the PagedPoolSize registry entry to a value that is smaller than the paged pool size that is permitted. For example, set the PagedPoolSize registry entry to a value 256 MB. If you want to configure a larger value, use the Performance Monitor utility to determine the computer's requirement for a larger paged pool size.

You can use the following values for PagedPoolSize registry entry: For Windows Server 2003 without a service pack:

Value Name: PagedPoolSize
Data Type: REG_DWORD
Data: 10000000
Base: Hexadecimal



For Windows Server 2003 with Service Pack 1 (SP1) installed:

Value Name: PagedPoolSize
Data Type: REG_DWORD
Data: FFFFFFFF
Base: Hexadecimal


Note After you make any of the registry changes that are described in this article, restart the computer to cause the changes to take effect.

If after you enable the Special Pool feature, the computer stops responding (hangs), and you receive an error message on a blue screen during startup, restart the computer by using the Last Known Good Configuration option. Enabling the Special Pool feature is not written to the Last Known Good Configuration entry in the registry until after a successful logon.