A more stable way to locate real KiServiceTable

Tan Chew Keong in his Win2K/XP SDT Restore 0.1 uses a simple way to find changed SDT entries - he just compares SDT from memory with SDT from ntoskrnl.exe file, assuming that KeServiceDescriptorTable.Base is not changed. If it is, Tan's code that locates KiServiceTable on disk will fail when KeServiceDescriptorTable.Base points somewhere outside the ntoskrnl.

I've found a way how to locate original KiServiceTable in the ntoskrnl file even if KeServiceDescriptorTable.Base has been changed. Method works both in the user mode and in the kmode.

This may be useful to bypass SDT-patching hooks, and not only to restore old SDT. For example, KAV uses SDT relocation ;)

KeServiceDescriptorTable is initialized by KiInitSystem():

    KeServiceDescriptorTable[0].Base = &KiServiceTable[0];
    KeServiceDescriptorTable[0].Count = NULL;
    KeServiceDescriptorTable[0].Limit = KiServiceLimit;
    KeServiceDescriptorTable[0].Number = &KiArgumentTable[0];
    for (Index = 1; Index < NUMBER_SERVICE_TABLES; Index += 1) {
        KeServiceDescriptorTable[Index].Limit = 0;

Thus, we can find KiServiceTable by examining all xrefs to KeServiceDescriptorTable in the kernel. We will search for

C7 05 ..8 bytes..   mov     ds:_KeServiceDescriptorTable.Base, offset _KiServiceTable

from which we will get _KiServiceTable rva.

It's easy to find KeServiceDescriptorTable xrefs by scanning the code, but this is dangerous and time-consuming. It's better to use ntoskrnl's relocation information - it is always present in all nt systems.
This "mov [mem32], imm32" instruction will have 2 relocs pointing in it, and the second is the one we're searching for. So, the usermode code will do these steps:

1. Load ntosknrl as a dll.
2. Locate KeServiceDescriptorTable - it is exported.
3. Enumerate all relocations to find xrefs to the KeServiceDescriptorTable.
4. Check these opcodes to be a "mov [mem32],imm32".
5. Get KiServiceTable - it's offset is +6 from the opcode beginning.

The example code dumps KiServiceTable data from file and does no comparings with the existing sdt.

So, it's a must to a SDT-patching rootkit to hook file system and show "patched" version of ntoskrnl to all readers. And of course, services hooks code must reside in the ntoskrnl region and have no codepaths outside it. Otherwise it may be tracked in seconds.

#include <windows.h>
#include <winnt.h>
#include <stdio.h>

#define RVATOVA(base,offset) ((PVOID)((DWORD)(base)+(DWORD)(offset)))
#define ibaseDD *(PDWORD)&ibase
#define NT_SUCCESS(Status) ((NTSTATUS)(Status) >= 0)

typedef struct {
    WORD    offset:12;
    WORD    type:4;


#ifdef __cplusplus
extern "C" {

    DWORD    SystemInformationClass,
    PVOID    SystemInformation,
    ULONG    SystemInformationLength,
    PULONG    ReturnLength

#ifdef __cplusplus

typedef struct _SYSTEM_MODULE_INFORMATION {//Information Class 11
    ULONG    Reserved[2];
    PVOID    Base;
    ULONG    Size;
    ULONG    Flags;
    USHORT    Index;
    USHORT    Unknown;
    USHORT    LoadCount;
    USHORT    ModuleNameOffset;
    CHAR    ImageName[256];

typedef struct {
    DWORD    dwNumberOfModules;

#define    SystemModuleInformation    11

DWORD GetHeaders(PCHAR ibase,
                PIMAGE_FILE_HEADER *pfh,
                PIMAGE_OPTIONAL_HEADER *poh,
                PIMAGE_SECTION_HEADER *psh)

    if    ((mzhead->e_magic!=IMAGE_DOS_SIGNATURE) ||        
        return FALSE;
    if (((PIMAGE_NT_HEADERS)*pfh)->Signature!=IMAGE_NT_SIGNATURE)
        return FALSE;
    if ((*poh)->Magic!=IMAGE_NT_OPTIONAL_HDR32_MAGIC)
        return FALSE;
    return TRUE;

DWORD FindKiServiceTable(HMODULE hModule,DWORD dwKSDT)
    PIMAGE_FIXUP_ENTRY    pfe;    
    DWORD    dwFixups=0,i,dwPointerRva,dwPointsToRva,dwKiServiceTable;
    BOOL    bFirstChunk;


    // loop thru relocs to speed up the search
    if ((poh->DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].VirtualAddress) &&
        (!((pfh->Characteristics)&IMAGE_FILE_RELOCS_STRIPPED))) {

        // 1st IMAGE_BASE_RELOCATION.VirtualAddress of ntoskrnl is 0
        while (bFirstChunk || pbr->VirtualAddress) {


            for (i=0;i<(pbr->SizeOfBlock-sizeof(IMAGE_BASE_RELOCATION))>>1;i++,pfe++) {
                if (pfe->type==IMAGE_REL_BASED_HIGHLOW) {
                    // DONT_RESOLVE_DLL_REFERENCES flag means relocs aren't fixed

                    // does this reloc point to KeServiceDescriptorTable.Base?
                    if (dwPointsToRva==dwKSDT) {
                        // check for mov [mem32],imm32. we are trying to find
                        // "mov ds:_KeServiceDescriptorTable.Base, offset _KiServiceTable"
                        // from the KiInitSystem.
                        if (*(PWORD)((DWORD)hModule+dwPointerRva-2)==0x05c7) {
                            // should check for a reloc presence on KiServiceTable here
                            // but forget it
                            return dwKiServiceTable;
                } else
                    if (pfe->type!=IMAGE_REL_BASED_ABSOLUTE)
                        // should never get here
                        printf("\trelo type %d found at .%X\n",pfe->type,pbr->VirtualAddress+pfe->offset);
    if (!dwFixups)
        // should never happen - nt, 2k, xp kernels have relocation data
        printf("No fixups!\n");
    return 0;

void main(int argc,char *argv[])
    HMODULE    hKernel;
    DWORD    dwKSDT;                // rva of KeServiceDescriptorTable
    DWORD    dwKiServiceTable;    // rva of KiServiceTable
    PMODULES    pModules=(PMODULES)&pModules;
    DWORD    dwNeededSize,rc;
    DWORD    dwKernelBase,dwServices=0;
    PCHAR    pKernelName;
    PDWORD    pService;

    // get system modules - ntoskrnl is always first there
    } else {
        printf("strange NtQuerySystemInformation()!\n");
    if (!NT_SUCCESS(rc)) goto strange;
    // imagebase
    // filename - it may be renamed in the boot.ini
    // map ntoskrnl - hopefully it has relocs
    if (!hKernel) {
        printf("Failed to load! LastError=%i\n",GetLastError());


    // our own export walker is useless here - we have GetProcAddress :)    
    if (!(dwKSDT=(DWORD)GetProcAddress(hKernel,"KeServiceDescriptorTable"))) {
        printf("Can't find KeServiceDescriptorTable\n");

    // get KeServiceDescriptorTable rva
    // find KiServiceTable
    if (!(dwKiServiceTable=FindKiServiceTable(hKernel,dwKSDT))) {
        printf("Can't find KiServiceTable...\n");

    printf("&KiServiceTable==%08X\n\nDumping 'old' ServiceTable:\n\n",
    // let's dump KiServiceTable contents        
    // MAY FAIL!!!
    // should get right ServiceLimit here, but this is trivial in the kernel mode
    for (pService=(PDWORD)((DWORD)hKernel+dwKiServiceTable);

    printf("\n\nPossibly KiServiceLimit==%08X\n",dwServices);



wbr, 90210