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 STATUS_INFO_LENGTH_MISMATCH ((NTSTATUS)0xC0000004L)
#define NT_SUCCESS(Status) ((NTSTATUS)(Status) >= 0)
typedef struct {
WORD offset:12;
WORD type:4;
} IMAGE_FIXUP_ENTRY, *PIMAGE_FIXUP_ENTRY;
typedef LONG NTSTATUS;
#ifdef __cplusplus
extern "C" {
#endif
NTSTATUS
WINAPI
NtQuerySystemInformation(
DWORD SystemInformationClass,
PVOID SystemInformation,
ULONG SystemInformationLength,
PULONG ReturnLength
);
#ifdef __cplusplus
}
#endif
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];
}SYSTEM_MODULE_INFORMATION,*PSYSTEM_MODULE_INFORMATION;
typedef struct {
DWORD dwNumberOfModules;
SYSTEM_MODULE_INFORMATION smi;
} MODULES, *PMODULES;
#define SystemModuleInformation 11
DWORD GetHeaders(PCHAR ibase,
PIMAGE_FILE_HEADER *pfh,
PIMAGE_OPTIONAL_HEADER *poh,
PIMAGE_SECTION_HEADER *psh)
{
PIMAGE_DOS_HEADER mzhead=(PIMAGE_DOS_HEADER)ibase;
if ((mzhead->e_magic!=IMAGE_DOS_SIGNATURE) ||
(ibaseDD[mzhead->e_lfanew]!=IMAGE_NT_SIGNATURE))
return FALSE;
*pfh=(PIMAGE_FILE_HEADER)&ibase[mzhead->e_lfanew];
if (((PIMAGE_NT_HEADERS)*pfh)->Signature!=IMAGE_NT_SIGNATURE)
return FALSE;
*pfh=(PIMAGE_FILE_HEADER)((PBYTE)*pfh+sizeof(IMAGE_NT_SIGNATURE));
*poh=(PIMAGE_OPTIONAL_HEADER)((PBYTE)*pfh+sizeof(IMAGE_FILE_HEADER));
if ((*poh)->Magic!=IMAGE_NT_OPTIONAL_HDR32_MAGIC)
return FALSE;
*psh=(PIMAGE_SECTION_HEADER)((PBYTE)*poh+sizeof(IMAGE_OPTIONAL_HEADER));
return TRUE;
}
DWORD FindKiServiceTable(HMODULE hModule,DWORD dwKSDT)
{
PIMAGE_FILE_HEADER pfh;
PIMAGE_OPTIONAL_HEADER poh;
PIMAGE_SECTION_HEADER psh;
PIMAGE_BASE_RELOCATION pbr;
PIMAGE_FIXUP_ENTRY pfe;
DWORD dwFixups=0,i,dwPointerRva,dwPointsToRva,dwKiServiceTable;
BOOL bFirstChunk;
GetHeaders((PBYTE)hModule,&pfh,&poh,&psh);
// loop thru relocs to speed up the search
if ((poh->DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].VirtualAddress) &&
(!((pfh->Characteristics)&IMAGE_FILE_RELOCS_STRIPPED))) {
pbr=(PIMAGE_BASE_RELOCATION)RVATOVA(poh->DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].VirtualAddress,hModule);
bFirstChunk=TRUE;
// 1st IMAGE_BASE_RELOCATION.VirtualAddress of ntoskrnl is 0
while (bFirstChunk || pbr->VirtualAddress) {
bFirstChunk=FALSE;
pfe=(PIMAGE_FIXUP_ENTRY)((DWORD)pbr+sizeof(IMAGE_BASE_RELOCATION));
for (i=0;i<(pbr->SizeOfBlock-sizeof(IMAGE_BASE_RELOCATION))>>1;i++,pfe++) {
if (pfe->type==IMAGE_REL_BASED_HIGHLOW) {
dwFixups++;
dwPointerRva=pbr->VirtualAddress+pfe->offset;
// DONT_RESOLVE_DLL_REFERENCES flag means relocs aren't fixed
dwPointsToRva=*(PDWORD)((DWORD)hModule+dwPointerRva)-(DWORD)poh->ImageBase;
// 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
dwKiServiceTable=*(PDWORD)((DWORD)hModule+dwPointerRva+4)-poh->ImageBase;
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);
}
*(PDWORD)&pbr+=pbr->SizeOfBlock;
}
}
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;
PIMAGE_FILE_HEADER pfh;
PIMAGE_OPTIONAL_HEADER poh;
PIMAGE_SECTION_HEADER psh;
// get system modules - ntoskrnl is always first there
rc=NtQuerySystemInformation(SystemModuleInformation,pModules,4,&dwNeededSize);
if (rc==STATUS_INFO_LENGTH_MISMATCH) {
pModules=GlobalAlloc(GPTR,dwNeededSize);
rc=NtQuerySystemInformation(SystemModuleInformation,pModules,dwNeededSize,NULL);
} else {
strange:
printf("strange NtQuerySystemInformation()!\n");
return;
}
if (!NT_SUCCESS(rc)) goto strange;
// imagebase
dwKernelBase=(DWORD)pModules->smi.Base;
// filename - it may be renamed in the boot.ini
pKernelName=pModules->smi.ModuleNameOffset+pModules->smi.ImageName;
// map ntoskrnl - hopefully it has relocs
hKernel=LoadLibraryEx(pKernelName,0,DONT_RESOLVE_DLL_REFERENCES);
if (!hKernel) {
printf("Failed to load! LastError=%i\n",GetLastError());
return;
}
GlobalFree(pModules);
// our own export walker is useless here - we have GetProcAddress :)
if (!(dwKSDT=(DWORD)GetProcAddress(hKernel,"KeServiceDescriptorTable"))) {
printf("Can't find KeServiceDescriptorTable\n");
return;
}
// get KeServiceDescriptorTable rva
dwKSDT-=(DWORD)hKernel;
// find KiServiceTable
if (!(dwKiServiceTable=FindKiServiceTable(hKernel,dwKSDT))) {
printf("Can't find KiServiceTable...\n");
return;
}
printf("&KiServiceTable==%08X\n\nDumping 'old' ServiceTable:\n\n",
dwKiServiceTable+dwKernelBase);
// let's dump KiServiceTable contents
// MAY FAIL!!!
// should get right ServiceLimit here, but this is trivial in the kernel mode
GetHeaders((PBYTE)hKernel,&pfh,&poh,&psh);
for (pService=(PDWORD)((DWORD)hKernel+dwKiServiceTable);
*pService-poh->ImageBase<poh->SizeOfImage;
pService++,dwServices++)
printf("%08X\n",*pService-poh->ImageBase+dwKernelBase);
printf("\n\nPossibly KiServiceLimit==%08X\n",dwServices);
FreeLibrary(hKernel);
}
