Table of Contents
- SST: References
- Algorithm
- Memory Mapped Files
- Implementation
- Demonstration
- How to Build
- History
1. SST: References
This article is a logical continuation of the article "Driver to Hide Processes and Files" by Ivan Romananko. You can find all the necessary information about System Service Table (SST) and its hooking in it.
In this article, I would like to present how to write your own unhooker that will restore original SST hooked by drivers like Ivan's one.
2. Algorithm
My goal is to write a simple driver for SST hooking detection and removing purposes.
This means that our driver should not use various Zw
-functions and SST table because I suppose that SST table is corrupted by unknown rootkits.
I do not care about filter drivers and function code splicers for now, but maybe I will come back to them in future.
The simplest way to detect and remove hooks is to compare SST that is placed in memory with the initial SST from ntoskernel.exe file.
So the goal is:
- to find ntoskernel module in memory
- to find the section of ntoskernel where SST is placed and to calculate relative offset of SST in the section
- to find this section in the ntoskernel.exe file
- to calculate the real address of SST in the file
- to read values from the file and to compare them with SST
But before the implementation, I would like to present some additional information.
3. Memory Mapped Files in Kernel Mode
"A memory-mapped file is a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource". (c) Wiki
Yeah, we want to parse the PE file and memory mapped files are very useful for this task.
And it is easy enough to use mapped files API from the kernel mode, because it is very similar to Win32 API. Instead of CreateFileMapping
and MapViewOfSection
functions in kernel mode driver should access:
NTSTATUS
ZwCreateSection(
OUT PHANDLE SectionHandle,
IN ACCESS_MASK DesiredAccess,
IN POBJECT_ATTRIBUTES ObjectAttributes OPTIONAL,
IN PLARGE_INTEGER MaximumSize OPTIONAL,
IN ULONG SectionPageProtection,
IN ULONG AllocationAttributes,
IN HANDLE FileHandle OPTIONAL
);
and
NTSTATUS
ZwMapViewOfSection(
IN HANDLE SectionHandle,
IN HANDLE ProcessHandle,
IN OUT PVOID *BaseAddress,
IN ULONG_PTR ZeroBits,
IN SIZE_T CommitSize,
IN OUT PLARGE_INTEGER SectionOffset OPTIONAL,
IN OUT PSIZE_T ViewSize,
IN SECTION_INHERIT InheritDisposition,
IN ULONG AllocationType,
IN ULONG Win32Protect
);
functions.
But if we use these functions, we will break our own rule not to use SST. Also, it is good for antirootkit to use extremely low level functions in the hope of being invisible to the possible rootkits.
With regard to this, we can use undocumented functions of Memory Manager (Mm), of course at our own risk:
NTSTATUS
MmCreateSection (
OUT PVOID *SectionObject,
IN ACCESS_MASK DesiredAccess,
IN POBJECT_ATTRIBUTES ObjectAttributes OPTIONAL,
IN PLARGE_INTEGER MaximumSize,
IN ULONG SectionPageProtection,
IN ULONG AllocationAttributes,
IN HANDLE FileHandle OPTIONAL,
IN PFILE_OBJECT File OPTIONAL
);
NTSTATUS
MmMapViewOfSection(
IN PVOID SectionToMap,
IN PEPROCESS Process,
IN OUT PVOID *CapturedBase,
IN ULONG_PTR ZeroBits,
IN SIZE_T CommitSize,
IN OUT PLARGE_INTEGER SectionOffset,
IN OUT PSIZE_T CapturedViewSize,
IN SECTION_INHERIT InheritDisposition,
IN ULONG AllocationType,
IN ULONG Protect
);
NTSTATUS
MmUnmapViewOfSection(
IN PEPROCESS Process,
IN PVOID BaseAddress
);
NTSTATUS drv_MapAllFileEx(HANDLE hFile OPTIONAL,
drv_MappedFile * pMappedFile,
LARGE_INTEGER * pFileSize,
ULONG Protect)
{
NTSTATUS status = STATUS_SUCCESS;
PVOID section = 0;
PCHAR pData=0;
LARGE_INTEGER offset;
offset.QuadPart = 0;
if (!pFileSize->QuadPart)
goto calc_exit;
status = MmCreateSection (§ion,
SECTION_MAP_READ,
0, pFileSize, Protect,
0x8000000,
hFile,
0
);
if (status!= STATUS_SUCCESS)
goto calc_exit;
status = MmMapViewOfSection(section,
PsGetCurrentProcess(),
(PVOID*)&pData,
0,
0,
&offset,
&pFileSize->LowPart,
ViewUnmap,
0,
Protect);
if (status!= STATUS_SUCCESS)
goto calc_exit;
calc_exit:
if (NT_SUCCESS(status))
{
pMappedFile->fileSize.QuadPart = pFileSize->QuadPart;
pMappedFile->pData = pData;
pMappedFile->section = section;
}
else
{
if (pData)
MmUnmapViewOfSection(PsGetCurrentProcess(),
pData);
if (section)
{
ObMakeTemporaryObject(section);
ObDereferenceObject(section);
}
}
return status;
}
This example demonstrates an alternative approach to the usage of mapped files through MmCreateSection
/MmMapViewOfSection
functions.
The presented approach is pretty good because it doesn’t utilize Zw*
functions and even handles at all, but it has one restriction. If you start this sample from DriverEntry
, it will work fine, but if you start it from the IRP_MJ_DEVICE_CONTROL
handler, you will see that MmCreateSection
function fails with STATUS_ACCESS_DENIED
. Why?
The answer is: Zw*
functions do one good thing - they set previous mode to KernelMode
and this allows to utilize kernel mode pointers and handles as parameters for them (for more information, see Nt vs. Zw - Clearing Confusion On The Native API article).
So, the presented above function can be called only from DriverEntry
or from the system thread.
4. Algorithm Implementation
I designed the following structure to save all ntoskernel
parsing results:
#define IMAGE_SIZEOF_SHORT_NAME 8
typedef struct _Drv_VirginityContext
{
drv_MappedFile m_mapped;
HANDLE m_hFile;
UCHAR m_SectionName[IMAGE_SIZEOF_SHORT_NAME+1];
ULONG m_sstOffsetInSection;
char * m_mappedSST;
ULONG m_imageBase;
char * m_pSectionStart;
char * m_pMappedSectionStart;
char * m_pLoadedNtAddress;
}Drv_VirginityContext;
And I implemented the chosen algorithm as follows:
static NTSTATUS ResolveSST(Drv_VirginityContext * pContext,
SYSTEM_MODULE * pNtOsInfo)
{
PIMAGE_SECTION_HEADER pSection = 0;
PIMAGE_SECTION_HEADER pMappedSection = 0;
NTSTATUS status = 0;
PNTPROC pStartSST = KeServiceDescriptorTable->ntoskrnl.ServiceTable;
char * pSectionStart = 0;
char * pMappedSectionStart = 0;
pContext->m_pLoadedNtAddress = (char*)pNtOsInfo->pAddress;
status = Drv_ResolveSectionAddress(pNtOsInfo->pAddress, pStartSST, &pSection);
if (!NT_SUCCESS(status))
goto clean;
memcpy(pContext->m_SectionName, pSection->Name, IMAGE_SIZEOF_SHORT_NAME);
pSectionStart = (char *)pNtOsInfo->pAddress + pSection->VirtualAddress;
pContext->m_sstOffsetInSection = (char*)pStartSST - pSectionStart;
status = Drv_FindSection(pContext->m_mapped.pData,
pSection->Name,
&pMappedSection);
if (!NT_SUCCESS(status))
goto clean;
pMappedSectionStart = (char *)pContext->m_mapped.pData +
pMappedSection->PointerToRawData;
pContext->m_mappedSST = pMappedSectionStart + pContext->m_sstOffsetInSection;
{ PIMAGE_DOS_HEADER dosHeader =
(PIMAGE_DOS_HEADER)pContext->m_mapped.pData;
PIMAGE_NT_HEADERS pNTHeader =
(PIMAGE_NT_HEADERS)((char*)dosHeader + dosHeader->e_lfanew);
pContext->m_imageBase = pNTHeader->OptionalHeader.ImageBase;
}
pContext->m_pSectionStart = pSectionStart;
pContext->m_pMappedSectionStart = pMappedSectionStart;
clean:
return status;
}
And here is the function that returns real value of SST:
void Drv_GetRealSSTValue(Drv_VirginityContext * pContext, long index, void ** ppValue)
{
char * pSST = pContext->m_mappedSST;
ULONG * pValue = ((ULONG *) pSST) + index;
*ppValue = (void*)(*pValue + (ULONG)pContext->m_pLoadedNtAddress –
pContext->m_imageBase);
}
After that, it is quite simple to implement main functionality:
virtual NTSTATUS ExecuteReal()
{
CAutoVirginity initer;
NT_CHECK(initer.Init(&m_virginityContext));
for(int i = 0, sstSize = Drv_GetSizeOfNtosSST();
i < sstSize;
++i)
{
void ** pCurrentHandler = Drv_GetNtosSSTEntry(i);
void * pRealHandler = 0;
Drv_GetRealSSTValue(&m_virginityContext, i, &pRealHandler);
if (pRealHandler != *pCurrentHandler)
{
Drv_HookSST(pCurrentHandler, pRealHandler);
}
}
return NT_OK;
}
This tiny cycle completely removes all SST hooks and brings SST to its initial state.
6. Demonstration
For testing purposes, I developed a simple console utility named unhooker.exe. This utility can be started without parameters; in this case, it shows information about its abilities:
- “
stat
” command shows statistics about SST hooking - “
unhook
” command cleans SST
This sample demonstrates how to use utility to detect and erase hooks:
Have fun!
6. How to Build
Build steps are the same as in the “Hide Driver” article. They are:
- Install Windows Driver Developer Kit 2003 - http://www.microsoft.com/whdc/devtools/ddk/default.mspx
- Set global environment variable "
BASEDIR
" to path of installed DDK. Go here: Computer -> Properties -> Advanced -> Environment variables ->System Variables -> New
And set it like this: BASEDIR
-> c:\winddk\3790
(You have to restart your computer after this.)
If you choose Visual Studio 2003, then you can simply open UnhookerMain.sln and build all.
7. History