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Prevent_File_Deletion

Record & prevent file deletion in kernel mode

Huge thanks to https://0x00sec.org/t/kernel-mode-rootkits-file-deletion-protection/7616

This guy has some awesome things! Thanks to the tutorials! https://github.com/NtRaiseHardError

Study Notes

What to do in DriverEntry

  1. Use FltRegisterFilter to register a minifilter.
  2. We can use 0x%08x format specifier in DbgPrint to print error codes.
  3. After FltRegisterFilter, use FltStartFiltering to start the minifilter.
  4. If FltStartFiltering fails, we should unregister the minifilter by calling FltUnregisterFilter.

Now here comes the tough things...

FltRegisterFilter function

For FltRegisterFilter function, we need three parameters:

(Reference: https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/fltkernel/nf-fltkernel-fltregisterfilter)

The code should be like this:

PFLT_FILTER Filter;
status = FltRegisterFilter(DriverObject, &FilterRegistration, &Filter);

The Registration parameter can be a little complicated. Other parameters are straightforward. The type of Registration is FLT_REGISTRATION, which will be something like this:

(Reference: https://github.com/NtRaiseHardError/Anti-Delete/blob/9fbe5ac0e46ba7a70b9e9983977d1bd38a3999a0/Anti%20Delete/Anti%20Delete/Driver.c#L24)

const FLT_REGISTRATION FilterRegistration = {
	sizeof(FLT_REGISTRATION),	// Size
	FLT_REGISTRATION_VERSION,	// Version
	0,							// Flags
	NULL,						// ContextRegistration
	Callbacks,					// OperationRegistration
	badgirlFilterUnload,		// FilterUnloadCallback
	NULL,						// InstanceSetupCallback
	NULL,						// InstanceQueryTeardownCallback
	NULL,						// InstanceTeardownStartCallback
	NULL,						// InstanceTeardownCompleteCallback
	NULL,						// GenerateFileNameCallback
	NULL,						// NormalizeNameComponentCallback
	NULL,						// NormalizeContextCleanupCallback
	NULL,						// TransactionNotificationCallback
	NULL,						// NormalizeNameComponentExCallback
	NULL						// SectionNotificationCallback
};

Note that we need to specify OperationRegistration and FilterUnloadCallback in this demo. I don't know what do other callbacks do so far.

Pay attention to OperationRegistration. Note that Callbacks should be FLT_OPERATION_REGISTRATION, which is an array. It binds major function and callbacks. It should be like this:

(Reference: https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/fltkernel/ns-fltkernel-_flt_operation_registration)

const FLT_OPERATION_REGISTRATION Callbacks[] = {
	{ IRP_MJ_CREATE, 0, badgirlFilterAntiDelete, NULL, NULL },
	{ IRP_MJ_SET_INFORMATION, 0, badgirlFilterAntiDelete, NULL, NULL },
    
    ...

	{ IRP_MJ_OPERATION_END }
};
  • The 1st element specifies MajorFunction of the operation.
  • The 2nd element is Flags, I don't know what's that, even after reading the documentation. Leave that with 0.
  • The 3rd element is PreOperation, which is the callback function that will be called before an operation.
  • The 4th element is PostOperation, which is the callback function that will be called after an operation. In this demo, we don't need PostOperation callbacks, so we set them to NULLs.
  • The 5th element is reserved. Set that to NULL.
  • The last element in the array must be { IRP_MJ_OPERATION_END }.

So... That's what you need to do to call FltRegisterFilter.

Registry

When FltRegisterFilter returns STATUS_OBJECT_NAME_NOT_FOUND (0xc0000034), it usually indicates there's something wrong with the registry. (Reference: https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/fltkernel/nf-fltkernel-fltregisterfilter#return-value) After searching for this problem, I found the reason. It is because I am too lazy and I didn't create an INF file for this driver. So the minifilter is not recognized by the system. To solve this, we need to write an appropriate INF file. (Reference: https://stackoverflow.com/questions/42389211/fltregisterfilter-not-working) The INF file for a minifilter is different from INF file for other drivers. (https://docs.microsoft.com/en-us/windows-hardware/drivers/ifs/creating-an-inf-file-for-a-minifilter-driver) There are several things to notice.

Service name

The service name that the driver loader passes to CreateService should be the same as the ServiceName specified in the INF file. Otherwise, the system won't recognize the minifilter.

INF file

  1. You need to specify CatalogFile to your_driver_name.cat, even though the documentation (https://docs.microsoft.com/en-us/windows-hardware/drivers/ifs/creating-an-inf-file-for-a-minifilter-driver?redirectedfrom=MSDN#version-section-required) says minifilter drivers except antivirus minifilter drivers should leave this entry blank (This demo is not an antivirus minifilter (320000 < Altitude < 329999), but an activity monitor minifilter (Altitude = 365000)). I don't know why, but my solution refuses to compile unless I specify the value of CatalogFile.
  2. Change Class and ClassGuid according to your purpose. (https://docs.microsoft.com/en-us/windows-hardware/drivers/ifs/file-system-filter-driver-classes-and-class-guids)
  3. Change Instance1.Altitude to an appropriate value according to your purpose. (https://docs.microsoft.com/en-us/windows-hardware/drivers/ifs/load-order-groups-and-altitudes-for-minifilter-drivers)
  4. An example INF file: https://github.com/Microsoft/Windows-driver-samples/blob/master/filesys/miniFilter/NameChanger/NameChanger.inf
  5. If the solution fails to build and says Section xxx should have an architecture decoration, go to project properties - Driver Settings - General - Set "Target Platform" to "Desktop". (microsoft/Windows-driver-samples#366) This may be a better solution, but I havn't tried that: https://community.osr.com/discussion/291286/wdk10-stampinf-returns-error-1420-defaultinstall-based-inf-cannot-be-processed-as-primitive

Linking

If the compiler says "unresolved external symbol _Flt*", you need to link fltMgr.lib. Go to project properties - Linker - Input, edit "Additional Dependencies" and add $(DDK_LIB_PATH)fltMgr.lib.

IRQL

I don't have much concept about IRQL. But I noticed that every minifilter callback function (like PreOperation, FilterUnloadCallback) has PAGED_CODE(); in its body. This is related to IRQL, but I still need to understand the concept.

Misc

  1. FalgOn(a, b) macro can be used to determine if a includes b. (i.e. a & b != 0 indicates a includes b)
  2. We can use FltGetFileNameInformation and FltParseFileNameInformation to get file name information from FltObjects.
  3. Data->Thread can be used to identify the process that's requesting the operation.
  4. To prevent an operation:
Data->IoStatus.Status = STATUS_ACCESS_DENIED;
Data->IoStatus.Information = 0;
ret = FLT_PREOP_COMPLETE;

UPDATE: Prevent file modification

To prevent file modification, we can check flags in Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess. The following code will prevent rename, delete, modify and property change (Not strictly tested though). If we want to prevent the file from being read, we can deny the access when related DesiredAccess flag is detected.

if (FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, FILE_WRITE_DATA) ||
    FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, FILE_WRITE_ATTRIBUTES) ||
    FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, FILE_WRITE_EA) ||
    FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, FILE_APPEND_DATA) ||
    FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, DELETE) ||
    FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, WRITE_DAC) ||
    FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, WRITE_OWNER) ||
    FlagOn(Data->Iopb->Parameters.Create.SecurityContext->DesiredAccess, GENERIC_WRITE)) {

    Data->IoStatus.Status = STATUS_ACCESS_DENIED;
    Data->IoStatus.Information = 0;
    ret = FLT_PREOP_COMPLETE;
}

UPDATE: Find out which process requested the operation

To find out which process requested the operation, we can use Data->Thread.

PEPROCESS TargetProcess = IoThreadToProcess(Data->Thread);
HANDLE Pid = PsGetProcessId(TargetProcess);
PUNICODE_STRING ProcessName = NULL;
SeLocateProcessImageName(TargetProcess, &ProcessName);

What to do next

  1. Currently, loading the driver will return an error code that indicates An instance of the service is already running. I don't know why, because there is no an intance of the service is already running.
  2. Loading the driver for a second time will fail to register the minifilter, I don't know why. (Update: I found that reinstall the service will solve the problem, but still don't know why).
  3. I will need to figure out how to prevent file creation and modification. (Solved)
  4. I found that the minifilter will be loaded even if I whatever path I specified in the driver loader. This is weird! Maybe it's because the system puts the newly installed minifilter driver into a "pending to load" list, and it will be loaded as soon as some program loads other drivers? I don't know.
  5. HRSword can delete/rename protected files without the specific DesiredAccess. Maybe it is because it simply passes the file handle to the kernel-mode driver and let the driver to finish the job? I am not sure.