File Filter Driver Development Tutorial390

Introduction

A file filter driver is a type of kernel-mode driver that can intercept and modify file system operations. This can be useful for a variety of purposes, such as data protection, data logging, and performance optimization. In this tutorial, we will show you how to develop a simple file filter driver in Windows.

Prerequisites
Windows Driver Kit (WDK)
Visual Studio
C++ compiler

Creating a New Driver Project
Open Visual Studio and create a new project.
Select the "Windows Driver" project template.
Enter a name for your project and click "OK".
In the "Driver Type" dialog box, select "File system filter driver" and click "Finish".

Implementing the Driver

The following code shows the implementation of the main driver function:```c++
NTSTATUS DriverEntry(PDRIVER_OBJECT DriverObject, PUNICODE_STRING RegistryPath)
{
// Register the filter driver with the I/O manager
NTSTATUS status = IoRegisterFileSystemFilterDriver(DriverObject);
if (!NT_SUCCESS(status))
{
return status;
}
// Create a dispatch table for the filter driver
PDRIVER_DISPATCH dispatchTable[] = {
{IRP_MJ_CREATE, FilterCreate},
{IRP_MJ_READ, FilterRead},
{IRP_MJ_WRITE, FilterWrite},
{IRP_MJ_CLOSE, FilterClose},
{IRP_MJ_CLEANUP, FilterCleanup},
{IRP_MJ_DIRECTORY_CONTROL, FilterDirectoryControl},
};
// Attach the dispatch table to the driver object
DriverObject->MajorFunction[IRP_MJ_CREATE] = dispatchTable[0];
DriverObject->MajorFunction[IRP_MJ_READ] = dispatchTable[1];
DriverObject->MajorFunction[IRP_MJ_WRITE] = dispatchTable[2];
DriverObject->MajorFunction[IRP_MJ_CLOSE] = dispatchTable[3];
DriverObject->MajorFunction[IRP_MJ_CLEANUP] = dispatchTable[4];
DriverObject->MajorFunction[IRP_MJ_DIRECTORY_CONTROL] = dispatchTable[5];
return STATUS_SUCCESS;
}
```

The following code shows the implementation of the filter create function:```c++
NTSTATUS FilterCreate(PDEVICE_OBJECT DeviceObject, PIRP Irp)
{
// Get a pointer to the file object
PFILE_OBJECT FileObject = IoGetCurrentIrpStackLocation(Irp)->FileObject;
// Open the file using the original IRP
NTSTATUS status = FltCreate(DeviceObject, Irp);
// If the file was opened successfully, add a filter context to the file object
if (NT_SUCCESS(status))
{
PFILTER_CONTEXT filterContext = ExAllocatePoolWithTag(NonPagedPool, sizeof(FILTER_CONTEXT), 'FLT');
if (filterContext != NULL)
{
filterContext->DeviceObject = DeviceObject;
filterContext->Irp = Irp;
IoSetFilterContext(FileObject, filterContext);
}
else
{
status = STATUS_INSUFFICIENT_RESOURCES;
}
}
return status;
}
```

The following code shows the implementation of the filter read function:```c++
NTSTATUS FilterRead(PDEVICE_OBJECT DeviceObject, PIRP Irp)
{
// Get the filter context from the file object
PFILTER_CONTEXT filterContext = IoGetFilterContext(IoGetCurrentIrpStackLocation(Irp)->FileObject);
// Call the original IRP
NTSTATUS status = FltReadFile(DeviceObject, Irp);
// If the read operation was successful, log the data that was read
if (NT_SUCCESS(status))
{
PVOID buffer = Irp->;
ULONG bufferSize = Irp->;
DbgPrint("Data read from file: %s", buffer);
}
return status;
}
```

The following code shows the implementation of the filter write function:```c++
NTSTATUS FilterWrite(PDEVICE_OBJECT DeviceObject, PIRP Irp)
{
// Get the filter context from the file object
PFILTER_CONTEXT filterContext = IoGetFilterContext(IoGetCurrentIrpStackLocation(Irp)->FileObject);
// Call the original IRP
NTSTATUS status = FltWriteFile(DeviceObject, Irp);
// If the write operation was successful, log the data that was written
if (NT_SUCCESS(status))
{
PVOID buffer = Irp->;
ULONG bufferSize = Irp->;
DbgPrint("Data written to file: %s", buffer);
}
return status;
}
```

The following code shows the implementation of the filter close function:```c++
NTSTATUS FilterClose(PDEVICE_OBJECT DeviceObject, PIRP Irp)
{
// Get the filter context from the file object
PFILTER_CONTEXT filterContext = IoGetFilterContext(IoGetCurrentIrpStackLocation(Irp)->FileObject);
// Free the filter context
ExFreePoolWithTag(filterContext, 'FLT');
// Call the original IRP
return FltCloseFile(DeviceObject, Irp);
}
```

The following code shows the implementation of the filter cleanup function:```c++
NTSTATUS FilterCleanup(PDEVICE_OBJECT DeviceObject, PIRP Irp)
{
// Get the filter context from the file object
PFILTER_CONTEXT filterContext = IoGetFilterContext(IoGetCurrentIrpStackLocation(Irp)->FileObject);
// Free the filter context
ExFreePoolWithTag(filterContext, 'FLT');
// Call the original IRP
return FltCleanup(DeviceObject, Irp);
}
```

The following code shows the implementation of the filter directory control function:```c++
NTSTATUS FilterDirectoryControl(PDEVICE_OBJECT DeviceObject, PIRP Irp)
{
// Call the original IRP
return FltDirectoryControl(DeviceObject, Irp);
}
```

Building and Installing the Driver
Build the driver project.
Sign the driver using a code signing certificate.
Copy the driver files to the target computer.
Install the driver using the Device Manager.

Testing the Driver
Create a file on the target computer.
Open the file using a text editor.
Write some data to the file.
Save the file.
Check the debug output to see if the filter driver logged the data.

Conclusion

In this tutorial, we showed you how to develop a simple file filter driver in Windows. This driver can be used to intercept and modify file system operations. You can use this knowledge to develop more complex drivers for your own purposes.

2025-01-03

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