Writing a Hyper-V “Bridge” for Fuzzing — Part 2 : Hypercalls & MDLs

Introduction

Previously, in Part 1, we were able to see how the Windows Driver Foundation (WDF) can significantly simplify the development of drivers, including even “research-type” non-PnP drivers. In this part, we will now enter the guts of Hyper-V and talk about how hypercalls work (a portmanteau of syscall (system call) as applied when talking about an OS->Hypervisor transition instead of an App->OS transition).

First, it’s important to learn that there are two kinds of hypercalls that Hyper-V supports, which are described in the Top Level Functional Specification (TLFS). The first, called regular or slow hypercalls, use physical addresses of RAM in order to send and receive input and output parameters, which are aligned to natural boundaries. We’ll have to learn about the Windows concept of Memory Descriptor Lists (MDLs) to explain how we can pass such data to Hyper-V, and this will be the topic of this part.

The second kind of hypercall is called an extended fast hypercall and uses registers, including XMM non-integer registers in order to pass in up to 112 bytes of data, aligned on and padded to 8 byte boundaries. This is much faster as memory allocations and mappings are not involved, but requires some more complex understanding of memory alignments, and we’ll leave this for the next part.

Hypercall Basics

Regardless of the type of hypercall being initiated, a hypercall input value and a hypercall result value are always used and returned. This data is used to both inform the hypervisor as to the calling convention and type of hypercall being attempted, as well as to provide any errors, intermediate state, or final result back to the caller. To begin with, here’s what the input value looks like:

typedef union _HV_X64_HYPERCALL_INPUT
{
    struct
    {
        UINT32 CallCode : 16;
        UINT32 IsFast : 1;
        UINT32 Reserved1 : 15;
        UINT32 CountOfElements : 12;
        UINT32 Reserved2 : 4;
        UINT32 RepStartIndex : 12;
        UINT32 Reserved3 : 4;
    };
    UINT64 AsUINT64;
} HV_X64_HYPERCALL_INPUT, *PHV_X64_HYPERCALL_INPUT;

While the first members should be self-evident (specifying the hypercall index and the calling convention used, the count and start index fields related to a concept not yet introduced — the repeated hypercall. You see, because processing a hypercall essentially results in the OS losing control of the processor (no interrupts and no scheduling), it’s important for the hypervisor to minimize this time.

Hyper-V employs an innovative idea of allowing more complex requests to be split up as “chunks of work” (i.e.: repetitions), and for the hypervisor to perform enough repetitions to fill up a timeslice (say, 50 microseconds), return back the current index where it left off, and allowing the OS some time to handle interrupts and scheduling needs, before re-issuing the hypercall at the updated start index. This works, by the way, both for input and output parameters.

Now let’s see what the result value has in store:

typedef union _HV_X64_HYPERCALL_OUTPUT
{
    struct
    {
        HV_STATUS CallStatus;
        UINT16 Reserved1;
        UINT32 ElementsProcessed : 12;
        UINT32 Reserved2 : 20;
    };
    UINT64 AsUINT64;
} HV_X64_HYPERCALL_OUTPUT, *PHV_X64_HYPERCALL_OUTPUT;

Once again, we can see both the obvious return value itself (similar to an NTSTATUS or HRESULT) as well as a specific repeated hypercall field, which allows the caller, as explained above, to correctly restart the hypercall with the start index pointing after the number of elements processed (or, in the case of an error, determine which element caused the error).

Memory Descriptor Lists

Memory Descriptor Lists are an extremely useful construct in the Windows kernel which allows driver writers to either take existing user-mode memory or kernel-mode memory (such as a nonpaged pool buffer) and lock it down in memory (pinning it so that it can not be reused nor freed), guaranteeing its access rights, and returning an array of corresponding Page Frame Numbers (PFNs) that map to the virtual buffers.

Obviously, this type of data structure is extremely useful when doing Direct Memory Access (DMA) with a Network Interface Card (NIC) and its driver, but it also has software-specific uses such as re-mapping an existing buffer with different permissions (by creating a new, secondary mapping, of the initial buffer whose pages are now locked down) — this specific use case being seen in certain rootkits, for example. Windows also provides APIs for directly requesting empty physical memory and attaching an MDL to it, followed by APIs which allow mapping the physical pages into a virtual buffer based on the requested mapping flags.

In our use case, the hypervisor expects a page aligned contiguous set of input physical pages that match the data (potentially repeated) specified by the hypercall input value and a similar set of pages for the output buffer. In the code snippet below, a helper function is used to take a user-mode buffer and convert it into an appropriate MDL for use with Hyper-V hypercalls, returning the underlying physical address.

Note that, as mentioned above, it is technically possible to directly construct an MDL around the initial user buffer without requiring a kernel-copy to be made, but this would assume that the user-buffer is on physically contiguous pages (or page-aligned within a single page). A safer approach is taken here, where the MDL is allocated and a copy of the user buffer is made. On output, this means that the caller must copy the MDL buffer back into the user buffer.

Also take note that the virtual buffer is not zeroed out for performance reasons, which means that the output buffer copied back into user-mode should only copy the exact number of bytes that the hypervisor returned back, in order to avoid leaking arbitrary kernel memory back to the user (in this particular implementation, this is a moot point, as the hypercalls used in the fuzzing interface regularly accept/return kernel pointers and assume a Ring 0 attacker to begin with).

NTSTATUS
MapUserBufferToMdl (
    _In_ PVOID UserBuffer,
    _In_ ULONG BufferSize,
    _In_ BOOLEAN IsInput,
    _Out_ PULONGLONG MappedPa,
    _Out_ PMDL* MdlToFree
    )
{
    PMDL hvMdl;
    PHYSICAL_ADDRESS low, high;
    PVOID mapBuffer;
    ULONG pageCount;
    ULONG idealNode;
    ULONG flags;
    NTSTATUS status;

    //
    // Allocate an MDL for the number of pages needed, in the
    // current NUMA node, and allow the processor to cache them.
    // In case more than a page of data is needed, make sure to
    // require contiguous pages, as the hypervisor only receives
    // the starting PFN, not an array. We also allow the memory
    // manager to look at other non local nodes if the current
    // one is unavailable, and we speed it up by not requesting
    // zeroed memory.
    //
    *MdlToFree = NULL;
    *MappedPa = 0;
    low.QuadPart = 0;
    high.QuadPart = ~0ULL;
    pageCount = ROUND_TO_PAGES(BufferSize);
    idealNode = KeGetCurrentNodeNumber();
    flags = MM_ALLOCATE_REQUIRE_CONTIGUOUS_CHUNKS |
            MM_ALLOCATE_FULLY_REQUIRED |
            MM_DONT_ZERO_ALLOCATION;

    //
    // Use the very latest 1809 API which also allows us to
    // pass in the Memory Partition from which to grab the
    // pages from -- in our case we pass NULL meaning use the
    // System Partition (0).
    //
    hvMdl = MmAllocatePartitionNodePagesForMdlEx(low,
                                                 high,
                                                 low,
                                                 pageCount,
                                                 MmCached,
                                                 idealNode,
                                                 flags,
                                                 NULL);
    if (hvMdl == NULL)
    {
        //
        // There is not enough free contiguous physical memory,
        // bail out
        //
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "Failed to allocate MDL\n");
        status = STATUS_INSUFFICIENT_RESOURCES;
        goto Cleanup;
    }

    //
    // Map the MDL pages in kernel-mode, with RWNX permissions
    //
    mapBuffer = MmGetSystemAddressForMdlSafe(hvMdl,
                                            MdlMappingNoExecute);
    if (mapBuffer == NULL)
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "Failed to map buffer\n");
        status = STATUS_INSUFFICIENT_RESOURCES;
        goto Cleanup;
    }

    //
    // Use SEH in case the user-mode buffer is invalid
    //
    __try
    {
        if (IsInput != FALSE)
        {
            //
            // Make sure the input buffer is aligned user-mode
            // memory, then copy it into the mapped kernel buffer
            //
            ProbeForRead(UserBuffer,
                         BufferSize,
                         __alignof(UCHAR));
            RtlCopyMemory(mapBuffer,
                          UserBuffer,
                          BufferSize);
        }
        else
        {
            //
            // Make sure the output buffer is aligned user-mode
            // memory and that it is writeable. The copy will be
            // done after the hypercall completes.
            //
            ProbeForWrite(UserBuffer,
                          BufferSize,
                          __alignof(UCHAR));
        }
    }
    __except(EXCEPTION_EXECUTE_HANDLER)
    {
        //
        // An exception was raised, bail out
        //
        status = GetExceptionCode();
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "Exception copying buffer : %lx\n",
                   status);
        goto Cleanup;
    }

    //
    // Hyper-V will want to know the starting physical address
    // for the buffer, so grab it
    //
    *MappedPa = *MmGetMdlPfnArray(hvMdl) << PAGE_SHIFT;
    *MdlToFree = hvMdl;
    status = STATUS_SUCCESS;

Cleanup:
    //
    // On failure, clean up the MDL if one was created/mapped
    //
    if (!(NT_SUCCESS(status)) && (hvMdl != NULL))
    {
        //
        // This also cleans up the mapping buffer if one exists
        //
        MmFreePagesFromMdlEx(hvMdl, 0);
        ExFreePool(hvMdl);
    }
    return status;
}

As a small addendum related to Windows Internals, however, it should be noted that Windows does not typically go through this heavy handed approach each time it wishes to issue a hypercall. Instead, two helper functions: HvlpAcquireHypercallPage and HvlpReleaseHypercallPage, are used to grab an appropriate physical page from one of the following possible locations:

  • An SLIST (Lock-Free Stack List) is used, which is used to cache a number of pre-configured cached hypercall pages stored in the KPRCB (HypercallPageList). 4 such pages are stored in the KPRCB HypercallCachedPages array, starting at index 2.
  • If no cached pages are available, and the stack buffer can be used, it is page aligned and its physical address is used for the hypercall. The Interrupt Request Level (IRQL) is raised to DISPATCH_LEVEL to avoid the kernel stack from being marked as non-resident.
  • If the stack buffer cannot be used, two hardcoded pages are used from the KPRCB HypercallCachedPages array — index 0 for the input page, index 1 for the output page.

For processor 0 (the bootstrap processor or BSP) the default and cached pages are allocated by the HvlpSetupBootProcessorEarlyHypercallPages function by using the HAL (Hardware Abstraction Layer) early allocation code (HalAllocateEarlyPages from the HAL Private Dispatch Table), which ends up grabbing memory from the so-called HAL Heap (advanced readers are advised that this is interesting), while for the Application Processors (APs), MmAllocateIndependentPages, the HvlInitializeProcessor function uses MmAllocateIndependentPages to grab per-NUMA-node local physical pages.

Issuing the Hypercall

Now that we know how to take the input and output buffer and convert them into appropriate physical addresses, we need to talk about how to actually talk to Hyper-V to issue this call. First, it’s important to note that the goal of this series is not to go into Hyper-V specifics as much as it is to talk about interfacing with Hyper-V on a Windows system, for security/fuzzing purposes — therefore, the details of how the actual VMCALL instruction works and how Hyper-V maps its “hypercall page” through the Hypercall Interface MSR will be left to readers curious enough to read the TLFS.

Our approach, instead, will be to re-use Windows’ existing capabilities, and avoid handcrafing assembly code (and conflicting with the memory manager, VSM, and Patchguard). In order to assist us, Windows 10 has a helpful exported kernel call which lets us do just that:

NTKERNELAPI
HV_X64_HYPERCALL_OUTPUT
NTAPI
HvlInvokeHypercall (
    _In_ HV_X64_HYPERCALL_INPUT InputValue,
    _In_ ULONGLONG InputPa,
    _In_opt_ ULONGLONG OutputPa
    );

As you may have expected, this API simply takes in the hypercall input value, comes back with the hypercall result value, and accepts the physical addresses of the input and output buffers (if any). Therefore, all we have to do is plug in a call to this export from our IOCTL handler (seen in Part 1), and correctly construct the MDLs with the copy of the input and output buffer.

Sample Code — Bridging User and Kernel

All the building blocks are now ready and we begin by first defining the IOCTL value itself as well as the data structure that will be used to communicate between the two worlds. Then, we add some code to the IOCTL event callback to execute our handler, which will build the MDLs and then issue the call. Afterwise, we copy any output data back to the caller, and the user-mode client displays the result.

Defining the IOCTL Value and Buffer

In this approach, we’ve decided to simply use the standard of beginning our IOCTL functions at 0x100, and picking the FILE_DEVICE_UNKNOWN device type instead of defining our own. The input data structure contains the actual pointers to the input and output buffers (and their size), as well as the hypercall input value and the hypercall result value.

#define IOCTL_ISSUE_HYPER_CALL   \
    CTL_CODE(FILE_DEVICE_UNKNOWN, 0x100, METHOD_BUFFERED, FILE_WRITE_ACCESS)

typedef struct _VTL_BRIDGE_ISSUE_HYPER_CALL
{
    HV_X64_HYPERCALL_INPUT InputDescriptor;
    _Field_size_(InputSize) _User_always_
        PVOID InputBuffer;
    ULONG InputSize;

    HV_X64_HYPERCALL_OUTPUT OutputDescriptor;
    _Field_size_opt_(OutputSize) _User_always_
        PVOID OutputBuffer;
    ULONG OutputSize;
} VTL_BRIDGE_ISSUE_HYPER_CALL, *PVTL_BRIDGE_ISSUE_HYPER_CALL;

Extending the IOCTL Event Callback

We take the stub code seen in Part 1 and we add the following block of code to the IOCTL switch statement, which now calls the handler itself.

    case IOCTL_ISSUE_HYPER_CALL:
    {
        //
        // The input and output buffer sizes are identical
        //
        if (InputLength != OutputLength)
        {
            DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                       DPFLTR_ERROR_LEVEL,
                       "Size mismatch: %llx %llx\n",
                       InputLength,
                       OutputLength);
            status = STATUS_INVALID_PARAMETER;
            goto Exit;
        }

        //
        // Handle a regular hypercall request
        //
        status = HandleRegularHvCall(inBuffer,
                                     InputLength,
                                     &resultLength);
        break;
    }

Implementing the Handler

The handler needs to take our request buffer, make sure it’s the expected size, and then construct MDLs for the input and output buffers that are referenced. Once they are constructed, the HVL interface can be used to communicate to Hyper-V, after which the result value can be written back in the buffer.

Recall that WDF takes care of probing and copying the request buffer, but the deep pointers to the input and output buffers are user-mode data for us to correctly handle.

NTSTATUS
HandleRegularHvCall (
    _In_ PVOID RequestBuffer,
    _In_ SIZE_T RequestBufferSize,
    _Out_ PULONG_PTR ResultLength
    )
{
    NTSTATUS status;
    PVTL_BRIDGE_ISSUE_HYPER_CALL hcCall;
    HV_X64_HYPERCALL_OUTPUT output;
    ULONGLONG inputPa, outputPa;
    PMDL inputMdl, outputMdl;

    //
    // Grab the hypercall buffer from the caller
    //
    hcCall = (PVTL_BRIDGE_ISSUE_HYPER_CALL)RequestBuffer;
    inputMdl = NULL;
    outputMdl = NULL;
    inputPa = 0;
    outputPa = 0;

    //
    // The request buffer must match the size we expect
    //
    *ResultLength = 0;
    if (RequestBufferSize != sizeof(*hcCall))
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "Buffer size mismatch: %lx\n",
                   RequestBuffer);
        status = STATUS_INVALID_BUFFER_SIZE;
        goto Cleanup;
    }

    //
    // Check if the hypercall has any input data
    //
    if (hcCall->InputSize != 0)
    {
        //
        // Make an MDL for it
        //
        status = MapUserBufferToMdl(hcCall->InputBuffer,
                                    hcCall->InputSize,
                                    TRUE,
                                    &inputPa,
                                    &inputMdl);
        if (!NT_SUCCESS(status))
        {
            DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                       DPFLTR_ERROR_LEVEL,
                       "Failed to create input MDL: %lx\n",
                       status);
            goto Cleanup;
        }
    }

    //
    // Check if the hypercall has output data
    //
    if (hcCall->OutputSize != 0)
    {
        //
        // Make an MDL for it too
        //
        status = MapUserBufferToMdl(hcCall->OutputBuffer,
                                    hcCall->OutputSize,
                                    FALSE,
                                    &outputPa,
                                    &outputMdl);
        if (!NT_SUCCESS(status))
        {
            DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                       DPFLTR_ERROR_LEVEL,
                       "Failed to create output MDL: %lx\n",
                       status);
            goto Cleanup;
        }
    }

    //
    // Issue the hyper call, providing the physical addresses
    //
    output = HvlInvokeHypercall(hcCall->InputDescriptor,
                                inputPa,
                                outputPa);
    hcCall->OutputDescriptor = output;

    //
    // Check if the caller expected an output buffer
    //
    if (hcCall->OutputSize != 0)
    {
        //
        // The user buffer may have become invalid,
        // guard against this with an exception handler
        //
        __try
        {
            NT_ASSERT(outputMdl != NULL);
            RtlCopyMemory(hcCall->OutputBuffer,
                          MmGetMdlVirtualAddress(outputMdl),
                          __alignof(UCHAR));
        }
        __except (EXCEPTION_EXECUTE_HANDLER)
        {
            status = GetExceptionCode();
            goto Cleanup;
        }
    }

    //
    // Return the data back to the user, who will deal with it
    //
    *ResultLength = sizeof(*hcCall);
    status = STATUS_SUCCESS;

Cleanup:
    //
    // If there was an input MDL, free it
    //
    if (inputMdl != NULL)
    {
        NT_ASSERT(hcCall->InputSize != 0);
        MmFreePagesFromMdlEx(inputMdl, 0);
        ExFreePool(inputMdl);
    }

    //
    // To the same for the output MDL
    //
    if (outputMdl != NULL)
    {
        NT_ASSERT(hcCall->OutputSize != 0);
        MmFreePagesFromMdlEx(outputMdl, 0);
        ExFreePool(outputMdl);
    }
    return status;
}

Issuing the IOCTL from User-Mode

Now let’s try an actual hypercall and see if the bridge works. For this example, we’ll use HvCallGetVpIndexFromApicId, which is a very simple call that returns the Virtual Processor (VP) index based on the physical Advanced Programmable Interrupt Controller (APIC) ID of the processor. From the host, this call should be allowed, and should return the identical number back — as on the root partition, there is a 1:1 mapping between VPs and APIC IDs.

You might note that this is actually a repeated call as is visible by the setting of the CountOfElements field. This is because an array of APIC IDs can be provided, which will result in an array of VPs. In this sample, though, we are only specifying a single element, so we don’t have the restarting logic that a repeated call normally requires.

DWORD
HcBridgeTest (
    _In_ HANDLE hFuzzer
    )
{
    HV_INPUT_GET_VP_INDEX_FROM_APIC_ID Input;
    HV_VP_INDEX Output;
    VTL_BRIDGE_ISSUE_HYPER_CALL hcCall;
    BOOL bRes;
    DWORD dwError;
    HV_STATUS hvStatus;

    //
    // Clear buffers
    //
    RtlZeroMemory(&Input, sizeof(Input));
    RtlZeroMemory(&Output, sizeof(Output));

    //
    // Issue a slow call with a single element
    //
    hcCall.InputDescriptor.AsUINT64 = HvCallGetVpIndexFromApicId;
    hcCall.InputDescriptor.CountOfElements = 1;
    hcCall.InputDescriptor.IsFast = 0;

    //
    // Request the VP Index for APIC ID 1
    //
    Input.PartitionId = HV_PARTITION_ID_SELF;
    Input.TargetVtl = 0;
    Input.ProcHwIds[0] = 1;

    //
    // Construct the request buffer
    //
    hcCall.InputSize = sizeof(Input);
    hcCall.OutputSize = sizeof(Output);
    hcCall.InputBuffer = &Input;
    hcCall.OutputBuffer = &Output;

    //
    // Issue the IOCTL to our bridge
    //
    bRes = DeviceIoControl(hFuzzer,
                           IOCTL_ISSUE_HYPER_CALL,
                           &hcCall,
                           sizeof(hcCall),
                           &hcCall,
                           sizeof(hcCall),
                           NULL,
                           NULL);
    if (bRes == FALSE)
    {
        //
        // The bridge failed in some way
        //
        dwError = GetLastError();
        printf("Bridge failed to issue call: %lx\n", dwError);
        goto Exit;
    }

    //
    // Bridge worked -- see what the hypervisor returned
    //
    hvStatus = hcCall.OutputDescriptor.CallStatus;
    if (hvStatus != HV_STATUS_SUCCESS)
    {
        //
        // The hypercall made it, but it failed for some reason
        //
        printf("Hypercall failure: %lx\n", hvStatus);
        dwError = RtlNtStatusToDosError(0xC0350000 | hvStatus);
        goto Exit;
    }

    //
    // Print the processor index and return 
    //
    printf("VP Index: %lx\n", Output);
    dwError = ERROR_SUCCESS;
Exit:
    return dwError;
}

Conclusion

By now, you have hopefully gained an insight into how the elementary hypercall interface works in Windows, for regular (slow) calls that are either simple or repeated, and additional clarity on how to interface with kernel-mode drivers from user-mode through an IOCTL-based interface on top of WDF (although a WDM driver would behave identically in this particular case).

In part 3, we will continue this series by looking at extended fast hypercalls, which will require some careful understanding of stack layouts and memory alignments due to the use of XMM registers. I’ve asked Azeria to help me with one or two diagrams which should hopefully make things easier to visualize, thanks to her amazing graphic skills.

Writing a Hyper-V “Bridge” for Fuzzing — Part 1: WDF

Introduction

After spending the better part of a weekend writing a specialized Windows driver for the purposes of allowing me to communicate with the Hyper-V hypervisor, as well as the Secure Kernel, from user-mode, I realized that there was a dearth of concise technical content on non-PnP driver development, and especially on how the Windows Driver Foundation (WDF) fundamentally changes how such drivers can be developed.

While I’ll eventually release my full tool, once better polished, on GitHub, I figured I’d share some of the steps I took in getting there. Unlike my more usual low-level super-technical posts, this one is meant more as an introduction and tutorial, so if you already consider yourself experienced in WDF driver development, feel free to wait for Part 2.

Writing a Traditional Non-PnP Driver

Having written non-PnP Windows Driver Model (WDM) style drivers for almost two decades, it’s almost become a mechanized skill that allows me to churn out a basic driver (and associated user-mode client application) in less than 15 minutes, where I always find myself following the same basic recipe:

  • Write the DriverEntry and DriverUnload function, including creating a device object, naming it, ACL’ing it correctly, and then creating a Win32 symbolic link under \DosDevices
  • Stub out an IRP_MJ_CREATE, IRP_MJ_CLOSE and IRP_MJ_DEVICE_CONTROL handler
  • Define some IOCTLs as METHOD_BUFFERED, to be safe
  • Implement the IRP_MJ_CREATE/IRP_MJ_CLOSE handlers to always return success and complete the IRP
  • Do the right IO_STACK_LOCATION manipulation in the IOCTL handler and use the appropriate WDM macros to read, parse, and complete the request(s)
  • Write a user-mode tool that calls CreateFile and then DeviceIoControl to communicate to the driver
  • Either use Sc.exe on the command-line to create a kernel-mode service entry and then start/stop the driver as needed, or write the equivalent C Code using CreateService, StartService and StopService

Countless online tutorials and updated samples on GitHub from Microsoft (such as those part of the Windows Driver Kit) explain these steps in detail for the curious minded — but this post isn’t about rehashing that, it’s about looking at the new.

I had long heard how the Windows Driver Foundation (WDF) is meant to provide a much easier model for writing true hardware device drivers — including, in Windows 10, allowing mostly cross-compilable user-mode drivers (vs. the older framework which required writing the driver in C++/COM).

But I had wrongly assumed that for writing Non-PnP research/academic drivers (or even for production), WDM was still the better, and easier choice. For example, I don’t know of a single anti-malware tool whose filter drivers are WDF based — and in fact, the framework is poorly suited for such use (or NDIS Light-Weight Filters (LWF), or Windows Filtering Platform (WFP) callout drivers, or etc…).

The truth is that, unless you’re truly plugging into some OS filtering stack still focused on WDM, a simple “process some IOCTLs” driver can be much more easily written in WDF, and the installation/uninstallation process can also be made more robust by following certain principles.

Writing a Modern “Non-PnP” Driver

With WDF — and more specifically, its kernel counterpart, the Kernel Mode Driver Framework (KMDF) — you’ll have two options at your disposal for writing a simple non-hardware driver that communicates with a user-space client (and note that KMDF was back-ported all the way back to Windows 2000, so this isn’t some sort of new Windows 10 functionality):

  1. You can write a true non-PnP driver by setting the correct flag on your WDF Driver Object, manually creating your WDF Device as a “Control Device”, naming it, creating the Win32 symbolic link, and securing it with an ACL. You don’t need an IRP_MJ_CREATE or IRP_MJ_CLOSE handler, and can immediately write your IOCTL handler. You must still provide a DriverUnload routine.
  2. Or you can develop a “root bus-enumerated” PnP driver by providing a simple .INF file, provide an AddDevice routine, and have WDF automatically call that when your ‘device’ is detected. In your AddDevice routine, construct an unnamed WDF Device, register an interface with a custom GUID (you can also do this in the INF and avoid a few extra lines of code), and provide your IOCTL handler. You do not need an IRP_MJ_CREATE, IRP_MJ_CLOSE, or IRP_MJ_PNP/IRP_MJ_POWER handler, and must still provide a DriverUnload routine.

While these two options appear similar, there is one crucial difference — in the first implementation, you must manually register, load, and unload this driver every time you wish to talk with it from user-mode. If you leave it loaded, the user’s only choice is to manually run command-line tools like Sc.exe or Net.exe to unload it. Without using forensic tools, the debugger, or power tools, the user does not know your driver is loaded. You must pick a static name for your device, and hope it does not collide with anyone else’s device name.

In the second implementation, your driver is registered with the system as a PnP driver that is automatically detected by virtue of being on the “root bus”. This means users see it in Device Manager, and can easily interrogate it for information, disable it, and even uninstall it. To communicate with your driver, your application uses a custom GUID that you’ve defined, and enumerates an interface associated with it — a much stricter and unique protocol than relying on a string. Such a driver can also be more easily signed by Microsoft’s Windows Hardware Quality Lab (WHQL) infrastructure, and can attest to its security better than a raw non-PnP driver without an .INF or .CAT file.

Clearly, for a pure “proof-of-concept” driver, the benefits of the second implementation may not seem worth writing an extra INF file and learning some new SetupAPI functions instead of CreateService. But for something a little more polished, more generally usable, a root bus-enumerated driver, is in my opinion, the way to go.

It’s worth noting that WDF doesn’t invent this concept, but what finally makes it (in my opinion) reachable to the researcher masses, is that unlike in WDM, where this option required 1500-4000 lines of boiler-plate PnP code to be correctly enumerated, installed, uninstalled, disabled, interrogated, and more, there is literally zero additional work required when using WDF — again, no IRP_MJ_PNP handler, no WMI, no Power Management, and none of the things you may have seen if you have ever attempted this in your past.

In fact, strictly speaking, it’s actually less line of code to write a root-bus enumerated PnP driver than a non-PnP driver, with the caveat that the latter needs an INF file. But let’s be honest, once you’ve written one, you can largely copy-paste it — and if you ever wanted to have your driver signed by Microsoft, you’ll need an INF anyway.

Interacting with a Root-Enumerated PnP Driver

Because you are not statically naming your device driver, and because it must be PnP enumerated, the user-mode code looks a bit different than the traditional way to install and talk to a non-PnP driver. There are 2 steps that might be new to you:

  1. Installing the driver is done by first creating a “fake” device node under the root bus. Typically, true hardware device drivers are installed when the PnP manager discovers a physical device on the machine, interrogates it and builds a unique device instance path for it (containing, among other things, information such as Device ID and/or Vendor ID), and finds a matching driver that is registered with some combination/part of that device instance path (through its INF file). In our case, we will manually create a device node pretending that PnP detected such a “device” on the root bus, and we will manually name this node in the exact same way our INF file indicates, while claiming that this “device” is of the same device class our INF file indicates.
  2. After we’ve created this fake device node, we’ll point Windows at our INF file, and tell it to do an update/refresh of the PnP device tree — which will make it discover our fake device node, see that there’s a perfectly matching INF that describes it, and load the indicated driver!

If this sounds scary and a lot of code — let me re-assure you: we are talking about 3 APIs and less than 20 lines of code, as you’ll shortly see in the sample code below!

Now that the driver has been installed, unless your code (or the user/some other code) uninstalls it, it will remain persistent on the system, and automatically reloaded every boot. In fact, if you repeat the steps above even in the face of an already-installed copy of the driver, you will simply be creating yet another fake device node, and load another copy of your driver.

Therefore, we must solve our last hurdle — figuring out if our driver is already loaded — both so that we can avoid multiple re-installations, as well as so we can figure out how to communicate with it from user-mode. This is achieved by using that device interface GUID that we mentioned a driver should register:

  1. First, begin by checking if there are any devices that expose our custom device interface GUID. If not, then our driver is not loaded, which means we must perform the installation steps described.
  2. If so, this will return a device interface data structure, which we can then query to obtain a device instance path name. This name can then directly be passed to CreateFile in order to obtain a handle to the device object and send IOCTLs.

Behind the scenes, what really happens is that the “unnamed” WDF/PnP device object that was created does actually have a name — say, for example \Device\0000005c (if you’ve seen such devices in your WinObj or WinDbg before, now you know what they are). In turn, under the \DosDevices namespace (aka \GLOBAL??), the I/O manager did create a symbolic link — but based on a string representation of the unique device instance path. The API that queries the device interface data mentioned in step #2 above essentially does this lookup, and returns that symbolic link.

Once again, while this may also sound like a lot of complex code, it’s actually achieved by a single API call and less than a dozen lines of code (and that includes error handling), as you’re about to see below!

Finally, you may also want to provide the option (or automatically do this every time your user-mode tool exits) to uninstall the driver. In this situation, we use the SetupAPI calls to enumerate for our device class GUID (instead of our interface GUID), and once found, we pass that information to the same installer API, but a different parameter that handles uninstallation in this situation.

With only 3 APIs and another 20 lines of code, the driver is automatically unloaded assuming there’s no longer any handles (otherwise, it will be unloaded when the handles are closed and/or when the machine reboots), and uninstalled from Device Manager so that the device node is not found and matched again at the next boot.

Sample Code

Enough theory — let’s take a look at a very simple root-bus enumerated driver to see the code in practice, including how the INF file should look like. Then, we’ll see how the associated user-mode client application looks like, and how it can install, uninstall, and communicate with our device by finding its interface and opening a handle to it.

DriverEntry and DriverUnload Routines

First, our DriverEntry function looks a bit different than in WDM, as we are not touching the DRIVER_OBJECT in any way. Instead, we use a WDF_DRIVER_CONFIG structure to initialize our AddDevice routine and our Unload routine, then use WdfDriverCreate to initialize a WDF Driver Object on top of the WDM/NT DRIVER_OBJECT.

NTSTATUS
DriverEntry (
    _Inout_ PDRIVER_OBJECT DriverObject,
    _In_ PUNICODE_STRING RegistryPath
    )
{
    NTSTATUS status;
    WDF_DRIVER_CONFIG config;

    //
    // Initialize our Driver Configuration, specifying an unload
    // routine and an AddDevice routine (making us a PnP driver)
    //
    WDF_DRIVER_CONFIG_INIT(&config, DeviceAdd);
    config.EvtDriverUnload = DriverUnload;

    //
    // Create the WDF Driver Object
    //
    status = WdfDriverCreate(DriverObject,
                             RegistryPath,
                             WDF_NO_OBJECT_ATTRIBUTES,
                             &config,
                             WDF_NO_HANDLE);
    if (!NT_SUCCESS(status))
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "WdfDriverCreate fail: %lx\n",
                   status);
    }

    //
    // Return back to OS
    //
    return status;
}

The DriverUnload function works just like in the WDM world, except that from now on, all routines (except the DriverEntry) will be receiving a WDFDRIVER, not a DRIVER_OBJECT.

VOID
DriverUnload (
    _In_ WDFDRIVER Driver
    )
{
    UNREFERENCED_PARAMETER(Driver);
    PAGED_CODE();

    //
    // Nothing to do for now
    //
    NOTHING;
}

AddDevice Routine

This is where, traditional PnP WDM drivers would create their DEVICE_OBJECT and do PCI/bus scanning and/or identification to confirm this is a device they can handle. Additional initialization would then usually happen in the driver’s IRP_MN_START_DEVICE handler as part of their IRP_MJ_PNP handler. But as we are not a true hardware driver, these concerns do not affect us.

Instead, in the WDF world, the only things we have to worry about are creating a WDF Device Object, register a custom device interface GUID so that we can talk to our device from user mode, and initialize a WDF Queue Object, which is how we’ll be able to receive IRPs — in our case, we register an IOCTL handler for IRP_MJ_DEVICE_CONTROL.

A WDF queue can either be serialized or parallelized, and we don’t have any specific restrictions that prevent multiple client apps from talking with us concurrently (but if we wanted to prevent that, we could).

DEFINE_GUID(HyprFuzzGuid,
            0x4056adb2,
            0x8e4e,
            0x4b6a,
            0x88, 0x2e, 0xff, 0x1, 0xc, 0x3a, 0x1c, 0x63);

NTSTATUS
AddDevice (
    _In_ WDFDRIVER Driver,
    _In_ PWDFDEVICE_INIT DeviceInit
    )
{
    NTSTATUS status;
    WDFDEVICE device;
    WDF_IO_QUEUE_CONFIG queueConfig;
    WDFQUEUE queue;
    UNREFERENCED_PARAMETER(Driver);

    //
    // Create a WDF Device 
    //
    status = WdfDeviceCreate(&DeviceInit,
                             WDF_NO_OBJECT_ATTRIBUTES,
                             &device);
    if (!NT_SUCCESS(status))
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "WdfDeviceCreate fail: %lx\n",
                   status);
        goto Exit;
    }

    //
    // Create a device interface so that UM can talk with us
    //
    status = WdfDeviceCreateDeviceInterface(device,
                                            &HyprFuzzGuid,
                                            NULL);
    if (!NT_SUCCESS(status))
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "WdfDeviceCreateDeviceInterface fail: %lx\n",
                   status);
        goto Exit;
    }

    //
    // Create a queue that handles IOCTLs
    //
    WDF_IO_QUEUE_CONFIG_INIT_DEFAULT_QUEUE(&queueConfig,
                                    WdfIoQueueDispatchParallel);
    queueConfig.EvtIoDeviceControl = IoDeviceControl;
    status = WdfIoQueueCreate(device,
                              &queueConfig,
                              WDF_NO_OBJECT_ATTRIBUTES,
                              &queue);
    if (!NT_SUCCESS(status))
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "WdfIoQueueCreate fail: %lx\n",
                   status);
        goto Exit;
    }

Exit:
    return status;
}

Device I/O Control (IOCTL) Routine

Well, that’s almost it! At this point, the only missing piece is the IOCTL handler which will consume request from user-mode.

With WDF, you do not need to worry about I/O Stack Locations, and the various macros to copy, skip, switch these locations. In addition, instead of having a generic IRP handler which requires you to manually read the appropriate arguments in the I/O Stack Location, each WDF event callback contains specifically the data associated with this request, making development easier.

An additional benefit of WDF is that regardless of the IOCTL mechanism, APIs exist to grab the input buffer and length, so that you do not have to remember which field in the IRP contains the input and output buffers.

VOID
IoDeviceControl (
    _In_ WDFQUEUE Queue,
    _In_ WDFREQUEST Request,
    _In_ SIZE_T OutputLength,
    _In_ SIZE_T InputLength,
    _In_ ULONG IoControlCode
    )
{
    NTSTATUS status;
    PVOID inBuffer;
    PVOID outBuffer;
    ULONG_PTR resultLength;
    PAGED_CODE();
    UNREFERENCED_PARAMETER(Queue);

    //
    // Assume we won't return anything
    //
    resultLength = 0;
    inBuffer = NULL;
    outBuffer = NULL;

    //
    // Grab the input buffer (this will fail if it's 0 bytes)
    //
    status = WdfRequestRetrieveInputBuffer(Request,
                                           InputLength,
                                           &inBuffer,
                                           NULL);
    if (!NT_SUCCESS(status))
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "WdfRequestRetrieveInputBuffer fail: %lx\n",
                   status);
        goto Exit;
    }

    //
    // Grab the output buffer (this will fail if it's 0 bytes)
    //
    status = WdfRequestRetrieveOutputBuffer(Request,
                                            OutputLength,
                                            &outBuffer,
                                            NULL);
    if (!NT_SUCCESS(status))
    {
        DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                   DPFLTR_ERROR_LEVEL,
                   "WdfRequestRetrieveOutputBuffer fail: %lx\n",
                   status);
        goto Exit;
    }

    //
    // Handle the possible IOCTLs
    //
    switch (IoControlCode)
    {
        case IOCTL_ISSUE_HYPER_CALL:
        {
            //
            // Implement this
            //
            status = STATUS_SUCCESS;
            break;
        }

        default:
        {
            DbgPrintEx(DPFLTR_IHVDRIVER_ID,
                       DPFLTR_ERROR_LEVEL,
                       "Invalid IOCTL: %lx\n",
                       IoControlCode);
            status = STATUS_INVALID_DEVICE_REQUEST;
            break;
        }
    }

Exit:
    //
    // Return back to caller
    //
    WdfRequestCompleteWithInformation(Request,
                                      status,
                                      resultLength);
}

The INF File

The last step in a proper WDF driver is having an INF file that marks us as a root-enumerated driver, sets a strong security descriptor, and populates some strings and icons for the user to see in their Device Manager.

The key part is the Device Class GUID, which we’ll need to remember for the installation of our driver, and the device node name, which here is Root\VtlBrdge. Everything else you see is mostly boilerplate, or done for readability and UI purposes.

[Version]
DriverVer = 01/15/2019,10.30.45.805
signature="$WINDOWS NT$"
Class=FuzzerClass
CatalogFile=vtlbrdge.cat
ClassGuid={0D833DAE-8619-11D3-C19B-B60B0E0FD4AB}
Provider=%Mfg%

[SourceDisksNames]
1=%DiskId%

[SourceDisksFiles]
vtlbrdge.sys = 1

[ClassInstall32]
AddReg=FuzzerClass

[FuzzerClass]
HKR,,,,%ClassName%
HKR,,Icon,,-8
HKR,,Security,,"D:P(A;;GA;;;SY)(A;;GA;;;BA)"

[DestinationDirs]
SYS.CopyList=10,system32\drivers

[Manufacturer]
%Mfg%=AlexIonescu,NTAMD64

[AlexIonescu.NTAMD64]
%DeviceDesc% = VtlBrdgeInstall, Root\VtlBrdge

[VtlBrdgeInstall]
CopyFiles=SYS.CopyList

[SYS.CopyList]
vtlbrdge.sys

[VtlBrdgeInstall.Services]
AddService = VtlBrdge,2,VtlBrdgeInstall_Service_Inst

[VtlBrdgeInstall_Service_Inst]
ServiceType   = 1
StartType     = 3
ErrorControl  = 1
LoadOrderGroup = "Base"
ServiceBinary = %12%\vtlbrdge.sys 

[Strings]
Mfg = "Alex Ionescu (@aionescu)"
DeviceDesc = "VTL Bridge and Hyper Call Connector"
DiskId = "Install disk (1 of 1)"
ClassName = "Fuzzer Devices"

So now we have a fully working driver that we are ready to talk to from user-mode, let’s look at how we can install our driver, obtain a handle to communicate with it, and finally, uninstall it.

Installing the Driver

First, installing the driver is a simple matter of creating the fake root device node, then pointing Windows at the INF to bind with it and load our driver. The trick is referencing the same device class GUID as in the INF, as well as the same root device instance path, as we mentioned above.

Note that there are more complex APIs that you can use to automatically parse the INF and extract this information if you’re dealing with someone else’s driver, but you ought to know (and hardcode) your own GUID and instance path for your own driver, in my opinion.

DEFINE_GUID(FuzzerClassGuid,
            0xd833dae,
            0x8619,
            0x11d3,
            0xc1, 0x9b, 0xb6, 0xb, 0xe, 0xf, 0xd4, 0xab);

LPWSTR g_DevPath = L"Root\\VtlBrdge\0\0";

LPWSTR g_InfPath = L"C:\\vtlbrdge\\vtlbrdge.inf";

DWORD
HvFuzzInstallDevice (
    VOID
    )
{
    HDEVINFO hDevInfo;
    SP_DEVINFO_DATA devInfo;
    BOOL bReboot, bRes;
    DWORD dwError;
    DWORD dwPathLen;

    //
    // Create a device info list for our class GUID
    //
    hDevInfo = SetupDiCreateDeviceInfoList(&FuzzerClassGuid,
                                           NULL);
    if (hDevInfo == INVALID_HANDLE_VALUE)
    {
        dwError = GetLastError();
        printf("SetupDiCreateDeviceInfoList fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Construct a device information structure for this device
    //
    ZeroMemory(&devInfo, sizeof(devInfo));
    devInfo.cbSize = sizeof(devInfo);
    bRes = SetupDiCreateDeviceInfo(hDevInfo,
                                   L"FuzzerClass",
                                   &FuzzerClassGuid,
                                   L"Fuzzer Class Devices",
                                   NULL,
                                   DICD_GENERATE_ID,
                                   &devInfo);
    if (bRes == FALSE)
    {
        dwError = GetLastError();
        printf("SetupDiCreateDeviceInfo fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Add the hardware ID for this specific fuzzing device
    //
    dwPathLen = ((DWORD)wcslen(g_DevPath) + 3) * sizeof(WCHAR);
    bRes = SetupDiSetDeviceRegistryProperty(hDevInfo,
                                            &devInfo,
                                            SPDRP_HARDWAREID,
                                            (LPBYTE)g_DevPath,
                                            dwPathLen);
    if (bRes == FALSE)
    {
        dwError = GetLastError();
        printf("SetupDiSetDeviceRegistryProperty fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Create the "fake" root device node for the device
    //
    bRes = SetupDiCallClassInstaller(DIF_REGISTERDEVICE,
                                     hDevInfo,
                                     &devInfo);
    if (bRes == FALSE)
    {
        dwError = GetLastError();
        printf("SetupDiCallClassInstaller fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Now install the INF file for the fuzzing device.
    //
    // It will be root enumerated because of the device node
    // that we created above, resulting in the driver loading.
    //
    bRes = UpdateDriverForPlugAndPlayDevices(NULL,
                                             g_DevPath,
                                             g_InfPath,
                                             INSTALLFLAG_FORCE,
                                             &bReboot);
    if (bRes == FALSE)
    {
        dwError = GetLastError();
        printf("UpdateDriverForPlugAndPlayDevices fail: %lx\n",
               dwError);
        goto Exit;
    }

    dwError = ERROR_SUCCESS;
Exit:
    return dwError;
}

Opening a Handle to the Driver

Next, or rather, typically, even before attempting installation, we must obtain a handle to our device, checking if anyone exposes our device interface GUID on this system, and if so, obtaining the symbolic link name of the interface and creating a file handle to it. Here, we must use the GUID of our device interface, not that of the device class.

DEFINE_GUID(HyprFuzzGuid,
            0x4056adb2,
            0x8e4e,
            0x4b6a,
            0x88, 0x2e, 0xff, 0x1, 0xc, 0x3a, 0x1c, 0x63);

DWORD
HvFuzzGetHandle (
    _Outptr_ PHANDLE phFuzzer
    )
{
    CONFIGRET cr;
    DWORD dwError;
    WCHAR pwszDeviceName[MAX_DEVICE_ID_LEN];
    HANDLE hFuzzer;

    //
    // Assume failure
    //
    *phFuzzer = NULL;

    //
    // Get the device interface -- we only expose one
    //
    pwszDeviceName[0] = UNICODE_NULL;
    cr = CM_Get_Device_Interface_List((LPGUID)&HyprFuzzGuid,
                                      NULL,
                                      pwszDeviceName,
                                      _countof(pwszDeviceName),
                                      CM_GET_DEVICE_INTERFACE_
                                      LIST_PRESENT);
    if (cr != CR_SUCCESS)
    {
        dwError = GetLastError();
        printf("CM_Get_Device_Interface_List fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Make sure there's an actual name there
    //
    if (pwszDeviceName[0] == UNICODE_NULL)
    {
        dwError = ERROR_NOT_FOUND;
        goto Exit;
    }

    //
    // Open the device
    //
    hFuzzer = CreateFile(pwszDeviceName,
                         GENERIC_WRITE | GENERIC_READ,
                         FILE_SHARE_READ | FILE_SHARE_WRITE,
                         NULL,
                         OPEN_EXISTING,
                         FILE_ATTRIBUTE_NORMAL,
                         NULL);
    if (hFuzzer == INVALID_HANDLE_VALUE)
    {
        dwError = GetLastError();
        printf("CreateFile fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Return a handle to the device
    //
    *phFuzzer = hFuzzer;
    dwError = ERROR_SUCCESS;
Exit:
    return dwError;
}

Uninstalling the Driver

Either by offering it up as a user action in a command-line tool, or doing it every single time your user-mode application exists, you may want to uninstall the driver without prompting the user to use Device Manager to do so. This is a simple matter of enumerating our driver to find it (this time using the device class GUID) and calling the same SetupAPI function as for installation, but with the DIF_REMOVE parameter instead.

DWORD
HvFuzzUninstallDevice (
    VOID
    )
{
    BOOL bRes;
    DWORD dwError;
    HDEVINFO hDevInfo;
    SP_DEVINFO_DATA devData;

    //
    // Open the device info list for our class GUID
    //
    hDevInfo = SetupDiGetClassDevs(&FuzzerClassGuid,
                                   NULL,
                                   NULL,
                                   0);
    if (hDevInfo == INVALID_HANDLE_VALUE)
    {
        dwError = GetLastError();
        printf("SetupDiGetClassDevs fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Locate our class device
    //
    ZeroMemory(&devData, sizeof(devData));
    devData.cbSize = sizeof(devData);
    bRes = SetupDiEnumDeviceInfo(hDevInfo, 0, &devData);
    if (bRes == FALSE)
    {
        dwError = GetLastError();
        printf("SetupDiEnumDeviceInfo fail: %lx\n",
               dwError);
        goto Exit;
    }

    //
    // Uninstall it
    //
    bRes = SetupDiCallClassInstaller(DIF_REMOVE,
                                     hDevInfo,
                                     &devData);
    if (bRes == FALSE)
    {
        dwError = GetLastError();
        printf("SetupDiCallClassInstaller fail: %lx\n",
               dwError);
        goto Exit;
    }

    dwError = ERROR_SUCCESS;
Exit:
    return dwError;
}

Conclusion

Now that you’ve written up both the driver code and application code, checking if everything works is fairly simple. Once the installation path executes, a nicely visible entry in Device Manager will show up as per below, with the various strings we specified in our INF file.

Furthermore, due to the negative Icon ID (-8) we specified in our INF file, this instructs Device Manager to look up Icon Group 8 in the resource section of SetupApi.dll, which I thought matched quite well with the “VTL Bridge and Hyper Call Connector” device we are trying to represent. I used Resource Hacker to go over the resources, since the icons are not formally documented anywhere. Note that using a positive Icon ID results in Device Manager looking up the resource in your driver binary (or DLL co-installer).

Our visible device in the Device Manager, under our custom device class (“Fuzzer Devices”)

And, once your application is talking to the driver, you’ll see a handle to some “unnamed”, or rather, as we’ve seen, a numbered device object, thanks to the instance path symbolic link exposed by our interface. By using hfiref0x’s great WinObjEx64 tool, you can easily see it, as in the screenshot below.

The device instance path symbolic link to the numbered device object

The astute reader may notice that this instance path is a simple combination of the root bus name, the device class name, an instance identifier for the device (0000), followed by the GUID of the HyprFuzz device interface we defined in our code above. As such, while not recommended, you can theoretically directly try to open a handle to your driver by hard-coding this information, but going through the appropriate API is certainly nicer.

This concludes Part 1 — while I apologize that no meaty technical details about Hyper-V were present, I still do hope that this content/tutorial was useful to some of the more junior readers, and check back next week for Part 2, where we’ll go over in detail into some hypervisor internals!