In this episode of the series, we are going to take a little side trip to explore contexts in a registry filter driver. There are two main types of context in a registry filter. The first is the most simple and is called a callback context. This is simply a pointer value that you can initialize during a pre-operation callback and it gets passed to you in your post-operation callback. The PVOID member is in all of the pre-op callback structures, such as REG_CREATE_KEY_INFORMATION, starting with Vista. The field name is CallContext, and you simply set it during pre-op and use it during post-op. Cm does not manage this object’s lifetime in any way for you. It doesn’t automatically behave one way if the call succeeds vs. fails, or anything like that. So you have to take care to handle those things yourself. Following is a very simple pre-create/open callback, all it does is allocate a structure and set the CallContext so that the post- callback will receive it.

NTSTATUS RfPreCreateOrOpenKeyEx( __in PVOID CallbackContext, __in PVOID Argument1, __in PREG_CREATE_KEY_INFORMATION CallbackData )
{
    UNREFERENCED_PARAMETER( CallbackContext );
    UNREFERENCED_PARAMETER( Argument1 );
    PAGED_CODE();

    //	Allocate a new callback context and have it passed back to us when the open key completes
    PMY_CONTEXT pContext = (PMY_CONTEXT) ExAllocatePoolWithTag( PagedPool, sizeof(MY_CONTEXT), 'xCyM' );
    if ( pContext )
    {
        //	Put some meaningless data in our context that we can verify later
        pContext->Data = 42;
        CallbackData->CallContext = pContext;
    }
    return STATUS_SUCCESS;
}

The code for the post- callback is almost as simple, but now we will examine the second kind of context: the object context. Registry filters can attach a bit of data to any registry object. This may be useful for creating some data during key-open that would be used during later operations such as a value enumerate. You can put anything in here, and Cm will pass it to you in the ObjectContext field of every subsequent callback for that object. Multiple registry filters can each store their own context, and the system differentiates between these by using the “Cookie” value that is returned when you register your filter with Cm.

For our example, we will take the context that was generated in the pre-create/open, and if the create/open was successful, we will attempt to attach this same context to the object. If the operation was not successful, or we were unable to set the object context, then we need to free the context ourselves.

NTSTATUS RfPostCreateOrOpenKeyEx( __in PVOID CallbackContext, __in PVOID Argument1, __in PREG_POST_OPERATION_INFORMATION CallbackData )
{
    UNREFERENCED_PARAMETER( CallbackContext );
    UNREFERENCED_PARAMETER( Argument1 );
    PAGED_CODE();

    if ( CallbackData->CallContext )
    {
        if ( STATUS_SUCCESS == CallbackData->Status )
        {
            //	Attach the context we allocated to the opened object
            NTSTATUS status;
            status = CmSetCallbackObjectContext( CallbackData->Object, &g_CmCookie, CallbackData->CallContext, NULL );
            if ( NT_SUCCESS( status ) )
            {
                InterlockedIncrement( &g_nObjectContexts );
            }
            else
            {
                ExFreePool( CallbackData->CallContext );
            }
        }
        else
        {
            ExFreePool( CallbackData->CallContext );
        }
    }
    return STATUS_SUCCESS;
}

Once the context is attached to the object, Cm now will manage it’s lifetime for us, giving us a callback whenever it needs the context to be cleaned up. (Cm makes this callback either when the object is closed, or when your filter is unloaded.)

NTSTATUS RfCallbackObjectContextCleanup( __in PVOID CallbackContext, __in PVOID Argument1, __in PREG_CALLBACK_CONTEXT_CLEANUP_INFORMATION CallbackData )
{
    UNREFERENCED_PARAMETER( CallbackContext );
    UNREFERENCED_PARAMETER( Argument1 );
    PAGED_CODE();

    if ( CallbackData->ObjectContext )
    {
        //	Make sure that our context looks like we expect it to
        PMY_CONTEXT pContext = (PMY_CONTEXT) CallbackData->ObjectContext;
        ASSERT( 42 == pContext->Data );
        ExFreePool( CallbackData->ObjectContext );
        InterlockedDecrement( &g_nObjectContexts );
    }
    return STATUS_SUCCESS;
}

The object context is a powerful way to help you generate information about an object that you will need later, and attach it to the object. It’s nothing you couldn’t have written yourself by managing your own storage of pointers to context, and linking the object pointer to each one, but it’s just so nice when Cm does it for you. ;)

One additional note should be made. Cm will also provide you the context of a root object when a relative create/open is done, in the RootObjectContext field.

Also note that I have put in some testing and debugging code, keeping track of how many contexts have been created vs. freed, and making sure that they all get cleaned up when the driver is unloaded. This test code is not strictly necessary, but helped to reassure me that the correct things were happening.

Registry Filter Driver Source (Part 4)

In Part 2 we looked at passively filtering a call to open a registry key (we were simply logging the name of keys opened), and we promised that next we would look at actively filtering the same kind of call. The most simple form of active filtering is simply completing the operation with some kind of error code. For example, if we wanted to prevent users from ever opening a certain registry key, we could just return access denied. This is as simple as returning the status code in your filter callback.

In the following example, we still need to get the full name of the registry key being opened as we did in Part 2, because we are determining whether to deny access based on the object name. If a caller tries to open HKLM\Software\MySecretTestKey they will receive an access denied error. There are some other side effects of putting this code in place. First, if we fail to get the name of the object being opened via our call to CmCallbackGetKeyObjectID, we will now be returning that failure code to the caller, with the result that the key will not be able to be opened even though it is not the key we are trying to block access to. This is a design decision, and probably has to do with whether a random failure should result in granting access.

NTSTATUS RfPreOpenKeyEx( __in PVOID CallbackContext, __in PVOID Argument1, __in PREG_OPEN_KEY_INFORMATION CallbackData )
{
    ...

    NTSTATUS status = STATUS_SUCCESS;

    ...

    PUNICODE_STRING pKeyNameBeingOpened = pLocalCompleteName ? pLocalCompleteName : CallbackData->CompleteName;

    //	Prevent callers from opening our secret registry key
    UNICODE_STRING TestKeyName;
    RtlInitUnicodeString( &TestKeyName, L"\\REGISTRY\\MACHINE\\SOFTWARE\\MySecretTestKey" );
    if ( !RtlCompareUnicodeString( pKeyNameBeingOpened, &TestKeyName, TRUE ) )
    {
        KdPrint( ( "RegFlt: PreOpenKeyEx for %wZ being denied!\n", pKeyNameBeingOpened ) );
        status = STATUS_ACCESS_DENIED;
        goto LExit;
    }

    ...

    return status;
}

The second side effect of our filter as written is that it handles open-key operations differently than create-key operations, which are two completely separate calls in the registry. This could result in a weird case where somebody could create the key, but then fail to open it in the future. (Similar to what can happen if you accidentally create a key with a security descriptor that doesn’t allow yourself access to the object.) In practice, I have found that it is generally best to have the create and open key callbacks perform all the same logic, and if possible to have them call the same code. I will change our sample to do this, but will not show the code here. I am going to being attaching the code file with each article so you can look there if you want to see how it’s done.

Now we put the driver onto a test system and sure enough we can pull up regedit and see that it will not let us open that key. But a strange thing is now happening on my Windows 7 test system. It doesn’t appear to prevent me from creating that key through regedit. If I create it through code, or from the command-line reg.exe, then the create fails as expected. What we are seeing here is that regedit actually creates a key named something like “New Key #1″, and then does a RENAME! (The rename-key functionality was added in Windows XP, and although very few applications use it, regedit is one that does.)

So what we are going to have to do now is filter the rename key call. This seems like it should be very simple. The REG_RENAME_KEY_INFORMATION structure provides a NewName member. We should just be able to test this against our secret key name and return access denied. But wait! Apparently, the NewName member only has the final path component (the rename key operation only allows the final component to be changed, i.e., renaming a key but not moving it to a different parent). So it looks like we are going to have to do our own name lookup and parsing to do our comparison; after all, we don’t want to block somebody from making a key named MySecretTestKey just anywhere!

Note that this is the kind of code that I really would like to avoid having to write. Pointer arithmetic and messing around with buffers directly is error prone, and great care must be taken, but sometimes it cannot be avoided. I decided to write a function that would take a UNICODE_STRING containing an object name and initialize two new UNICODE_STRING structures using the same buffer as the original, but only referring to the parent part of the name and the relative part of the name. I won’t say that the following is perfect code, but it is good enough to pass my cursory testing, and gives you an idea of what has to be done.

NTSTATUS _RfSplitParentAndRelativeNames( __in PCUNICODE_STRING pFullObjectName, __out_opt PUNICODE_STRING pParentObjectName, __out_opt PUNICODE_STRING pRelativeObjectName )
{
    //	Defensive programming. These names may come from somebody elses code, so be careful!
    if ( !pFullObjectName->Buffer || 0 == pFullObjectName->Length )
    {
        return STATUS_INVALID_PARAMETER;
    }

    //	Search backward through full object name for the path separator, taking care not to underflow!
    const wchar_t* pCh = &pFullObjectName->Buffer[ (pFullObjectName->Length / sizeof(wchar_t)) - 1 ];
    while ( pCh && pCh >= pFullObjectName->Buffer && *pCh != OBJ_NAME_PATH_SEPARATOR )
    {
        --pCh;
    }

    //	Again, be defensive. The string provided may not have had a backslash.
    if ( pCh <= pFullObjectName->Buffer )
    {
        return STATUS_INVALID_PARAMETER;
    }

    //	Everything before the character we stopped on is the parent, everything after is the relative name
    USHORT cbOffset = (USHORT) PtrDiff( pCh, pFullObjectName->Buffer );
    if ( pParentObjectName )
    {
        pParentObjectName->Length = pParentObjectName->MaximumLength = cbOffset;
        pParentObjectName->Buffer = pFullObjectName->Buffer;
    }

    if ( pRelativeObjectName )
    {
        cbOffset += sizeof( wchar_t );
        pRelativeObjectName->Length = pRelativeObjectName->MaximumLength = pFullObjectName->Length - cbOffset;
        pRelativeObjectName->Buffer = (PWCH) Add2Ptr( pFullObjectName->Buffer, cbOffset );
    }

    return STATUS_SUCCESS;
}

Given this new function we can split up the name of the object passed to us in the rename key callback, and pass the parent of the original object name, along with the new name, and test to see if it matches our secret key. So our rename key callback is fairly simple, as follows.

NTSTATUS RfPreRenameKey( __in PVOID CallbackContext, __in PVOID Argument1, __in PREG_RENAME_KEY_INFORMATION CallbackData )
{
    UNREFERENCED_PARAMETER( CallbackContext );
    UNREFERENCED_PARAMETER( Argument1 );

    NTSTATUS status = STATUS_SUCCESS;

    //	Get the full name of the new key
    PCUNICODE_STRING pObjectName;
    status = CmCallbackGetKeyObjectID( &g_CmCookie, CallbackData->Object, NULL, &pObjectName );
    if ( !NT_SUCCESS( status ) )
    {
        goto LExit;
    }

    //	Find the last path separator character and get a "parent object" key name
    UNICODE_STRING ParentObjectName;
    status = _RfSplitParentAndRelativeNames( pObjectName, &ParentObjectName, NULL );
    if ( !NT_SUCCESS( status ) )
    {
        goto LExit;
    }

    //	Prevent callers from opening our secret registry key
    if ( _RfIsKeyMySecretKey( &ParentObjectName, CallbackData->NewName ) )
    {
        KdPrint( ( "RegFlt: Rename of key to %wZ being denied!\n", CallbackData->NewName ) );
        status = STATUS_ACCESS_DENIED;
        goto LExit;
    }

LExit:
    return status;
}

Now, a few additional comments. First, as I was working on the code for this, I found a small bug in my code to get the key object name from part 2. The code attached with this post will contain a fix, and I will post a comment to the previous article with details.

Another strange behavior I noted while testing this is that when a key is renamed multiple times, the rename key callback often gets the ORIGINAL key name when it calls CmCallbackGetKeyObjectID. For example, in regedit you create a new key “New Key #1″ and rename it to “TestKey”. Then you rename it again to “TestKey2″. The rename key callback will still get something like “\REGISTRY\MACHINE\SOFTWARE\New Key #1″ for the object, and the NewName would contain TestKey2. So it appears that you may not be able to entirely rely on CmCallbackGetKeyObjectID to return the current name of the object. It may be cached for the handle/object, or it may be cached for a longer lifetime, I couldn’t tell for sure from my limited testing.

RegFlt Source Part 3

In Part 1 we created a basic registry filter driver. For a sample, the code that we created was fine, but for production code, there are some parts of it that could become a little unwieldy. In this article we will do a little cleanup on the code, and then start looking at simple filtering of the open key operation.

Referring back to Part 1, the RfRegistryCallback function, which actually receives all the callbacks from the CM, has a nice little switch statement with three cases. In practice, there may be a good ten to twenty cases that you need to handle in order to implement a functional product. This switch statement gets a little difficult to maintain, and then you break things out into separate functions for each operation callback type. A good way to do this is just to convert our switch statement into some code that uses an array of function pointers, and calls the right operation callback depending on the operation class provided. So after these changes the RfRegistryCallback looks as follows:

NTSTATUS RfRegistryCallback( __in PVOID CallbackContext, __in PVOID Argument1, __in PVOID Argument2 )
{
    REG_NOTIFY_CLASS Operation = (REG_NOTIFY_CLASS) (ULONG_PTR) Argument1;

    //	If we have no operation callback routine for this operation then just return to Cm
    if ( !g_RegistryCallbackTable[ Operation ] )
    {
        return STATUS_SUCCESS;
    }

    //	Call our operation callback routine
    return g_RegistryCallbackTable[ Operation ]( CallbackContext, Argument1, Argument2 );
}

We also have a global array of function pointers for operation callback routines, and we initialize that during DriverEntry (prior to registering our callback):

PEX_CALLBACK_FUNCTION g_RegistryCallbackTable[ MaxRegNtNotifyClass ] = { 0 };

extern "C"
NTSTATUS DriverEntry( __in PDRIVER_OBJECT pDriverObject, __in PUNICODE_STRING pRegistryPath )
{
    //	... Set up unload routine, etc.

    //	Set up our registry callback table
    g_RegistryCallbackTable[ RegNtPreCreateKeyEx ] = (PEX_CALLBACK_FUNCTION) RfPreCreateKeyEx;
    g_RegistryCallbackTable[ RegNtPreOpenKeyEx ] = (PEX_CALLBACK_FUNCTION) RfPreOpenKeyEx;
    g_RegistryCallbackTable[ RegNtKeyHandleClose ] = (PEX_CALLBACK_FUNCTION) RfKeyHandleClose;

    //	... Register callback with CM
}

And then we have a few operation callback routines that all look similar, mainly differing in the type of the CallbackData parameter:

NTSTATUS RfPreCreateKeyEx( __in PVOID CallbackContext, __in PVOID Argument1, __in PREG_CREATE_KEY_INFORMATION CallbackData );

Now that we have a callback routine that is a bit cleaner and easier to maintain, let’s dive into filtering the open key operation. One of the most common things to need to do while filtering an open or create key operation is to examine the name of the object being opened. Maybe you have a list of keys you want to filter (or a list NOT to filter), or some other similar requirement. If we look at the CallbackData structure, which is of type REG_OPEN_KEY_INFORMATION, there is a CompleteName field. This field is not very well named, because it may or may not actually be the “complete” name. If the name begins with a backslash then it has the complete name, otherwise it is a name relative to the RootObject field, and we have to figure it out for ourselves. (Note that there is an API call CmCallbackGetKeyObjectID that purports to get the name of a key object. This API will not work in pre-Open/Create calls, because it is designed to work on an object, and cannot build us the complete name from an object and relative name. We will use this API to get the string name of the RootObject.)

The following sample function shows how to construct the full object name if necessary, and use either the constructed name or the one provided by Cm to display what key is being opened.

NTSTATUS RfPreOpenKeyEx( __in PVOID CallbackContext, __in PVOID Argument1, __in PREG_OPEN_KEY_INFORMATION CallbackData )
{
    UNREFERENCED_PARAMETER( CallbackContext );
    UNREFERENCED_PARAMETER( Argument1 );

    NTSTATUS status;
    PUNICODE_STRING pLocalCompleteName = NULL;

    //	Get the complete name of the key being opened
    if ( CallbackData->CompleteName->Length > 0 && *CallbackData->CompleteName->Buffer != OBJ_NAME_PATH_SEPARATOR )
    {
        PCUNICODE_STRING pRootObjectName;
        status = CmCallbackGetKeyObjectID( &g_CmCookie, CallbackData->RootObject, NULL, &pRootObjectName );
        if ( NT_SUCCESS( status ) )
        {
            //	Build the new name
            USHORT cbBuffer = pRootObjectName->Length;
            cbBuffer += sizeof( wchar_t );
            cbBuffer += CallbackData->CompleteName->Length;
            ULONG cbUString = sizeof(UNICODE_STRING) + cbBuffer;

            pLocalCompleteName = (PUNICODE_STRING) ExAllocatePoolWithTag( PagedPool, cbUString, 'tlFR' );
            if ( pLocalCompleteName )
            {
                pLocalCompleteName->Length = 0;
                pLocalCompleteName->MaximumLength = cbBuffer;
                pLocalCompleteName->Buffer = (PWCH) ( (PCCH) pLocalCompleteName + sizeof( UNICODE_STRING ) );

                RtlCopyUnicodeString( pLocalCompleteName, pRootObjectName );
                RtlAppendUnicodeToString( pLocalCompleteName, L"\\" );
                RtlAppendUnicodeStringToString( pLocalCompleteName, CallbackData->CompleteName );
            }
        }
        else
        {
            //	Could not get the complete name since we have a relative name and were
            //	unable to get the name of the root object
            goto LExit;
        }
    }

    KdPrint( ( "RegFlt: PreOpenKeyEx for %wZ\n", pLocalCompleteName ? pLocalCompleteName : CallbackData->CompleteName ) );

LExit:
    if ( pLocalCompleteName )
    {
        ExFreePool( pLocalCompleteName );
    }

    return STATUS_SUCCESS;
}

We now have a filter that get called for every open-key on the system and prints out in the debugger what is being opened. These techniques could be useful if you were building a monitoring product of some kind, but the real fun work comes when you want to change the behavior. In the next part we will look at actively filtering a registry open operation.

For the past number of years I have been working on driver projects that are file system filter drivers. Many people are familiar with these kinds of drivers, and there is lots of information out on the internet about how to create them. My drivers have also been register filter drivers: they do much the same thing that file system filter drivers do, but for the registry. Unfortunately, there is not a lot of information out there about writing one of these. There is a brief article on osronline.com (written by a coworker of mine of one of these drivers), and there is a sample driver include as part of the Microsoft Windows Driver Kit samples. The lack of information out there leaves me feeling like I am the only developer foolish enough to be working on one of these drivers. I recently decided that I should help to remedy that lack of information available about these drivers, and hopefully entice a few more foolish developers into trying their hand at this.

This first article will be a very basic intro on building a registry filter driver that does nothing! But never fear, dear reader, there will be more in-depth information coming soon.

First, let’s talk generally about motivation. What is “filtering” the registry, and why in the world would I want to do that? A filter driver lets you see operating system calls for the registry before (and after) they actually get processed by the Windows registry code (Configuration Manager or CM) in the kernel. A filter driver lets you examine the calls passively, for example you might create a registry call logging utility like Sysinternals’ RegMon or Process Monitor. It also lets you actively filter the calls, changing the results if you feel inclined. For example, you might want to have a security product that prevents people from writing to certain keys. (Granted, you could do this with ACLs as well, but you get the idea.)

In the old days of yore (pre-Vista) this functionality was not really baked. In early operating systems it was not there at all, and you would be forced to hook into the system call table in order to try to write your filtering product. This could lead to all sorts of potential problems, of course, and Microsoft decided that to support these products they needed new APIs for registry filtering. These were added in Windows XP and later, but the initial releases only supported passive filtering. The ability to do active filtering came along in Vista and later, and continues to see improvements in later operating systems.

The basics of building a registry filter driver are that you need your driver to call CmRegisterCallbackEx during its DriverEntry routines, and then to call CmUnRegisterCallback during your DriverUnload routines. Registry filters are considered minifilters and are loaded based on Load Order Groups and Altitude, just as file system minifilters are. We won’t really discuss this much in this series, but to create your own you need to request an altitude from Microsoft which helps them determine in which order filters should be called.

So to build a bare-bones registry filter, first we call the registration function during our driver initialization as follows. Note that we have defined a callback routine (which we will implement later), and made up an altitude just for testing. We get back from the registry a “cookie” that we must use later on when we want to unregister our filter.

NTSTATUS RfRegistryCallback( __in PVOID CallbackContext, __in PVOID Argument1, __in PVOID Argument2 );
void RfUnload( __in PDRIVER_OBJECT pDriverObject );
LARGE_INTEGER g_CmCookie = { 0 };

extern "C"
NTSTATUS DriverEntry( __in PDRIVER_OBJECT pDriverObject, __in PUNICODE_STRING pRegistryPath )
{
    UNREFERENCED_PARAMETER( pRegistryPath );

    //	Set up our unload routine
    pDriverObject->DriverUnload = RfUnload;

    //	Register our callback with the system
    UNICODE_STRING AltitudeString = RTL_CONSTANT_STRING( L"360000" );
    NTSTATUS status = CmRegisterCallbackEx( RfRegistryCallback, &AltitudeString, pDriverObject, NULL, &g_CmCookie, NULL );
    if ( !NT_SUCCESS( status ) )
    {
        //	Failed to register - probably shouldn't succeed the driver intialization since this is critical
    }

    return status;
}

Next we need to unregister our registry callback when our driver unloads:

void RfUnload( __in PDRIVER_OBJECT pDriverObject )
{
    UNREFERENCED_PARAMETER( pDriverObject );
    PAGED_CODE();

    NTSTATUS status = CmUnRegisterCallback( g_CmCookie );
    if ( !NT_SUCCESS( status ) )
    {
        //	Failed to unregister - try to handle gracefully
    }
}

Finally, we need a routine for the registry to call in our driver when something happens. For this first article, this routine isn’t really going to do anything, but it exists as a sample of what the callback function looks like, and what some of the parameters look like and how to handle them. In a future article we will look at a more graceful way to handle this callback routine.

NTSTATUS RfRegistryCallback( __in PVOID CallbackContext, __in PVOID Argument1, __in PVOID Argument2 )
{
    UNREFERENCED_PARAMETER( CallbackContext );
    UNREFERENCED_PARAMETER( Argument2 );

    REG_NOTIFY_CLASS Operation = (REG_NOTIFY_CLASS) (ULONG_PTR) Argument1;
    switch ( Operation )
    {
    case RegNtPreCreateKeyEx:
        break;
    case RegNtPreOpenKeyEx:
        break;
    case RegNtKeyHandleClose:
        break;
    }

    return STATUS_SUCCESS;
}

That’s it! At this point you have a working registry filter driver. You can install it on a machine (I just do this with the command line “sc create” and regedit for testing purposes). If you connect a kernel debugger and put a breakpoint on the RfRegistryCallback routine, you will see it’s getting called for all kinds of registry operations (there are a LOT of these going on in the system).

I hit a pretty frustrating problem the other day when I renewed my digital certificate for code signing and my drivers stopped working. When I renewed the certificate through VeriSign (Symantec), I was given the option of choosing a SHA-1 or a SHA-2 (with 256-bit digest) hashing algorithm for the certificate. After reading through the background information on these options, and understanding that the new and improved SHA-2 was supported on Vista and above, I determined that SHA-2 would meet my requirements and clicked the button.

I got the new cert, put it in my cert store, and reconfigured all my projects in Visual Studio to sign with the new cert. Everything was working perfectly. It installed and run fine on my test machine, but then one of my co-workers complained that the driver wouldn’t load on his machine. After a short while I realized that I had the same problem on my machine, I just wasn’t seeing it because I had a kernel debugger attached. (Note that there are some registry options that can enable mandatory kernel mode code signing even when the debugger is attached. Read more at MSDN.)

So now that I could reproduce the problem, I started troubleshooting the signing itself. Using “signtool.exe verify /kp”, I verified that the driver was signed, all certificates were valid, and the cross-certificate from Microsoft was also in the signing chain. Signtool claimed that everything was just fine, but the darn thing still wouldn’t load. I was getting the error ERROR_INVALID_IMAGE_HASH (0×80070241): “Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.” After searching the event logs for a bit, I found the Microsoft-Windows-CodeIntegrity log which contained an Event 3004 with basically the same message as the error code above.

After exhausting my Google-Fu skills with no results, we started brainstorming about what was different between the build yesterday that worked and the one today. Somebody mentioned encryption algorithms, and the light bulbs went on in my head. Adding the specific case of SHA-2 to my searching yielded a couple of pages: Practical Windows Code and Driver Signing and PiXCL: Signing Windows 8 Drivers.

Some of my own testing showed that I couldn’t get a driver built with Visual Studio and a SHA-2 certificate to load on both Windows 7 and Windows 8. Theoretically you could sign it twice with each algorithm (using the /fd switch of signtool), but Visual Studio with the WDK is not really set up to do that, I didn’t want to go monkeying with MSBuild stuff too much.

After calling Symantec support and describing the problem we were basically told, “oh yeah, the support for SHA-2 is not really there for a lot of systems.” You think? Maybe you could have mentioned this when we were renewing the certificate?

The moral of the story, dear reader, is stick with the SHA-1 hash algorithm for signing your kernel mode drivers, at least for the time being.

I recently had a co-worker ask me what the best way to find an object matching certain criteria in an STL vector. We talked about the “old-school” ways of doing that, like a for loop from zero to the number of items, or a for loop using the being and end iterators. Each of these would do the comparison and then break out of the loop if a match was found. Now these ways work just fine, but there are some newer, more “modern” methods of accomplishing the same things that use STL algorithms. In case you haven’t seen them before I thought I would share…

The first method uses a structure/class to wrap the comparison function and is usually called a “functor”. Essentially this is just an object that implements operator(), which allows you to treat the object somewhat like a function. (And you can pass it to other functions like STL algorithms.)

The second method uses the more modern “lambda” function syntax. It allows you to just define the comparison function right inline with the code. This is the one that I prefer because it keeps the comparison logic with the calling of the find algorithm. I think one of the most important aspects of good code is that it’s easy to follow and understand: you shouldn’t have to go skipping all over the code to figure out what some piece of code is doing.

Of course at first glance, a programmer who is unfamiliar with either of these methods is going to respond “huh?” But once you get used to seeing a functor or a lambda expression, they become pretty easy to read and understand.

So without further ado, on to the code, which demonstrates a very simple example of each method:
Continue reading

I ran into an interesting case while working on a registry filter driver the other day. Boiled down to its most basic form, my driver is blocking access to registry keys by name. I watch for a name that matches one in my list then fail the open/create. The interesting case is when a registry key is a symbolic link (I.e., was created with REG_CREATE_LINK and has a REG_LINK value named SymbolicLinkValue that points to a second key).

All my filtering work takes place in the Pre-Open/Create path, and because the name coming in doesn’t actually represent what really will be opened, I can’t exactly rely on the name. So after digging into how these operations get handled, I have found some very odd behavior. I see two different behaviors for an open/create under these conditions. I created a sample filter driver to test this and tested on Windows 8 x64 and Windows 7 x64.

  1. When I try to open the link source (the key that links to a second key), I see the following:
  1. RegNtPreOpenKeyEx with CompleteName containing the name of the key – I return STATUS_SUCCESS to allow processing to continue.
  2. RegNtPostOpenKeyEx with PreInformation->CompleteName containing the key name from the SymbolicLinkValue. Status contains STATUS_REPARSE.
  3. RegNtPreOpenKeyEx with Complete Name containing the key name of the link target (from step b).
  4. RegNtPostOpenKeyEx with Status equal to STATUS_SUCCESS.
  • After a small number of repetitions (usually 2) the above behavior stops, and is replaced with the following:
    1. RegNtPreOpenKeyEx with CompleteName containing the name of the key – I return STATUS_SUCCESS to allow processing to continue.
    2. RegNtPostOpenKeyEx with Status equal to STATUS_SUCCESS. PreInformation->CompleteName contains the original name from step a, but the Object contains a pointer to a registry object for the link target key.

    Additionally, the behaviors above seem to revert after some period of time. If I run a simple registry open command for the link source a hundred times, I see behavior #1 twice and then #2 the remainder of the time. But after doing that if I let the machine sit idle for 4-5 minutes and run it again, I will again see behavior #1 twice, followed by #2 for as many times as I care to repeat it.

    I e-mailed some contacts at Microsoft and they confirmed what I have been seeing. The registry maintains a key lookup cache that is intended to speed up open/create operations. One thing that the cache can do is remember symbolic links and avoid the need to reparse them every time they are opened. This is a bit frustrating for me, but it probably makes a significant difference in registry performance (which, incidentally is a heck of a lot faster than file system performance).

    Unfortunately it would seem that the way to solve this problem is to store some of the data I need internally to my driver based on watching the systems behavior. So, for example, I can watch for the STATUS_REPARSE result to come back, and do something with the CompleteName value. So I could add a context to the call in the pre-open, put the original complete name in the context, then in the post-open when I see STATUS_REPARSE, I can add an entry to a list that says “Name A links to Name B”, which I can thereafter use in my pre-open. I then also have to deal with cases where the link key gets removed (I can watch for a pre-open with OBJ_OPENLINK, place a context on the object, then watch for the delete, and clean up my internal cache). I also have to deal with the case where the link gets retargeted to something else (I can watch for the pre-open with OBJ_OPENLINK, context the object, watch for the pre-set-value for REG_LINK named SymbolicLinkValue and update my cache).

    I won’t show the solution code here, since its fairly complicated, and it is the intellectual property of my company, but the solution is fairly straight-forward once you understand what needs to be done. I will, however, show some of the code that I actually used to test this and grok the behavior.

    Continue reading

    A while back I wrote a tool called NativeTest and open-sourced it on CodePlex. You can view the original post about this at the FSLogix Blog. Today I was troubleshooting a problem with registry links that required me to look at what was really going on in the registry, without it redirecting me through symbolic links. So I turned to the NativeTest project, which was able to help out.

    For those who don’t already know, the registry can have symbolic links where a key actually is just a pointer to another key. So, for example, the HKLM\Software\CurrentControlSet key is a symbolic link created during system boot that points to HKLM\Software\CurrentControlSet001 (or some other numbered control set, depending on your hardware configuration).

    Viewing a registry symbolic link involves opening the key with a special flag (REG_OPTION_OPEN_LINK) and then querying the value named “SymbolicLinkValue”. The value’s data will be a fully qualified native path to another registry key. The only difficult part about doing this with NativeTest is figuring out that the numeric option for REG_OPTION_OPEN_LINK is equivalent to 8, which you can find in the Windows SDK header files.

    The NativeTest session for viewing the link at HKLM\System\ControlSet001\Control\Print\Printers is as follows:

    > openkeyex -name \registry\machine\system\controlset001\control\print\printers -access kr -options 8
    Status = 0x00000000
    Handle = 40 (AUTO-0)
    
    > enumvaluekey -handle AUTO-0 -index 0 -class full -bufferlen 256
    AUTO-HANDLE: 40
    Status = 0x00000000
    ResultLength = 210
    00 00 00 00 06 00 00 00     ........
    38 00 00 00 9a 00 00 00     8.......
    22 00 00 00 53 00 79 00     "...S.y.
    6d 00 62 00 6f 00 6c 00     m.b.o.l.
    69 00 63 00 4c 00 69 00     i.c.L.i.
    6e 00 6b 00 56 00 61 00     n.k.V.a.
    6c 00 75 00 65 00 46 00     l.u.e.F.
    5c 00 52 00 65 00 67 00     \.R.e.g.
    69 00 73 00 74 00 72 00     i.s.t.r.
    79 00 5c 00 4d 00 61 00     y.\.M.a.
    63 00 68 00 69 00 6e 00     c.h.i.n.
    65 00 5c 00 53 00 6f 00     e.\.S.o.
    66 00 74 00 77 00 61 00     f.t.w.a.
    72 00 65 00 5c 00 4d 00     r.e.\.M.
    69 00 63 00 72 00 6f 00     i.c.r.o.
    73 00 6f 00 66 00 74 00     s.o.f.t.
    5c 00 57 00 69 00 6e 00     \.W.i.n.
    64 00 6f 00 77 00 73 00     d.o.w.s.
    20 00 4e 00 54 00 5c 00      .N.T.\.
    43 00 75 00 72 00 72 00     C.u.r.r.
    65 00 6e 00 74 00 56 00     e.n.t.V.
    65 00 72 00 73 00 69 00     e.r.s.i.
    6f 00 6e 00 5c 00 50 00     o.n.\.P.
    72 00 69 00 6e 00 74 00     r.i.n.t.
    5c 00 50 00 72 00 69 00     \.P.r.i.
    6e 00 74 00 65 00 72 00     n.t.e.r.
    73 00                       s.
    
    > closekey -handle auto-0
    AUTO-HANDLE: 40
    Status = 0x00000000

    As you can see, this key is really just a pointer to HKLM\Software\Microsoft\Windows NT\CurrentVersion\Print\Printers.

    The next task is how can I actually create one of these links for my own testing, just so I don’t accidentally mess up one of the system keys. Creating a symbolic registry link is pretty easy. You just have to create they key with a certain option (REG_OPTION_CREATE_LINK), and then set the value named “SymbolicLinkValue”, which needs to be of type REG_LINK. Again, the hard part is figuring out that REG_OPTION_CREATE_LINK is the equivalent of 2.

    The session looks like this:

    > createkey -name \registry\machine\software\Test -access ka -options 2
    Status = 0x00000000
    Disposition = Created
    Handle = 48 (AUTO-0)
    
    > setvaluekey -handle auto-0 -name SymbolicLinkValue -type link -data \\Registry\\Machine\\Software\\Test2
    AUTO-HANDLE: 48
    Status = 0x00000000
    
    > queryvaluekey -handle auto-0 -name SymbolicLinkValue -class full -bufferlen 512
    AUTO-HANDLE: 48
    Status = 0x00000000
    ResultLength = 120
    00 00 00 00 06 00 00 00     ........
    38 00 00 00 44 00 00 00     8...D...
    22 00 00 00 53 00 79 00     "...S.y.
    6d 00 62 00 6f 00 6c 00     m.b.o.l.
    69 00 63 00 4c 00 69 00     i.c.L.i.
    6e 00 6b 00 56 00 61 00     n.k.V.a.
    6c 00 75 00 65 00 00 00     l.u.e...
    5c 00 52 00 65 00 67 00     \.R.e.g.
    69 00 73 00 74 00 72 00     i.s.t.r.
    79 00 5c 00 4d 00 61 00     y.\.M.a.
    63 00 68 00 69 00 6e 00     c.h.i.n.
    65 00 5c 00 53 00 6f 00     e.\.S.o.
    66 00 74 00 77 00 61 00     f.t.w.a.
    72 00 65 00 5c 00 46 00     r.e.\.T.
    53 00 4c 00 6f 00 67 00     e.s.t.2.
    
    > closekey -handle auto-0
    AUTO-HANDLE: 48
    Status = 0x00000000

    Once you have created it, it’s kind of fun to poke around at the key with regedit. If you, like me, decide to clean up your test key and delete it in regedit, you will find that deleting the link source actually follows the link and deletes the target. The link source remains and now displays an error about the system cannot find the file specified. In my testing, it seems to be impossible to delete the link source in regedit, so we need to look at one more thing in NativeTest: deleting the link source key. The only trick here is that you have to open the key with REG_OPTION_OPEN_LINK so that the system doesn’t try to follow the link (which, incidentally is what regedit is doing).

    The session looks like this:

    > openkeyex -name \registry\machine\software\test -access ka -options 8
    Status = 0x00000000
    Handle = 52 (AUTO-0)
    
    > deletekey -handle auto-0
    AUTO-HANDLE: 52
    Status = 0x00000000
    
    > closekey -handle auto-0
    AUTO-HANDLE: 52
    Status = 0x00000000

    A recent feature addition to my product at work is logging to a custom event log channel using ETW. I have used technology built on top of ETW before, such as WPP tracing, and while it is a bit cumbersome, the advantages are great, and once it’s set up, it’s not hard at all to use. I have also done classic Event Logging before.

    With this recent feature work, I wanted to use the current technology, and I wanted to be able to log from my service and driver. I thought I had everything working. I created a manifest .xml file that defined my custom channels, and the events I wanted to log. I wrote some code to register/unregister with ETW, and to log various messages with their accompanying data.

    But in my testing, I was able to log events from my service (running as system), but not from my driver. The calls to EtwRegister and EtwWrite looked like they were working perfectly. They returned a success code, but nothing every appeared in the log. The documentation says they will behave this way (returning success but not logging anything) if the event is not enabled.

    I spend the next few hours trying to figure out why my events were not enabled. The logging level (Error, Warning, Information) was fine. The keywords were fine. The manifest file and the message resources it pointed to were okay. I even went so far as to write a notification callback so I could see what the system was enabling. It all looked fine.

    I finally decided to try using a built-in channel instead of my own, and I selected the SYSTEM channel. Bam! Everything started working. Then I started to think that this might be a security issue. This was a bit counter-intuitive because the APIs weren’t failing. I would think that if you tried to write an event but didn’t have rights, you should get an access denied error, but apparently the system just happily eats your event and gives you a thumbs up.

    The key turned out to be the isolation attribute on my custom channel. Setting the isolation=”system” attribute on the channel made everything start working.

    I don’t understand why the logging worked from my service, but not from my driver, since both were running in the system context. But there it is. A very simple solution to a maddening problem (at least for a day).

    Windows Installer stores information about components, products, etc. that are installed on the system in the registry. Most of these elements are identified by a GUID, but the Installer doesn’t always store the GUID directly in the registry. Instead it uses what I call SQUIDs, or Squished GUIDs. Basically they just reformat the GUID a bit and remove some of the extra characters that don’t add anything (hyphens and braces). You can read more about this here: A Brief Note on Installer GUIDs.

    Periodically when I am working on some installer code I can end up needing to search the registry for stuff that is related to a component I am working on. It is very helpful to be able to convert between SQUIDs and GUIDs. I usually just do this by hand, but today I decided I needed a bit more help doing this and wrote a tiny vbscript piece of code that does the conversion. On the chance that it might provide value for someone else, I have decided to post it here.

    Function SquidToGuid(s)
      SquidToGuid = _
        StrReverse(Mid(s, 1, 8)) & "-" & _
        StrReverse(Mid(s, 9, 4)) & "-" & _
        StrReverse(Mid(s, 13, 4)) & "-" & _
        StrReverse(Mid(s, 17, 2)) & StrReverse(Mid(s, 19, 2)) & "-" & _
        StrReverse(Mid(s, 21, 2)) & StrReverse(Mid(s, 23, 2)) & StrReverse(Mid(s, 25, 2)) & _
        StrReverse(Mid(s, 27, 2)) & StrReverse(Mid(s, 29, 2)) & StrReverse(Mid(s, 31, 2))
    End Function
    
    Function GuidToSquid(g)
      Replace g, "{", ""
      Replace g, "}", ""
      GuidToSquid = _
        StrReverse(Mid(g, 1, 8)) & _
        StrReverse(Mid(g, 10, 4)) & _
        StrReverse(Mid(g, 15, 4)) & _
        StrReverse(Mid(g, 20, 2)) & StrReverse(Mid(g, 22, 2)) & _
        StrReverse(Mid(g, 25, 2)) & StrReverse(Mid(g, 27, 2)) & StrReverse(Mid(g, 29, 2)) & _
        StrReverse(Mid(g, 31, 2)) & StrReverse(Mid(g, 33, 2)) & StrReverse(Mid(g, 35, 2))
    End Function