injdrv

injdrv is a proof-of-concept Windows Driver for injecting DLL into user-mode processes using APC.

Motivation

Even though APCs are undocumented to decent extent, the technique of using them to inject a DLL into a user-mode process is not new and has been talked through many times. Such APC can be queued from regular user-mode process (seen in Cuckoo) as well as from kernel-mode driver (seen in Blackbone).

Despite its popularity, finding small, easy-to-understand and actually working projects demonstrating usage of this technique isn't very easy. This project tries to fill this gap.

Features

• Support for Windows 7 up to Windows 10
• Support for x86 & x64 architectures
• Ability to inject WoW64 processes
  • ...with x86 DLL and even with x64 DLL
• DLL is injected in very early process initialization stage
  • Injection is performed from the PsSetLoadImageNotifyRoutine callback
  • Native processes (x86 on Windows x86, x64 on Windows x64) are injected on next load of DLL after ntdll.dll
  • WoW64 processes are injected on next load of DLL after following system DLLs are loaded: ntdll.dll (both x64 and WoW64), wow64.dll, wow64cpu.dll and wow64win.dll
• Because of that, injected DLL is dependent only on ntdll.dll
• Demonstrative DLL performs hooking of few ntdll.dll functions
  • Achieved using DetoursNT
• Detoured functions use ETW to inform which hooked function has been called

Compilation

Because DetoursNT project is attached as a git submodule, which itself carries the Detours git submodule, you must not forget to fetch them:

git clone --recurse-submodules git@github.com:wbenny/injdrv.git

After that, compile this project using Visual Studio 2017. Solution file is included. The only required dependency is WDK.

Implementation

When the driver is loaded, it'll register two callbacks:

• For process create/exit notification (PsSetCreateProcessNotifyRoutineEx)
• For image load notification (PsSetLoadImageNotifyRoutine)

When a new process is created, the driver allocates small structure, which will hold information relevant to the process injection, such as:

• Which DLLs are already loaded in the process
• Addresses of important functions (such as LdrLoadDll in ntdll.dll)

Start of a new Windows process is followed by mapping ntdll.dll into its address space and then ongoing load of DLLs from the process's import table. In case of WoW64 processes, the following libraries are loaded immediately after native ntdll.dll: wow64.dll, wow64cpu.dll, wow64win.dll and second (WoW64) ntdll.dll. The driver is notified about load of these DLLs and marks down this information.

When these DLLs are loaded, it is safe for the driver to queue the user-mode APC to the process, which will load our DLL into the process.

Challenges

Although such project might seem trivial to implement, there are some obstacles you might be facing along the way. Here I will try to summarize some of them:

• Injection of x86 (or WoW64) DLL requires a small allocation inside of the user-mode address space. This allocation holds path to the DLL to be injected and a small shellcode, which basically calls LdrLoadDll with the DLL path as a parameter. It is obvious that this memory requires PAGE_EXECUTE_READ protection, but the driver has to fill this memory somehow - and PAGE_EXECUTE_READWRITE is unacceptable security concern.

  It might be tempting to use ZwAllocateVirtualMemory and ZwProtectVirtualMemory but unfortunatelly, the second function is exported only since Windows 8.1.

  The solution used in this driver is to create section (ZwCreateSection), map it (ZwMapViewOfSection) with PAGE_READWRITE protection, write the data, unmap it (ZwUnmapViewOfSection) and then map it again with PAGE_EXECUTE_READ protection.

• With usage of sections another problem arises. Since this driver performs injection from the image load notification callback - which is often called from the NtMapViewOfSection function - we'd be calling MapViewOfSection recursively. This wouldn't be a problem, if mapping of the section wouldn't lock the EPROCESS->AddressCreationLock. Because of that, we would end up in deadlock.

  The solution used in this driver is to inject kernel-mode APC first, from which the ZwMapViewOfSection is called. This kernel-mode APC is triggered right before the kernel-to-user-mode transition, so the internal NtMapViewOfSection call won't be on the callstack anymore (and therefore, AddressCreationLock will be unlocked).

• Injection of our DLL is triggered on first load of DLL which happens after all important system DLLs (mentioned above) are already loaded.

  In case of native processes, the codeflow is following:

  • process.exe is created (process create notification)
  • process.exe is loaded (image load notification)
  • ntdll.dll is loaded (image load notification)
  • kernel32.dll is loaded (image load notification + injection happens here)

  In case of WoW64 processes, the codeflow is following:

  • process.exe is created (process create notification)
  • process.exe is loaded (image load notification)
  • ntdll.dll is loaded (image load notification)
  • wow64.dll is loaded (image load notification)
  • wow64cpu.dll is loaded (image load notification)
  • wow64win.dll is loaded (image load notification)
  • ntdll.dll is loaded (image load notification - note, this is 32-bit ntdll.dll)
  • kernel32.dll is loaded (image load notification + injection happens here)
  
  

  Note that load of the kernel32.dll was used as an example. In fact, load of any DLL will trigger the injection. But in practice, kernel32.dll is loaded into every Windows process, even if:

  • it has no import table
  • it doesn't depend on kernel32.dll
  • it does depend only on ntdll.dll (covered in previous point, I just wanted to make that crystal-clear)
  • it is a console application

  Also note that the order of loaded DLLs mentioned above might not reflect the exact order the OS is performing.

  The only processes that won't be injected by this method are:

  • native processes (such as csrss.exe)
  • pico processes (such as applications running inside Windows Subsystem for Linux)

  Injection of these processes is not in the scope of this project.

  The injected user-mode APC is then force-delivered by calling KeTestAlertThread(UserMode). This call internally checks if any user-mode APCs are queued and if so, sets the Thread->ApcState.UserApcPending variable to TRUE. Because of this, the kernel immediately delivers this user-mode APC (by KiDeliverApc) on next transition from kernel-mode to user-mode.

  If we happened to not force the delivery of the APC, the APC would've been delivered when the thread would be in the alertable state. (There are two alertable states per each thread, one for kernel-mode, one for user-mode; this paragraph is talking about Thread->Alerted[UserMode] == TRUE.) Luckily, this happens when the Windows loader in the ntdll.dll finishes its job and gives control to the application - particularly by calling NtAlertThread in the LdrpInitialize (or _LdrpInitialize) function. So even if we happened to not force the APC, our DLL would still be injected before the main execution would take place.

  NOTE: This means that if we wouldn't force delivery of the APC on our own, the APC would be delivered BEFORE the main/WinMain is executed, but AFTER all TLS callbacks are executed. This is because TLS callbacks are executed also in the early process initialization stage, within the LdrpInitialize function.

  This behavior is configurable in this project by the ForceUserApc variable (by default it's TRUE).

  NOTE: Some badly written drivers try to inject DLL into processes by queuing APC at wrong time. For example:

  • Queuing an APC for injecting DLL that doesn't depend only on ntdll.dll right when ntdll.dll is mapped
  • Queuing an APC for injecting DLL that depends on kernel32.dll right when kernel32.dll is mapped (but not loaded!)

  Such injection will actually work as long as someone won't try to forcefully deliver user-mode APCs. Because this driver triggers immediate deliver of user-mode APCs (all of them, you can't pick which should be delivered), it might happen that APC of other driver will be triggered. If such APC consisted, let's say, of calling LoadLibraryA from kernel32.dll and the kernel32.dll won't be fully loaded (just mapped), such APC would fail. And because this injection happens in early process initialization stage, this error would be considered critical and the process start would fail. Also because basically every process is being injected, if start of every process would fail, it would make the system very unusable.

  The reason why our DLL is not injected immediately from the ntdll.dll image load callback is simple: the image load callback is called when the DLL is mapped into the process - and at this stage, the DLL is not fully initialized. The initialization takes place after this callback (in user-mode, obviously). If we would happen to inject LdrLoadDll call before ntdll.dll is initialized, the call would fail somewhere in that function, because some variable it relies on would not be initialized.

• Injection of protected processes is simply skipped, as it triggers code-integrity errors. Such processes are detected by the PsIsProtectedProcess function. If you're curious about workaround of this issue (by temporarily unprotecting these processes), you can peek into Blackbone source code. Keep in mind that unprotecting protected processes requires manipulation with undocumented structures, which change dramatically between Windows versions.

• Injection of x86 DLL into WoW64 processes is handled via PsWrapApcWow64Thread(&NormalContext, &NormalRoutine) call. This function essentially alters provided arguments in a way (not covered here) that KiUserApcDispatcher in x64 ntdll.dll is able to recognize and handle such APCs differently. Handling of such APCs is internally resolved by calling Wow64ApcRoutine (from wow64.dll). This function then emulates queuing of "32-bit APC" and resumes its execution in KiUserApcDispatcher in the x86 ntdll.dll.

• Injection of x64 DLL into WoW64 processes is tricky on its own, and SentinelOne wrote an excellent 3-part blogpost series on how to achieve that:

  • https://www.sentinelone.com/blog/deep-hooks-monitoring-native-execution-wow64-applications-part-1
  • https://www.sentinelone.com/blog/deep-hooks-monitoring-native-execution-wow64-applications-part-2
  • https://www.sentinelone.com/blog/deep-hooks-monitoring-native-execution-wow64-applications-part-3

  In short, if you try to use the same approach as mentioned above (injecting small stub which calls LdrLoadDll) for injecting x64 DLL into WoW64 process, you will run into problems with Control Flow Guard on Windows 10.

  • On x64 system, CFG maintains 2 bitmaps for WoW64 processes
    • One for "x86 address space" (used when checking execution of < 4GB memory)
    • One for "x64 address space" (used when checking execution of >= 4 GB memory)
  • You cannot allocate memory in > 4GB range (even from the kernel-mode), because of VAD that reserves this memory range
    • You can theoretically unlink such VAD from EPROCESS->VadRoot and decrement EPROCESS->VadCount, but that's highly unrecommended
  • That means, when you allocate memory inside of WoW64 process (even from the kernel-mode) or change its protection, the x86 CFG bitmap is used.
  • x64 ntdll.dll is mapped above 4GB, therefore, the KiUserApcDispatcher function is also located in > 4GB address.
  • Before KiUserApcDispatcher calls (indirectly) the NormalRoutine provided to the KeInitializeApc function, it checks whether NormalRoutine can be executed via CFG
  • Because KiUserApcDispatcher is called from > 4GB address, this CFG check is performed on x64 CFG bitmap, but this check will fail, because the allocated memory of ours is in < 4GB memory
    • You can theoreticaly work around this by disabling the CFG with various hacks, but that's also highly unrecommended
  • ZwProtectVirtualMemory and even ZwSetInformationVirtualMemory won't help you, because these APIs will operate on x86 CFG bitmap as well, if you feed them with < 4GB address

  The solution outlined in the SentinelOne blogpost rests in calling LdrLoadDll of x64 ntdll.dll directly from the user APC dispatcher - effectively, making NormalRoutine point to the address of the LdrLoadDll. The issue here is that PKNORMAL_ROUTINE takes only 3 parameters, while LdrLoadDll takes 4.

  typedef
  VOID
  (NTAPI *PKNORMAL_ROUTINE) (
    _In_ PVOID NormalContext,
    _In_ PVOID SystemArgument1,
    _In_ PVOID SystemArgument2
    );
  
  NTSTATUS
  NTAPI
  LdrLoadDll (
    _In_ PWSTR SearchPath OPTIONAL,
    _In_ PULONG DllCharacteristics OPTIONAL,
    _In_ PUNICODE_STRING DllName,
    _Out_ PVOID *BaseAddress
    );

  Note that 4th parameter of the LdrLoadDll must point to some valid address, where the BaseAddress will be stored. The devil is always in the details - the solution takes advance of "couple of lucky coincidences":

  • KiUserApcDispatcher is a function expecting RSP to point to the CONTEXT structure

  • From this structure, values P1Home ... P4Home are fetched:

    • P1Home (moved to RCX) represent NormalContext
    • P2Home (moved to RDX) represent SystemArgument1
    • P3Home (moved to R8) represent SystemArgument2
    • P4Home (moved to RAX) represent NormalRoutine
    • Also, R9 is set to point to the RSP (the CONTEXT structure)
    • Note that RCX, RDX, R8 and R9 are used as first four function parameters in Microsoft x64 calling convention

    

  • KiUserApcDispatcher calls KiUserCallForwarder

    • KiUserCallForwarder checks whether RAX points to valid execution target (in x64 CFG bitmap)
    • KiUserCallForwarder calls function pointed by RAX with parameters RCX, RDX, R8 and R9
    • This is basically equivalent of calling APC's PKNORMAL_ROUTINE
      • NormalRoutine(NormalContext, SystemArgument1, SystemArgument2)
    • ...except that, because R9 is set, it is in fact called like this:
      • NormalRoutine(NormalContext, SystemArgument1, SystemArgument2, ContinueContext)

    

  • Therefore, if we queue the user-mode APC like this:

    • NormalRoutine = address of LdrLoadDll in 64-bit ntdll.dll
    • NormalContext = NULL (translates to 1st param. of LdrLoadDll (SearchPath))
    • SystemArgument1 = NULL (translates to 2nd param. of LdrLoadDll (DllCharacteristics))
    • SystemArgument2 = pointer to UNICODE_STRING DllName (translates to 3rd param. of LdrLoadDll (DllName))
    • (as mentioned above, the 4th parameter (BaseAddress) will be provided automatically by the KiUserApcDispatcher)

  • ...it will effectively result in the following call: LdrLoadDll(NULL, 0, &DllName, &ContinueContext)

  • LdrLoadDll overwrites first 8 bytes of the CONTEXT structure, which happens to be its P1Home field

  • It doesn't break anything, because this field has been already used (when fetching NormalContext) and is no longer accessed (not even by ZwContinue)

  NOTE: Not all function calls from x86 NTDLL end up in x64 NTDLL. This is because some functions are fully implemented on its own in both x86 and x64 NTDLL. This applies mainly on functions that does not require any syscall - i.e. Rtl* functions. For example, if you wanted to hook RtlDecompressBuffer in WoW64 process, hooking that function in x64 NTDLL wouldn't have any effect and such hooked function would be never called.

• Injection of x86 processes on x86 Windows is handled exactly the same way as injection of WoW64 processes with x86 DLL on x64 Windows (with the exception of PsWrapApcWow64Thread call).

• Injection of x64 processes is handled exactly the same way as injection of WoW64 processes with x64 DLL.

• Finally, as mentioned in the beginning, the injected DLL performs logging of hooked functions with ETW. Because functions such as EventRegister, EventWriteString, ... are located in the advapi32.dll, we can't use them from our NTDLL-only dependent DLL. Luckily, ETW support is hardwired in the ntdll.dll too. In fact, most of the Event* functions in the advapi32.dll are simply redirected to the EtwEvent* functions in ntdll.dll without any change to the arguments! Therefore, we can simply mock the Event* functions and just include the <evntprov.h> header:

  //
  // Include support for ETW logging.
  // Note that following functions are mocked, because they're
  // located in advapi32.dll.  Fortunatelly, advapi32.dll simply
  // redirects calls to these functions to the ntdll.dll.
  //
  
  #define EventActivityIdControl  EtwEventActivityIdControl
  #define EventEnabled            EtwEventEnabled
  #define EventProviderEnabled    EtwEventProviderEnabled
  #define EventRegister           EtwEventRegister
  #define EventSetInformation     EtwEventSetInformation
  #define EventUnregister         EtwEventUnregister
  #define EventWrite              EtwEventWrite
  #define EventWriteEndScenario   EtwEventWriteEndScenario
  #define EventWriteEx            EtwEventWriteEx
  #define EventWriteStartScenario EtwEventWriteStartScenario
  #define EventWriteString        EtwEventWriteString
  #define EventWriteTransfer      EtwEventWriteTransfer
  
  #include <evntprov.h>

  ...easy, wasn't it?

Usage

Following example is performed on Windows 10 x64

Enable Test-Signing boot configuration option (note that you'll need administrative privileges to use bcdedit) and reboot the machine:

bcdedit /set testsigning on
shutdown /r /t 0

Now open administrator command line and run following command:

injldr -i

The -i option installs the driver. After the driver is installed, it waits for newly created processes. When a new process is created, it is hooked. Prepare some x86 application, for example, PuTTY and run it. With Process Explorer we can check that indeed, our x64 DLL is injected in this x86 application.

Also, immediately after injldr is started, it starts an ETW tracing session and prints out information about called hooked functions:

You can exit injldr by pressing Ctrl+C. Now you can run injldr without any parameters to just start the tracing session. If you wish to uninstall the driver, run injldr -u.

This driver by default inject x64 DLL into WoW64 processes. If you wish to change this behavior and inject x86 DLL instead, set UseWow64Injection to TRUE. Also, do not forget to compile injdll for x86 architecture and place it in the same directory as injldr.exe.

License

This software is open-source under the MIT license. See the LICENSE.txt file in this repository.

Dependencies are licensed by their own licenses.

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