Overview

EfiGuard is a portable x64 UEFI bootkit that patches the Windows boot manager, boot loader and kernel at boot time in order to disable PatchGuard and Driver Signature Enforcement (DSE).

Features

• Currently supports all EFI-compatible versions of Windows x64 ever released, from Vista SP1 to Server 2019.

• Easy to use: can be booted from a USB stick or the Windows EFI partition via a loader that automatically finds and boots Windows. The driver can also be loaded and configured manually using either the UEFI shell or the loader.

• Makes extensive use of the Zydis disassembler library for fast runtime instruction decoding to support more robust analysis than what is possible with signature matching, which often requires changes with new OS updates.

• Works passively: the driver does not load or start the Windows boot manager. Instead it acts on a load of bootmgfw.efi by the firmware boot manager via the boot selection menu or an EFI application such as the loader. If a non-Windows OS is booted, the driver will automatically unload itself.

• Supports four-stage patching for when bootmgfw.efi starts bootmgr.efi rather than winload.efi. This is the case when a WIM file is loaded to boot WinPE, Windows Setup or Windows Recovery mode.

• Graceful recovery: in case of patch failure, the driver will display error information and prompt to continue booting or to reboot by pressing ESC. This is true even up to the final kernel patch stage, because the last patch stage happens before ExitBootServices is called. Many UEFI Windows bootkits hook OslArchTransferToKernel which, while easy to find by pattern matching, is a function that executes in protected mode after ExitBootServices. This means no boot services are available to tell the user that something went wrong.

  Simulated patch failure with error information

• Debuggable: can output messages to a kernel debugger and to the screen (albeit buffered) during the kernel patching stage, and to a serial port or unbuffered to the screen during the boot manager and boot loader patching stages. If the driver is compiled with PDB debug information, it is possible to load the debug symbols at any point after HAL initialization by specifying the virtual DXE driver base and debugging it as you would a regular NT driver.

• DSE bypasses: available as either a straightforward UPGDSED-style DSE disable at boot time or as a hook on the SetVariable() EFI runtime service. The latter serves as an arbitrary kernel mode read/write backdoor that can be called from Windows using NtSetSystemEnvironmentValueEx and allows setting g_CiEnabled/g_CiOptions to the desired value. A small DSEFix-style application named EfiDSEFix.exe is provided that can be used to do this. It is also possible to leave DSE enabled and to disable only PatchGuard. The loader will use the SetVariable hook method by default, due to the fact that some anti-cheat and anti-virus programs do not understand the difference between cheats or malware and self-signed drivers in general and target the UPGDSED fix.

• Supports on-disk modified kernels and boot loaders by patching ImgpValidateImageHash at every stage as well as ImgpFilterValidationFailure, which may silently rat out some classes of violations to a TPM or the SI log file.

• Allows Secure Boot to work with Windows 7 (not a joke!). Windows 7 itself is oblivious to Secure Boot as it does not support it, or (officially) even booting without CSM. This is useful for people who want to use Windows 7 on a locked down device that requires WHQL Secure Boot. Wiki entry on how to get this to work here.

  WinObjEx64 on Windows 7 with Secure Boot enabled

Issues and limitations

• EfiGuard can not disable Hypervisor-enforced Code Integrity (HVCI or HyperGuard) due to HVCI running at a greater privilege level. EfiGuard can coexist with HVCI and even successfully disables PatchGuard in the normal kernel, but this is not useful in practice because HVCI will catch what PatchGuard did previously. Both types of DSE bypass are rendered useless by HVCI: the boot time patch has no effect because the kernel defers to the secure kernel for integrity checks, and the SetVariable hook will cause a SECURE_KERNEL_ERROR bugcheck if it is used to write to g_CiOptions.
• Checked kernels are not supported due to the differences in PatchGuard and DSE initialization code caused by disabled optimizations and added asserts, as well as additional changes to PatchGuard in checked kernels. This should not be an issue as checked kernels are not generally useful without a kernel debugger attached, which disables PatchGuard.

How to use

There are two ways to use EfiGuard: booting the loader (easiest), or using the UEFI shell to load the driver. In both cases it is possible to install EfiGuard on a secondary boot medium such as a USB stick or on the EFI system partition. Using the EFI partition has the advantage of not requiring a second boot disk, but this method is more complex to set up. It is advised to try one of the methods below first, and read the instructions in issue #2 if you want to install EfiGuard on the EFI partition.

Booting the loader

1. Download or compile EfiGuard, go to EFI/Boot and rename one of Loader.efi or Loader.config.efi to bootx64.efi. The two are identical, except Loader.efi boots without user interaction whereas Loader.config.efi will prompt you to configure the DSE patch method used by the driver (if you want to change this).
2. Place the files on a boot drive such as a USB stick (for physical machines) or an ISO/virtual disk (for VMs). The paths should be /EFI/Boot/{bootx64|EfiGuardDxe}.efi. It is recommended to use FAT32 formatted USB sticks.
3. Boot the machine from the new drive instead of booting Windows. Most firmwares provide a boot menu to do this (accessible via F10/F11/F12). If not, you will need to configure the BIOS to boot from the new drive.
4. If you are using the default loader, Windows should now boot, and you should see EfiGuard messages during boot. If you are using the configurable loader, answer the configuration prompts and Windows will boot.
5. If you booted with the SetVariable hook (the default), run EfiDSEFix.exe -d from a command prompt after boot to disable DSE. Run EfiDSEFix.exe to see the full list of options.

Using the UEFI shell to load the driver

1. Follow the steps 1 and 2 as above, but do not rename the loader to bootx64.efi. Instead, either use the BIOS-provided shell (if you have one), or download the EDK2 UEFI Shell and rename it to bootx64.efi.
2. Boot the machine to the UEFI shell.
3. cd to /EFI/Boot on the correct filesystem and run load EfiGuardDxe.efi to load the driver.
4. (Optional) Run either Loader.efi or Loader.config.efi from the same directory to boot Windows. You can also continue working in the shell, or exit to go back to the BIOS/boot menu and boot from there.
5. After boot, apply the DSE fix as above if applicable.

Compilation

Compiling EfiGuardDxe and the loader

EfiGuard requires EDK2 to build. If you don't have EDK2 installed, follow the steps in Getting Started with EDK2 first as the EDK2 build system is fairly complex to set up. This section assumes you have a workspace directory that your WORKSPACE environment variable points to, with a copy of EDK2 checked out in workspace/edk2. Supported compilers are MSVC, Clang, GCC and ICC.

1. Clone the EfiGuard repository into workspace/edk2/EfiGuardPkg.
2. Open a prompt or shell that sets up the environment variables for EDK2.
3. Run build -a X64 -t VS2017 -p EfiGuardPkg/EfiGuardPkg.dsc -b RELEASE, substituting your toolchain for VS2017.

This will produce EfiGuardDxe.efi and Loader.efi in workspace/Build/EfiGuard/RELEASE_VS2017/X64. To build the interactively configurable loader, append -D CONFIGURE_DRIVER=1 to the build command.

Compiling EfiDSEFix

EfiDSEFix requires Visual Studio to build.

1. Open EfiGuard.sln and build the solution.

The output binary EfiDSEFix.exe will be in Application/EfiDSEFix/bin.

The Visual Studio solution also includes projects for EfiGuardDxe.efi and Loader.efi which can be used with VisualUefi, but these projects are not built by default as they will not link without additional code, and the build output will be inferior (bigger) than what EDK2 produces. Loader.efi will not link at all due to VisualUefi missing UefiBootManagerLib. These project files are thus meant as a development aid only and the EFI files should still be compiled with EDK2. To set up VisualUefi for this purpose, clone the repository into workspace/VisualUefi and open EfiGuard.sln.

Architecture

While EfiGuard is a UEFI bootkit, it did not start out as one. EfiGuard was originally an on-disk patcher running on NT (similar to UPGDSED), intended to test the viability of a disassembler-based aproach, as opposed to using PDB symbols and version-specific signatures. PatchNtoskrnl.c still looks very much like this original design. Only after this approach proved successful, with no modifications to code needed in over a year of Windows updates, did UEFI come into the picture as a way to further improve capabilities and ease of use.

Some of the benefits provided by a bootkit approach include:

• No on-disk modifications to kernels or bootloaders needed.
• No need to modify the boot configuration store using bcdedit.
• No need to patch ImgpValidateImageHash (although this is still optionally done).
• Ironically, the use of a bootkit allows enabling Secure Boot, provided you own the Platform Key and are able to add your personal certificate to the db store.

The initial incarnation of EfiGuard as a bootkit was an attempt to get dude719's UEFI-Bootkit to work with recent versions of Windows 10, because it had become dated and no longer works on the latest versions (like UPGDSED, often caused by version-sensitive pattern scans). While I did eventually get this to work, I was unsatisfied with the result mostly due to the choice of hooking OslArchTransferToKernel, which as noted above executes in protected mode and after ExitBootServices has been called. Apart from this, I was not satisfied with only being able to patch some versions of Windows 10; I wanted the bootkit to work on every EFI-compatible version of Windows x64 released to date. Because of this, I rewrote the bootkit from scratch with the following aims:

• To provide patch information at every stage of boot including the kernel patch itself.
• To support all EFI-compatible versions of Windows versions (at the time of writing).
• To enable lazy instantiation of the bootkit and optionally a kernel backdoor, achieved by EFI System Table hooks.

A big picture overview of the final EfiGuard boot flow is shown in the diagram above. For the individual component-specific hooks and patches, see EfiGuardDxe/PatchXxx.c in the source files. For driver initialization/unloading and the EFI Boot and Runtime Services hooks, see EfiGuardDxe.c.

Credits

• UPGDSED by hfiref0x and Fyyre
• Zydis by zyantific
• Uninformed articles on PatchGuard v1, v2 and v3 by Skywing
• UEFI-Bootkit by dude719
• ReactOS

License

EfiGuard is licensed under the GPLv3. Files in the EfiGuardDxe/Zydis submodule are licensed under the MIT license.