PatchBoot

This repository provides a guide for patching AMI Aptio V UEFI firmware to circumvent Secure Boot checks and allow loading of unsigned executables.

Disclamer

Flashing modified firmware onto your motherboard carries the risk of damaging it. On certain motherboards, this damage could be irreparable. Please proceed with caution - you have been warned.

Reasons for patching

I use a single computer for both gaming and development, and my daily drivers are Arch Linux (yes, I felt the need to mention it) and Windows 10 LTSC/Windows 11. The issue is that certain intrusive anti-cheat systems require Secure Boot to be enabled. Though dual-booting with Secure Boot on is technically possible, it's an incredibly tedious process (and I would need to use different bootloader). So, I decided to straight up patch the firmware. I've previously attempted simpler solutions, but these anti-cheat systems have already adapted to detect and block such methods, preventing me from playing.

Guide

Before starting with the entire process, ensure there is a way to flash modded firmware onto your motherboard. I have ASUS ROG Strix X570-E Gaming which has USB BIOS FlashBack functionality that conveniently does not validate provided files. Other motherboards should be able to use AMI AFU. You can always search the internet for motherboard name + modded BIOS to see if there are any guides on flashing it and what tools are needed.

Getting the original firmware

While it's possible to directly dump the firmware from your motherboard, in most cases, you can simply use the update .CAP file provided by your motherboard manufacturer. In my case, I simply downloaded the latest one here.

Extracting EFI binaries

Once you have the firmware file, open it in UefiTool (make sure to use the latest stable version, as the nightly rewrite does not support module replacing).

After loading the file, navigate to 'File > Search', and execute a case-insensitive text search for the string 'image verification'.

You should multiple results from SecurityStubDxe (GUID: F80697E9-7FD6-4665-8646-88E33EF71DFC). You can navigate to the image by double-clicking on the entry Unicode text "image verification" found in PE32 image section at offset XXXXh. Then, right-click on the 'PE32 image section', and select 'Extract body...'.

Finding image verification handler

Now comes the most challenging part. To find the function responsible for image signature verification, you'll need to open the file in your preferred reverse engineering tool. In my case, that tool is IDA Pro.

Once you've opened the file, search for the EFI_SECURITY_VIOLATION status code. This code corresponds to 0x800000000000001A in hex format. In IDA Pro, you can execute this search by navigating to 'Search > Immediate value...'. Check 'Find all occurences'.

You'll get a list of a few hits. Examine these functions one by one. All of them will be short, except for one. This longer function will also feature a policy check at the top and that will be the handler function.

Note: I have renamed the function to 'ImageVerficationHandler' myself. You have to go through them one by one and figure out which is which.

You can identify this as the handler because it checks the Secure Boot policy status as defined by the UEFI specification. If this policy is disabled, the handler simply returns EFI_SUCCESS.

Now, you just need to copy the function bytes from the hex view for future use. Mine were 48 8B C4 48 89 58 08 48 89 68 18 48 89 70 20 48 89 50 10 57 41 54 41 55 41 56 41 57 48 81 EC for example.

Patching

Patching itself is incredibly easy. All we have to do is ensure the EFI_SUCCESS (zero) status is always returned. The Microsoft ABI, which is used here, returns the value from the function in the rax register.

For instance, the assembly code that zeroes out the rax register and then returns could be as follows:

xor rax, rax
ret

Compiled it would then be 48 31 c0 c3.

Open the file in a hex editor, such as HxD. Search for the function bytes that you saved earlier and replace the starting bytes with the 48 31 c0 c3 sequence.

After making the necessary edits, remember to save the file.

Rebuilding

Reopen UefiTool if you've closed it, and load your firmware file. Follow the same process to search for the binaries as you did before. This time, instead of saving them, select 'Replace body...' and choose the file that you previously patched.

Once you've replaced the file, save the firmware file and then close UefiTool.

Flashing

Now, the real fun begins. As mentioned earlier, I was lucky enough to have a motherboard with USB BIOS FlashBack. All I had to do was place the patched .CAP file onto a FAT32-formatted USB drive. Then, by pressing and holding the FlashBack button, I initiated the flashing process on the motherboard, and simply waited for it to complete.

However, you may not be as fortunate to have this functionality. If that's the case, I recommend searching the internet for guides on how to flash a modified BIOS on your specific motherboard model. There's also a general guide available that could prove helpful.

Result

Stock firmware:

Modded:

Video:

Detection vectors

This is perhaps my favorite part. If we ignore the fact that people will use this to load random garbage, the detectability of this patch is practically non-existent. That is, unless the anti-cheat system deploys its own EFI application to check for the patch or attempts to load an unsigned image. By the time the OS boots up (more specifically, after the call to ExitBootServices - which occurs long before any boot-time Windows driver is loaded) the DXE driver will already have been erased from memory.