Disabling Intel Boot Guard on MEv11

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This covers Intel Skylake, Kaby Lake and Kaby Lake Refresh / Coffeelake machines; note that Coffeelake includes KabyLake Refresh and may have MEv12. This page concerns only those platforms that have Intel MEv11, not MEv12. The facts on this page are applicable to both mobile and desktop platforms.

The Intel Boot Guard is a security mechanism implemented by intel, but not all vendors enable it. If enabled, the bootblock in the flash is protected at boot time by cryptographic signature verification; this means only the vendor can update the flash.

On systems with MEv11, a bug exists in older versions (of MEv11) that allows for unsigned code execution, at a very early stage in the boot process, to the point that almost all of the ME firmware in flash can be fully replaced. The ME is also what implements Boot Guard, and the hack is possible before Boot Guard is enforced, allowing for it to be disabled.

See: CVE-2017-5705

Mate Kukri, who authored the ThinkPad T480/T480 and OptiPlex 3050 Micro ports, wrote a tool called deguard, which Libreboot uses on these boards. This tool reconfigures the ME, exploiting it so as to disable the Boot Guard.

You can find it hosted by the coreboot project:

https://review.coreboot.org/deguard (git repository)

and Libreboot provides a mirror of this repository:

https://codeberg.org/libreboot/deguard

More information is available there.

This is done by externally flashing an older version. Libreboot’s build system automatically downloads this older version, runs me_cleaner on it, and applies the deguard hack; this includes machine-specific ME configuration, which is added per machine by extracting it from a dump of the original flash. The resulting configuration (for the MFS partition in the ME) is then inserted into the generic ME image.

Note that the deguard utility can also be used on MEv11 setups that don’t have Boot Guard, if you simply want to auto-download and neuter a generic ME image, and then provide machine configuration. It is essentially doing the same thing that Intel’s own FITC utility does (Intel Flash Image Tool), which is normally only available to vendors; the deguard utility written by Mate Kukri is available under a free software license, and included by default in Libreboot releases. It can be used for any MEv11-based system.

To download deguard in lbmk (Libreboot’s build system), do this:

./mk -f deguard

Then go in src/deguard/ and the README.html file in there tells you more information about how it works, and how to use it. You do not need to run this tool yourself, unless you’re adding a new board, because Libreboot is programmed to use it automatically, during the build process (or during insertion of vendor files after the fact, on release images).

With deguard, the machine is operating in a state as though the Boot Guard keys were never fused, even if they were. Previous work has been done by others, related to the Boot Guard, but nothing quite so thorough and easy to use as deguard existed previously!

Mate Kukri was able to figure this out and implement deguard, using existing work done by PT Research and Youness El Alaoui, exploiting the Intel SA 00086 bug which you can read more about here:

https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00086.html

https://www.intel.com/content/www/us/en/support/articles/000025619/software.html

Note that Intel refers to this as a means of a so-called attacker running so-called malicious code; while this may also be possible in the strictest sense, flash write protection is possible on these machines, which you can read about on the GRUB hardening page. Intel made the Boot Guard without giving users control of it, so people have worked for years to try to hack around it, as a matter of user freedom. So remember: when Intel is talking about security, they mean their security, not yours. To them, you are simply flashing malicious code. But they are the ones with malice.

Mate Kukri and others who work on such hacks are heroes, and they have done a great service to the Libreboot project.

Many more machines are now possible to port to coreboot, thanks to this hack.

Markdown file for this page: https://libreboot.org/docs/install/deguard.md

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