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Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!

Uses of the Management Engine

The Intel Management Engine (abbreviated "ME") is a CPU which:

permits out of band management of the computer. See the Wikipedia AMT article for example use cases.
On recent hardware, it also initializes the hardware, before the boot firmware(BIOS/EFI/UEFI/Coreboot/etc...).

Freedom and security issues

The code that is running inside the management engine is proprietary and signed. Therefore, it cannot easily be audited, tested, or replaced, except by those people with access to the relevant private keys, i.e. a handful of Intel staff (and possibly government agents).
The ME has access to a lot of things, see "physical capabilities" column below for more details.
In addition to obvious attack vectors (the ME could be used by an adversary to spy on the PC user, tamper with their documents, etc), it could also potentially be used to alter the contents of the motherboard's BIOS flash chip, thereby polluting Coreboot builds based upon extracting the contents of that flash chip.

Where

Board	Firmware	Microarchitecture	ME location and physical capabilities	ME restrictions
Lenovo X60/X60s/X60T	None. <ref name="nic-amt">The Ethernet controller is capable of running some fimrwares( like AMT 1.0), but the hardware is not configured to do it on that machine. So no firmwares are loaded. See Intel_82573_Ethernet_controller for more details.</ref>	I945 + ICH7	Inside the ethernet controller, disabled: no Ethernet controller fimrware. <ref name="nic-amt"></ref>	Disabled: No Ethernet controller fimrware. <ref name="nic-amt"></ref>
Lenovo T60		I945 + ICH7
Lenovo x200	Me firmware with AMT and other modules	GM45/GS45	The ME is inside the PCH, it: Has access to the computer's memory/RAM Controls the computer's original networking adapters	Signed firmware The ME can be disabled (no Fimrware is run by it).
Lenovo x201	Me firmware with AMT and other modules	Nehalem		Signed firmware If ME firmware is absent, the computer freezes about 30min after boot.
Packard Bell EasyNote LM85 (MS2290)	?	Nehalem
Samsung Series 5 550 Chromebook	me.bin	Sandy Bridge		Signed firmware
Samsung Series 3 Chromebox	me.bin
Lenovo t520	Me firmware with AMT and other modules
Google/HP Pavilion Chromebook 14	me.bin	Ivy Bridge		Signed firmware
Google Chromebook Pixel	me.bin
Google/Acer C7 Chromebook	me.bin
Google/Lenovo Thinkpad X131e Chromebook	me.bin
Lenovo t530	Me firmware with AMT and other modules
Lenovo x230	Me firmware with AMT and other modules
Kotron KTQM77/mITX	?
Google/Acer C720 Chromebook	?	Haswell		Signed firmware
Google/HP Chromebook 14	?	Haswell		Signed firmware

Why there is no replacement for it yet

Replacing the ME firmware is not that easy because:

The ME bootrom checks the firmware signature.
On recent chipset its RAM region is locked while it is allocated.
Power glitches(by the ec) while the ME is checking its firmware is probably not practically doable.

So even if some people partially documented some ME firmware format, there is very few probability of having a free software replacement for it one day.

However coreboot also support other systems than the ones with recent intel CPU/chipsets. The List of supported mainboard list some of them.

Some of theses don't have a management engine.
Some ships without it enabled(that means that the hardware is not used).
Some ships with it enabled, but it can be disabled not to use it at all, like on the Lenovo x200.

Neutralizing the ME

A collaborative effort to neutralize the ME has found some success, see here. This tool has been included in coreboot and can be enabled with the option "Strip down the Intel ME/TXE firmware" (CONFIG_USE_ME_CLEANER).

This can free up most of the space used by ME, allowing you to use a larger CBFS. See here.

Using a smaller version of the Intel ME

Most PCs ship a 5MiB version of ME firmware. It is possible to use a smaller version (2MiB), but you have to make sure that it matches the chipset you are running on. You may want to use a smaller version to increase the maximum payload size by 3MiB. Search on the web for BIOS updates of different vendors with the same chipset and extract the ME using available tools. Once you found a smaller ME, you have to update your Intel flash descriptor and decrease the region that is used for ME.

References

Intel Management Engine