5.8 KiB
% GM45 no-microcode bug mitigations re-added (ThinkPad X200, T400 etc) % Leah Rowe % 17 April 2023
UPDATE: This also applies to the Dell Latitute E6400 port, added on 19 April 2023 to Libreboot. See: E6400 announcement
The change described in this article is not present in Libreboot 20221214, 20230319 or 20230413 - UPDATE: Libreboot 20230423 is out, and includes this change on all GM45 targets
If you want a no-microcode setup, either build the latest Libreboot from source via lbmk and remove the microcode updates using the instructions on this page, or if you need pre-built ROM images, Libreboot 20220710 had no-microcode ROMs by default and it has the mitigation patches described by this article.
Introduction
Microcode updates provide stability and security fixes, using the files provided by coreboot, which are assembled at boot time. CPUs have microcode in them which configures the logic gates that implement an instruction set architecture (the x86/x86_64 architecture), and CPUs provide a volatile mechanism for hot-patching it at boot time, which coreboot makes use of.
Libreboot includes CPU microcode updates by default, on all x86 hardware.
This includes boards that it supported prior to the new policy enacted last
year; before the policy change, Libreboot excluded these updates (but the
code for loading them was retained, so you could manually add them yourself
if you wanted to). Adding and removing them can be done by coreboot, at build
time, or you can add/remove them from the compiled ROM image by
running cbfstool; you can learn how to add/remove microcode updates here:
How to add/remove microcode on coreboot images
You can learn why they're needed, and about microcode in general, by reading
the Binary Blobs Reduction Policy - it is a pragmatic policy,
enacted on 15 November 2022 with a view to supporting as much hardware as
possible from coreboot, reducing or eliminating binary blobs in the boot
flash and mitigating any issues that arise - for example, Libreboot makes use
of me_cleaner on some newer Intel platforms. You can read more about how the
policy is executed by reading the Freedom Status page.
I've received some feedback about this, specifically by users of GM45-era ThinkPads (X200, T400, T500, W500, R500, R400, X301, X200S/X200T, T400S and R500). On those machines, if you remove microcode updates, it would break SpeedStep (CPU frequency scaling) and the machine won't reboot properly (turning off and on would work). Additionally, the machine would crash on lots of I/O (CPU cycles and memory usage), due to a Machine Check Exception state leading to kernel panic in your operating system; this last issue does not have a known fix, except to update the microcode (which newer Libreboot releases do, by default).
Libreboot contained mitigations for broken SpeedStep/reboot, in older releases, but it was decided that these mitigations would be excluded from the new releases which include microcode updates by default; why mitigate a problem that no longer exists in Libreboot?
Why indeed.
Mitigations patches restored!
I've restored these mitigations to Libreboot, as of today.
The patch re-adding these mitigations can be seen here:
https://browse.libreboot.org/lbmk.git/commit/?id=bd4ea9a02845b22a09b73ebb015ce134234d100b
Why?
Free choice, that's why. We don't lecture anyone. We just help people.
One aspect of Libreboot policy is that Libreboot must be configurable. In practise, some users want to remove microcode updates regardless of the problems that it would cause. Even if I disagree with their reasoning, I respect their decision and this change is designed to accomodate them.
in practise, such users had a choice before but they currently do not, in recent Libreboot releases. This criticism was raised with me, and I've taken it to heart. Therefore, users now have a choice; they always did, but remember:
Libreboot is aimed at non-technical users. Libreboot needs to be usable by them. It needs to work. It does work, very well, when microcode updates are included, as they are by default these days. This patch is about harm reduction, to at least smooth things over for a small handful of users who decide to remove microcode updates (as is their choice).
This change, today, fixes a practical violation of Libreboot policy by once again restoring such practical choice to the user. It is already said how to do so elsewhere, but I'll repeat it here in this news article for posterity:
How to add/remove microcode updates from ROM
Extract to file on disk:
cbfstool libreboot.rom extract -n cpu_microcode_blob.bin -f cpu_microcode_blob.bin
Remove:
cbfstool libreboot.rom remove -n cpu_microcode_blob.bin
Add (if not present):
cbfstool libreboot.rom add -f cpu_microcode_blob.bin -n cpu_microcode_blob.bin -t microcode
The cbfstool program can be compiled from util/cbfstool/ in the coreboot
Git repository. In all recent Libreboot releases, you can find it
under coreboot/default/util/cbfstool/.
Regardless, the mitigations are largely harmless and so, I've restored them. They are now included once again, in the Libreboot git repository and they will be available in ROM images when the next release comes out.
I'll probably re-work the mitigation patch at some point, probably before the next release, so that the mitigations are only applied if microcode updates are absent. This would be checked for during machine initialisation, by modifying the current patches accordingly.
That is all.