[Bug 2130658] Re: hashed microcode updates

If I understand this right, this change has to do with CVE-2024-36347:

"Improper signature verification in AMD CPU ROM microcode patch loader
may allow an attacker with local administrator privilege to load
malicious microcode, potentially resulting in loss of integrity of x86
instruction execution, loss of confidentiality and integrity of data in
x86 CPU privileged context and compromise of SMM execution environment."

Which resulted in hardcoding microcode hashes in the kernel with the
following commit:

https://github.com/torvalds/linux/commit/50cef76d5cb0e199cda19f026842560f6eedc4f7

But I don't understand why this would help in any way. If you are a
local administrator, you can just disable the check with
"microcode.amd_sha_check=off", so how is this an improvement in any way?

I'm not sure it's worth the effort in trying to match the microcode with
the kernel for a fix that doesn't actually improve anything, unless I'm
not quite understanding the issue here.

** CVE added: https://cve.org/CVERecord?id=CVE-2024-36347

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https://bugs.launchpad.net/bugs/2130658

Title:
hashed microcode updates

Status in amd64-microcode package in Ubuntu:
New
Status in dracut package in Ubuntu:
New
Status in initramfs-tools package in Ubuntu:
New
Status in linux package in Ubuntu:
New

Bug description:
Apologies that this isn't better -- I've been putting this off for too
long as it is, and doing a poor job seems like it's better than
waiting until I can do a good job.

AMD has some problems with microcode updates -- the signature
verification is not right. (Google put together a microcode update to
recreate the xkcd random comic.)

The AMD kernel developers have pushed a mitigation to the Linux kernel
that hardcodes a single SHA256 hash of the latest microcode update.
When userspace tries to patch the microcode, the kernel driver will
compare the proposed new microcode and if it matches, then it is
loaded. If it doesn't match, then it is not loaded.

This is simple and easy. I'm afraid it isn't responsive to how we
deliver updates to our users:

- the kernel is delivered on its own timeframe; our updates even more so, perhaps over the course of months
- the amd64-microcode package is delivered on its own timeframe; our updates can be timed with more precision than the kernel updates, but it would be nice to follow AMD closely without stress.

If we deliver a new kernel with an updated hash from upstream before
we have delivered the microcode, users will probably be reverted to
running the factory firmware.

If we deliver a new microcode package before we have delivered an
updated kernel, users will probably be reverted to running the factory
firmware.

Running the factory firmware might be a performance, safety, security,
or feature regression vs the user's immediately previous boot. Users
might not even notice anything anything is wrong.

I don't pretend to have the best possible solution, so treat this idea
as something to be improved:

We should amend our systems to be more flexible in which microcodes we
will load, to allow loading any of the previous N microcode packages
that we have shipped.

We should amend our microcode packages to allow installing multiple
versions and manage the multiple versions similarly to the Linux
kernel "keep most recent three" sort of behavior.

We should amend our kernel to expose in the filesystem, even when not
running, which microcodes it can load.

We should amend our initramfs generators to include the newest
microcode that a kernel can load, for each initramfs that we generate.


Ideally, we would collaborate with Debian on this issue. Probably Debian's situation is better than ours, since they have significantly fewer kernels to wrangle, over fewer supported releases, but I suspect their users would also benefit from some de-coupling.

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