x86/cpu: Help users notice when running old Intel microcode

Old microcode is bad for users and for kernel developers.

For users, it exposes them to known fixed security and/or functional
issues. These obviously rarely result in instant dumpster fires in
every environment. But it is as important to keep your microcode up
to date as it is to keep your kernel up to date.

Old microcode also makes kernels harder to debug. A developer looking
at an oops need to consider kernel bugs, known CPU issues and unknown
CPU issues as possible causes. If they know the microcode is up to
date, they can mostly eliminate known CPU issues as the cause.

Make it easier to tell if CPU microcode is out of date. Add a list
of released microcode. If the loaded microcode is older than the
release, tell users in a place that folks can find it:

	/sys/devices/system/cpu/vulnerabilities/old_microcode

Tell kernel kernel developers about it with the existing taint
flag:

	TAINT_CPU_OUT_OF_SPEC

== Discussion ==

When a user reports a potential kernel issue, it is very common
to ask them to reproduce the issue on mainline. Running mainline,
they will (independently from the distro) acquire a more up-to-date
microcode version list. If their microcode is old, they will
get a warning about the taint and kernel developers can take that
into consideration when debugging.

Just like any other entry in "vulnerabilities/", users are free to
make their own assessment of their exposure.

== Microcode Revision Discussion ==

The microcode versions in the table were generated from the Intel
microcode git repo:

	8ac9378a8487 ("microcode-20241112 Release")

which as of this writing lags behind the latest microcode-20250211.

It can be argued that the versions that the kernel picks to call "old"
should be a revision or two old. Which specific version is picked is
less important to me than picking *a* version and enforcing it.

This repository contains only microcode versions that Intel has deemed
to be OS-loadable. It is quite possible that the BIOS has loaded a
newer microcode than the latest in this repo. If this happens, the
system is considered to have new microcode, not old.

Specifically, the sysfs file and taint flag answer the question:

	Is the CPU running on the latest OS-loadable microcode,
	or something even later that the BIOS loaded?

In other words, Intel never publishes an authoritative list of CPUs
and latest microcode revisions. Until it does, this is the best that
Linux can do.

Also note that the "intel-ucode-defs.h" file is simple, ugly and
has lots of magic numbers. That's on purpose and should allow a
single file to be shared across lots of stable kernel regardless of if
they have the new "VFM" infrastructure or not. It was generated with
a dumb script.

== FAQ ==

Q: Does this tell me if my system is secure or insecure?
A: No. It only tells you if your microcode was old when the
   system booted.

Q: Should the kernel warn if the microcode list itself is too old?
A: No. New kernels will get new microcode lists, both mainline
   and stable. The only way to have an old list is to be running
   an old kernel in which case you have bigger problems.

Q: Is this for security or functional issues?
A: Both.

Q: If a given microcode update only has functional problems but
   no security issues, will it be considered old?
A: Yes. All microcode image versions within a microcode release
   are treated identically. Intel appears to make security
   updates without disclosing them in the release notes.  Thus,
   all updates are considered to be security-relevant.

Q: Who runs old microcode?
A: Anybody with an old distro. This happens all the time inside
   of Intel where there are lots of weird systems in labs that
   might not be getting regular distro updates and might also
   be running rather exotic microcode images.

Q: If I update my microcode after booting will it stop saying
   "Vulnerable"?
A: No. Just like all the other vulnerabilies, you need to
   reboot before the kernel will reassess your vulnerability.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "Ahmed S. Darwish" <darwi@linutronix.de>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/all/20250421195659.CF426C07%40davehans-spike.ostc.intel.com
(cherry picked from commit 9127865b15eb0a1bd05ad7efe29489c44394bdc1)

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