[coreboot] FYI: Reverse Engineering x86 Processor Microcode

[Rudolf Marek]

Hi all,

I think some of you might already noticed a nice paper:

Microcode is an abstraction layer on top of the physical
components of a CPU and present in most generalpurpose
CPUs today. In addition to facilitate complex and
vast instruction sets, it also provides an update mechanism
that allows CPUs to be patched in-place without requiring
any special hardware. While it is well-known that CPUs
are regularly updated with this mechanism, very little is
known about its inner workings given that microcode and
the update mechanism are proprietary and have not been
throughly analyzed yet.

In this paper, we reverse engineer the microcode semantics
and inner workings of its update mechanism of conventional
COTS CPUs on the example of AMD’s K8 and
K10 microarchitectures. Furthermore, we demonstrate
how to develop custom microcode updates. We describe
the microcode semantics and additionally present a set of
microprograms that demonstrate the possibilities offered
by this technology. To this end, our microprograms range
from CPU-assisted instrumentation to microcoded Trojans
that can even be reached from within a web browser
and enable remote code execution and cryptographic implementation
attacks.

https://www.usenix.org/system/files/conference/usenixsecurity17/sec17-koppe.pdf

which is about unencrypted AMD K8 microcode. It should throw some light on how
the microcode works, and also why is it good to apply the fixes. Please note
that I don't want another flamewar about why not doing the microcode updates
would make my system more free.

There seems to be just one detail missing, the microcode triads are somehow
obfuscated, and the paper does not tell how.

Happy hacking,
Rudolf