Unsafe HyperThreading in Skylake and Kaby Lake intel CPUs

[LigH]

[WARNING] Intel Skylake/Kaby Lake processors: broken hyper-threading (Debian mailing list)

This warning advisory is relevant for users of systems with the Intel processors code-named "Skylake" and "Kaby Lake". These are: the 6th and 7th generation Intel Core processors (desktop, embedded, mobile and HEDT), their related server processors (such as Xeon v5 and Xeon v6), as well as select Intel Pentium processor models.

TL;DR: unfixed Skylake and Kaby Lake processors could, in some situations, dangerously misbehave when hyper-threading is enabled.
Disable hyper-threading immediately in BIOS/UEFI to work around the problem. Read this advisory for instructions about an Intel-provided fix.

[burfadel]

Surprising it has taken this long to find it. I guess until now people have been either blaming the software (in the case of Linux stuff) or Microsoft.

[LigH]

Responsible bug reporters will try to find a reliable way to reproduce an issue before blaming, so the harder a bug is to reproduce, the longer people may hesitate to report it with confidence.

[lvqcl]

Problem:
Under complex micro-architectural conditions, short loops
of less than 64 instructions that use AH, BH, CH or DH
registers as well as their corresponding wider register
(e.g. RAX, EAX or AX for AH) may cause unpredictable
system behavior. This can only happen when both logical
processors on the same physical processor are active.

Probably very few programs use AH/BH/CH/DH nowadays.