Intel Skylake & Kaby Lake Hyperthreading Flaw Uncovered

👤by Tim Harmer Comments 📅26.06.2017 17:04:09

Intel probably hoped that this week's tech. news would focus on their new Core X range of CPUs, but this weekend a moderately significant issue with their mainstream lineup has reared its head. Specifically, Skylake and Kaby Lake processors appear to be suffering from a critical flaw in Hyperthreading which could cause "unpredictable system behavior" - code for 'very bad things' - and unfortunately the process for getting it fixed is far from elementary.

The issue was highlighted in a security advisory to the Linux Debian user list ominously titled '[WARNING] Intel Skylake/Kaby Lake processors: broken hyper-threading', detailing the conditions which cause the issue and affected processors. Diagnosis of this bug originated in the OCaml community, who it appears were instrumental in developing a fix. The Intel Errata for this bug is as follows:


Short Loops Which Use AH/BH/CH/DH Registers May Cause
Unpredictable System Behavior.


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.


Due to this erratum, the system may experience
unpredictable system behavior.

The Debian security advisory makes it clear that although their statement refers specifically to the Debian linux environment, other OSes (including non-Linux OSes) are believed to be affected.

CPU microcode updates that address the problem are believed to have have been released to motherboard manufacturers, but will need to be rolled out via UEFI BIOS updates. Users should therefore be on the lookout for BIOS packages that reference the Intel Errata codes. Until a fix is applied the rather stark recommendation is that users disable Hyperthreading on their system.


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