Serious Hyperthreading bug affects Intel Skylake and Kaby Lake architectures

The bug affects all Kaby Lake and Skylake chips with Hyperthreading. (Source: Intel)

A serious bug in the microcode for Intel's Kaby Lake and Skylake CPUs has been found. The bug can cause misbehavior of software and loss of data. It affects all 6th and 7th-gen CPUs with Hyperthreading enabled.

Douglas Black, Published 06/26/2017

Delays, getting shown up by AMD, having Qualcomm nipping at their heels — it's been a rough year for Intel, and now it's about to get worse: a major flaw in both Intel's Skylake and Kaby Lake architectures was announced on the Debian Linux user forum alongside a warning. The flaw, however, affects every OS, not just Linux.

The bug is documented in Intel's errata as follows:

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 (eg 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.

In English, this means that both Skylake and Kaby Lake platforms have inherent bugs in their microcoding. As a result, systems running on Kaby Lake or Skylake platforms could "dangerously misbehave when hyper-threading is enabled", according to the advisory. Damage could include software misbehavior and corruption or loss of data, but the worst part is that the fix is to simply to disable Hyperthreading — a key component of the higher performance Intel's Core i7-series chips enjoy over their less expensive i5 counterparts.

A real fix (not disabling Hyperthreading) will require a BIOS or UEFI update to remedy the bug, but this must be disseminated by motherboard vendors. In the case of notebooks, this will require an update by the OEM themselves. For more information on this serious bug, the original thread is available to read here.