This can only happen when both logical processors on the same physical processor are active.
In the meantime, Debian advises all users of Kaby Lake and Skylake CPUs to disable hyperthreading immediately via either BIOS or UEFI settings. The defect also affects related Intel server processors, such as the Xeon v5 and v6, as well as "select Intel Pentium processor models", Holschuh added.
Intel's errata list for the recent Skylake-X processors (unearthed by Hot Hardware), provide a bit more insight into the nuts and bolts of the issue.
Implication: "Due to this erratum, the system may experience unpredictable system behaviour".
It appears the problem is confined to the sixth-generation Skylake and seventh-generation Kaby Lake processors, but it spans from desktop and mobile processors to Xeon models.
Debian developer Henrique de Moraes Holschuh is now warning users of a widely known GNU/Linux distribution about a new security error which is affecting Intel's 6 and 7 generation Skylake and Kabylake processors supporting HyperThreading.More news: 'Unprecedented Constitutional violations': Two attorneys general sue Trump
The issue came to light when Mark Shinwell, a developer working on the OCaml toolchain, contacted the Debian team to explain that the OCaml compiler triggered weird behavior on Intel processors.
The defect only affects processors on which hyperthreading has been enabled, so a quick fix is to turn off this feature in the system BIOS, although this may require a BIOS/UEFI update. However, until they do that, it would be to disable hyper-threading or even shift to AMD.
The report from Debian recommends disabling Hyper-Threading until it gets fixed, which is where the fixed microcode and BIOS/UEFI updates.
Kaby Lake is a little bit of a mixed bag, with Intel's latest April 2017 microcode update now a few months old, but has revisions 0x5d/0x5e that might fix these issues for Kaby Lake CPUs with signatures of 0x806e9 and 0x906e9.
Other users will need platform-specific fixes, or for BIOS/UEFI upgrades to land from their system vendors. We've sent along the requisite request to Intel for more information and will update accordingly.