Microcode Bug Affects Intel Skylake and Kaby Lake CPUs

Cesar Mills
June 26, 2017

The project member claims the erratum in question is called "Short Loops Which Use AH/BH/CH/DH Registers May Cause Unpredictable System Behavior" in Intel documentation.

According to the Debian blog, Intel is aware of the flaw and has apparently supplied patches, but the firm neglected to inform the OCaml researchers who originally reported the bug about this. This can only happen when both logical processors on the same physical processor are active.

Intel Skylake and Kaby Lake processors without HyperThreading capabilities are not affected by the flaw.

A warning advisory has gone out on Debian mailing lists claiming that Skylake and Kaby Lake processors may "dangerously misbehave" when Hyper-Threading is enabled: the writer advises that users should disable the feature immediately in their BIOS/UEFI to work around the problem. The errata apply to any operating system, so it can also impact Windows users.

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For now, Holschuh recommends disabling hyperthreading to circumvent the issue, but that isn't an acceptable long-term fix.

It might never have been noticed if Mark Shinwell, a developer working on the OCamlL toolchain, had not contacted the Debian team to explain that the OCaml compiler triggered an Intel microcode issue. Intel did issue microcode updates in the interim.

What all of this means in real world terms is that owners of Intel's 6th or 7th generation processors should turn off hyperthreading and wait for a UEFI/BIOS update from their motherboard vendor, or else they will run the risk of basically ruining their entire system. Skylake debuted in August 2015, so if there were a considerable number of mainstream desktop applications that trigger the errata, it would have likely already been thrust into the spotlight. We've sent along the requisite request to Intel for more information and will update accordingly.

The two solutions highlighted by the devloper are to either update the processor microcode or, perhaps more simply, disable hyperthreading.

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