Developers at Debian have discovered a critical Hyper-Threading flaw affecting Intel's 6th generation Skylake and 7th generation Kaby Lake processors. If left unaddressed, the flaw could cause "unpredictable system behavior," Intel says.
The flaw has to do with the actual micro-architectures of both chip families. With that being the case, it could potentially affect everything from low-power mobile processors all the way up to high-end desktop CPUS, and everything in between. However, it does not seem to present a security issue of any kind.
Intel details the errata in its own documentation as such:
"Short Loops Which Use AH/BH/CH/DH Registers May Cause Unpredictable System Behaviour."
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 behaviour. This can only happen when both logical processors on the same physical processor are active."
Implication: "Due to this erratum, the system may experience unpredictable system behavior."
To be clear, the flaw does not only affect Debian and other Linux platforms, but all operating systems, including Windows. That said, we have not observed any obvious wonky behavior ourselves, nor is it clear what exact "complex micro-architectural conditions" might need to exist for this flaw to crop its unpredictable head.
Intel has already issued microcode updates for its Skylake and Kaby Lake processors, though it is up to motherboard makers to implement them into BIOS/UEFI updates.
If you're experiencing weird behavior since upgrading to Skylake or Kaby Lake, check your motherboard manufacturer for updated firmware. Alternately, you can disable Hyper-Threading if you think it is negatively affecting your system.