The OpenBSD project announced today plans to disable support for Intel CPU hyper-threading due to security concerns regarding the theoretical threat of more "Spectre-class bugs."

Hyper-threading (HT) is Intel's proprietary implementation of Simultaneous Multithreading (SMT), a technology that enables multiple computing threads to run on each CPU core.

The feature has been added to most Intel CPUs released since 2002 and has come enabled by default, with Intel citing its performance boost as the main reason for its inclusion.

Intel HT susceptible to more timing attacks

But today, Mark Kettenis of the OpenBSD project, said the OpenBSD team was removing support for Intel HT because, by design, this technology just opens the door for more timing attacks.

Timing attacks are a class of cryptographic attacks through which a third-party observer can deduce the content of encrypted data by recording and analyzing the time taken to execute cryptographic algorithms.

The recently disclosed Meltdown and Spectre CPU vulnerabilities, along with their many variations [1, 2, 3] are all timing attacks at their core.

"[Intel HT] can make cache timing attacks a lot easier and we strongly suspect that this will make several spectre-class bugs exploitable," Kettenis said.

New sysctl available for some OpenBSD versions

The OpenBSD team is now stepping in to provide a new setting to disable HT support because "many modern machines no longer provide the ability to disable hyper-threading in the BIOS setup."

"This can [now] be controlled through a new hw.smt sysctl," Kettenis said. "For now this only works on Intel CPUs when running OpenBSD/amd64. But we're planning to extend this feature to CPUs from other vendors and other hardware architectures."

Kettenis says that SMT doesn't really have a positive effect on performance, as Intel and other CPU vendors have advertised, and the change shouldn't bring a big performance hit.

According to the infosec community, the reason why OpenBSD disabled Intel HT is related to a research paper detailing a new vulnerability named TLBleed, which will be presented at the Black Hat security conference that will be held in Las Vegas in August.

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