After recommending customers not use its microcode fix for Broadwell and Haswell chips, Intel has issued a new microcode update for Skylake processors that gives operating systems the ability to protect against the Spectre flaw revealed earlier this year.
The Spectre attacks work by persuading the processor's branch predictor into making a specific bad prediction. This bad prediction can then be used to infer the value of data stored in memory, in turn giving an attacker information that they shouldn't otherwise have. The microcode update is designed to give operating systems greater control over the branch predictor, enabling them to prevent one process from influencing the predictions made in another process.
Intel's first microcode update, developed late last year, was included in system firmware updates for machines with Broadwell, Haswell, Skylake, Kaby Lake, and Coffee Lake processors. It was subsequently discovered that the update was causing systems to crash and reboot; initially, only Broadwell and Haswell systems were confirmed to be affected, but subsequently it was determined that Skylake, Kaby Lake, and Coffee Lake systems were rebooting too. In response, people were advised not to use the new microcode, and operating system features that leveraged the new capabilities were disabled.