Intel’s Spectre variant 4 patch impacts CPU performance

Intel recently announced their fix for Spectre variant 4 attack that would significantly decrease CPU performance. While working on this fix, Intel anticipated some performance questions that were around the combined software and firmware microcode updates that helps mitigate Spectre variant 4.

As discovered by Jann Horn of Google Project Zero and Ken Johnson of Microsoft Spectre variant 4 is a speculative store bypass. Speculative bypass is a variant 4 vulnerability, with this an attacker can leverage variant 4 to read older memory values in a CPU’s stack or other memory locations. This vulnerability allows less privileged code to read arbitrary privileged data and run older commands speculatively. Intel call its mitigation of this Spectre attack as Speculative Store Bypass Disable (SSBD). Intel delivers this as a microcode update to appliance manufacturers, operating system vendors and other ecosystem partners.

According to Intel, this patch will be ‘off” by default but if enabled Intel has observed an impact on the the performance from 2%-8% approximately but this would all depend on the overall scores from benchmarks such as SPECint, SYSmark® 2014 SE, and more.

Back in January, Intel was less forthcoming in communicating about the CPU performance impact caused by Spectre variant 2 mitigation. They just waved-off such concerns with claiming that the performance would vary depending on the workload. However, Google pushed back stating the impact was severe and ended-up developing its very own Retpoline software alternative.

Recently, Intel tested the impact of SSBD running it on an unspecified Intel reference hardware and 8th Gen Intel Core desktop microprocessor. The results on the performance impact of the overall score are as follows:

• SYSmark 2014 SE: 4%
• SPECint_rate_base2006 (n copy): 2%
• SPECint_rate_base2006 (1 copy): 8%

These benchmark results are similar even on a Skylake architecture Xeon processor.

Intel has clearly stated that this mitigation will be set to ‘off’ by default giving customers a choice to enable it. This is because Intel speculates that most industry software partners will go with the default option to avoid overall performance degradation.

They also noted that SSBD would add an extra layer of protection to the hardware of consumers and original equipment manufacturers to prevent the Speculative Store Bypass from occurring. They also stated that the existing browser mitigations against Spectre variant 1 will help to an extend in mitigating variant 4.

You can know more about the latest security updates on Intel products form Intel security center.

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