• The Linux mitigation for Retbleed is invasive at nearly two thousand lines of new code and nearly 400 lines removed, across dozens of files.
• In the Retbleed whitepaper by ETH Zurich COMSEC researchers, they characterized the mitigations as result in 14~39% overhead.
• The Retbleed mitigations are some of the most performance-wrenching mitigations seen by the author of this article in a few years going back to the early Spectre/Meltdown days.
• Researchers and the hardware vendors believe Retbleed affects AMD Zen 1, Zen 1+, and Zen 2 processors -- but not the latest Zen 3 CPUs.
• When trying out the patched Linux kernel on Zen 3 hardware, there are no Retbleed mitigations applied. Over on the Intel side, Core 6th Generation through Core 8th Generation CPUs are impacted -- Skylake through Coffee Lake.
• The Retbleed mitigations were merged to Linux 5.19 Git as of yesterday and working their way currently to the various stable/supported Linux series right now. Those Linux stable point releases will be out shortly.
• When running on a patched Linux kernel, the Retbleed mitigations are applied automatically by default on the affected AMD/Intel CPUs as mentioned.