AMD Publishes Latest Linux Patch To Toggle Predictive Store Forwarding (PSF)

Written by Michael Larabel in AMD on 18 May 2021 at 03:00 AM EDT. 5 Comments
It's been a month and a half since AMD published a security analysis of their new Zen 3 "Predictive Store Forwarding" feature that while helping performance could theoretically lead to a new side-channel attack. While they published a Linux patch to allow disabling PSF if desired for increased security, to this day they remain in the works and have yet to be mainlined.

Days after that security white-paper was published AMD began posting the Linux patches to disable PSF optionally for users concerned around the possible security implications of Predictive Store Forwarding. As of Monday, there have now been six rounds of these patches for simply offering up the PSF user control.

The revisions consolidated the work, renamed PSF to a "control" rather than a "mitigation", fixed up some MSR handling, and now has been revved the sixth time. With this latest revision it shifts around the code, simplifies the PSF "mitigation" (but I thought a few rounds ago was changed to not be called a mitigation...), and changes the kernel parameter to be called predictive_store_fwd_disable= with accepted values of on/off (yes, so "on" means to disable).

With the latest patch, On kernels with the latest patch, booting with predictive_store_fwd_disable=on will disable Predictive Store Forwarding for AMD Zen 3 processors (Ryzen 5000 and EPYC 7003 processors).

Given the timing of this patch being past the Linux 5.13 merge window, it remains to be seen if they will try to mainline it this cycle as a security fix or rather just punt it off for Linux 5.14 as a feature during the next merge window, assuming no further rounds of this patch are needed for providing this PSF control that would further delay its mainlining.
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