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SubjectRe: [tip: x86/urgent] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

> >> x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h
> >>
> >> There have been reports of RDRAND issues after resuming from suspend on
> >> some AMD family 15h and family 16h systems. This issue stems from a BIOS
> >> not performing the proper steps during resume to ensure RDRAND continues
> >> to function properly.
> >
> > There are quite a few unanswered questions here.
> >
> > a) Is there/should there be CVE for this?
> >
> > b) Can we perform proper steps in kernel, thus making RDRAND usable
> > even when BIOS is buggy?
> >
> The kernel should at least be able to set its internal "CPUID" bit, visible
> through /proc/cpuinfo.

Actually, with hindsight I see two possible improvements here:

1) Not having enabled s2ram in config does not mean machine was not
suspended/resumed, then new kernel executed via kexec.

2) We really can continue using the RDRAND: we know how it fails
(constant pattern) so we can check for the failure in kernel, and can
continue to use it... It will certainly work until first suspend, and
there's good chance it will work after that, too. (We still need to
prevent userspace from using it).

Best regards,
(cesky, pictures)
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