Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

[Borislav Petkov]

On Wed, Oct 06, 2021 at 03:15:51PM +0100, Andrew Cooper wrote:
> The case which goes wrong is this:
> 
> 1. Zen1 (or earlier) and Zen2 (or later) in a migration pool
> 2. Linux boots on Zen2, probes and finds the absence of X86_BUG_NULL_SEL
> 3. Linux is then migrated to Zen1
> 
> Linux is now running on a X86_BUG_NULL_SEL-impacted CPU while believing
> that the bug is fixed.
> 
> The only way to address the problem is to fully trust the "no longer
> affected" CPUID bit when virtualised, because in the above case it would
> be clear deliberately to indicate the fact "you might migrate to
> somewhere which really is affected".

Yap, makes sense.

Thanks for taking the time - that's what I was looking for.

Please add to the commit message of the next version.

-- 
Regards/Gruss,
    Boris.

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