linux-kernel - Re: [PATCH Part2 v6 05/49] x86/sev: Add RMP entry lookup helpers

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Message-ID: <Ytr5ndnlOQvqWdPP@zn.tnic>
Date:   Fri, 22 Jul 2022 21:25:17 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Kalra, Ashish" <Ashish.Kalra@....com>,
        Dave Hansen <dave.hansen@...el.com>,
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Subject: Re: [PATCH Part2 v6 05/49] x86/sev: Add RMP entry lookup helpers

On Fri, Jul 22, 2022 at 07:04:23PM +0000, Sean Christopherson wrote:
> I disagree.  Running an old kernel on new hardware with a different RMP layout
> should refuse to use SNP, not read/write garbage and likely corrupt the RMP and/or
> host memory.

See my example below.

> And IMO, hiding the non-architectural RMP format in SNP-specific code so that we
> don't have to churn a bunch of call sites that don't _need_ access to the raw RMP
> format is a good idea regardless of whether we want to be optimistic or pessimistic
> about future formats.

I don't think I ever objected to that.

> > This is nothing else but normal CPU enablement work - it should be done
> > when it is really needed.
> > 

<--- this here.

> > Because the opposite can happen: you can add a model check which
> > excludes future model X, future model X comes along but does *not*
> > change the RMP format and then you're going to have to relax that model
> > check again to fix SNP on the new model X.

So constantly adding new models to a list which support a certain
version of the RMP format doesn't scale either.

If you corrupt the RMP because your kernel is old, you'll crash and burn
very visibly so that you'll be forced to have to look for an updated
kernel regardless.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette