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

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Message-ID: <Ytshp+D+IT8eaevH@google.com>
Date:   Fri, 22 Jul 2022 22:16:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Kalra, Ashish" <Ashish.Kalra@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
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Subject: Re: [PATCH Part2 v6 05/49] x86/sev: Add RMP entry lookup helpers

On Fri, Jul 22, 2022, Borislav Petkov wrote:
> 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.

Yar, just wanted to be make sure we're all on the same page, I wasn't entirely
sure what was get nacked :-)

> > > 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.

Yeah, but either we get AMD to give us an architectural layout or we'll have to
eat that cost at some point in the future.

> 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.

Heh, you're definitely more optimistic than me.  I can just see something truly
ridiculous happening like moving the page size bit and then getting weird behavior
only when KVM happens to need the page size for some edge case.

Anyways, it's not a sticking point, and I certainly am not volunteering to
maintain the FMS list...