linux-kernel - Re: [PATCH v6 08/42] x86/sev-es: initialize sev

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Message-ID: <YXGnxs0hV3IKGDwP@work-vm>
Date:   Thu, 21 Oct 2021 18:47:50 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Michael Roth <michael.roth@....com>,
        Brijesh Singh <brijesh.singh@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features
 within #VC handler

* Borislav Petkov (bp@...en8.de) wrote:
> On Thu, Oct 21, 2021 at 06:12:53PM +0100, Dr. David Alan Gilbert wrote:
> > OK, so that bit is 8...21 Eax ext2eax bit 6 page 1-109
> > 
> > then 2.1.5.3 CPUID policy enforcement shows 8...21 EAX as
> > 'bitmask'
> > 'bits set in the GuestVal must also be set in HostVal.
> > This is often applied to feature fields where each bit indicates
> > support for a feature'
> > 
> > So that's right isn't it?
> 
> Yap, AFAIRC, it would fail the check if:
> 
> (GuestVal & HostVal) != GuestVal
> 
> and GuestVal is "the CPUID result value created by the hypervisor that
> it wants to give to the guest". Let's say it clears bit 6 there.
                                               ^^^^^^^

> Then HostVal comes in which is "the actual CPUID result value specified
> in this PPR" and there the guest catches the HV lying its *ss off.
> 
> :-)

Hang on, I think it's perfectly fine for it to clear that bit - it just
gets caught if it *sets* it (i.e. claims to be a chip unaffected by the
bug).

i.e. if guestval=0 then (GustVal & whatever) == GuestVal
  fine

?

Dave

> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK