linux-kernel - Re: [PATCH] KVM: SVM: Do not terminate SEV-ES guests on GHCB validation failure

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Date:   Wed, 21 Jul 2021 20:09:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Peter Gonda <pgonda@...gle.com>,
        kvm list <kvm@...r.kernel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH] KVM: SVM: Do not terminate SEV-ES guests on GHCB
 validation failure

On Wed, Jul 21, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
> > On Thu, May 20, 2021, Tom Lendacky wrote:
> >> On 5/20/21 2:16 PM, Sean Christopherson wrote:
> >> > On Mon, May 17, 2021, Tom Lendacky wrote:
> >> >> On 5/14/21 6:06 PM, Peter Gonda wrote:
> >> >>> On Fri, May 14, 2021 at 1:22 PM Tom Lendacky <thomas.lendacky@....com> wrote:
> >> >>>>
> >> >>>> Currently, an SEV-ES guest is terminated if the validation of the VMGEXIT
> >> >>>> exit code and parameters fail. Since the VMGEXIT instruction can be issued
> >> >>>> from userspace, even though userspace (likely) can't update the GHCB,
> >> >>>> don't allow userspace to be able to kill the guest.
> >> >>>>
> >> >>>> Return a #GP request through the GHCB when validation fails, rather than
> >> >>>> terminating the guest.
> >> >>>
> >> >>> Is this a gap in the spec? I don't see anything that details what
> >> >>> should happen if the correct fields for NAE are not set in the first
> >> >>> couple paragraphs of section 4 'GHCB Protocol'.
> >> >>
> >> >> No, I don't think the spec needs to spell out everything like this. The
> >> >> hypervisor is free to determine its course of action in this case.
> >> > 
> >> > The hypervisor can decide whether to inject/return an error or kill the guest,
> >> > but what errors can be returned and how they're returned absolutely needs to be
> >> > ABI between guest and host, and to make the ABI vendor agnostic the GHCB spec
> >> > is the logical place to define said ABI.
> >> 
> >> For now, that is all we have for versions 1 and 2 of the spec. We can
> >> certainly extend it in future versions if that is desired.
> >> 
> >> I would suggest starting a thread on what we would like to see in the next
> >> version of the GHCB spec on the amd-sev-snp mailing list:
> >> 
> >> 	amd-sev-snp@...ts.suse.com
> >
> > Will do, but in the meantime, I don't think we should merge a fix of any kind
> > until there is consensus on what the VMM behavior will be.  IMO, fixing this in
> > upstream is not urgent; I highly doubt anyone is deploying SEV-ES in production
> > using a bleeding edge KVM.
> 
> Sorry for resurrecting this old thread but were there any deveopments
> here? I may have missed something but last time I've checked a single
> "rep; vmmcall" from userspace was still crashing the guest.

I don't think it went anywhere, I completely forgot about this.  I'll bump this
back to the top of my todo list, unless someone else wants the honors :-)

> The issue, however, doesn't seem to reproduce with Vmware ESXi which probably
> means they're just skipping the instruction and not even injecting #GP
> (AFAIR, I don't have an environment to re-test handy).