linux-kernel - Re: [RFC PATCH 08/35] KVM: SVM: Prevent debugging under SEV-ES

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Message-ID: <20200916164923.GC10227@sjchrist-ice>
Date:   Wed, 16 Sep 2020 09:49:23 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     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>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        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: [RFC PATCH 08/35] KVM: SVM: Prevent debugging under SEV-ES

On Wed, Sep 16, 2020 at 11:38:38AM -0500, Tom Lendacky wrote:
> 
> 
> On 9/16/20 11:02 AM, Sean Christopherson wrote:
> > On Wed, Sep 16, 2020 at 10:11:10AM -0500, Tom Lendacky wrote:
> >> On 9/15/20 3:13 PM, Tom Lendacky wrote:
> >>> On 9/15/20 11:30 AM, Sean Christopherson wrote:
> >>>> I don't quite follow the "doesn't mean debugging can't be done in the future".
> >>>> Does that imply that debugging could be supported for SEV-ES guests, even if
> >>>> they have an encrypted VMSA?
> >>>
> >>> Almost anything can be done with software. It would require a lot of
> >>> hypervisor and guest code and changes to the GHCB spec, etc. So given
> >>> that, probably just the check for arch.guest_state_protected is enough for
> >>> now. I'll just need to be sure none of the debugging paths can be taken
> >>> before the VMSA is encrypted.
> >>
> >> So I don't think there's any guarantee that the KVM_SET_GUEST_DEBUG ioctl
> >> couldn't be called before the VMSA is encrypted, meaning I can't check the
> >> arch.guest_state_protected bit for that call. So if we really want to get
> >> rid of the allow_debug() op, I'd need some other way to indicate that this
> >> is an SEV-ES / protected state guest.
> > 
> > Would anything break if KVM "speculatively" set guest_state_protected before
> > LAUNCH_UPDATE_VMSA?  E.g. does KVM need to emulate before LAUNCH_UPDATE_VMSA?
> 
> Yes, the way the code is set up, the guest state (VMSA) is initialized in
> the same way it is today (mostly) and that state is encrypted by the
> LAUNCH_UPDATE_VMSA call. I check the guest_state_protected bit to decide
> on whether to direct the updates to the real VMSA (before it's encrypted)
> or the GHCB (that's the get_vmsa() function from patch #5).

Ah, gotcha.  Would it work to set guest_state_protected[*] from time zero,
and move vmsa_encrypted to struct vcpu_svm?  I.e. keep vmsa_encrypted, but
use it only for guiding get_vmsa() and related behavior.