linux-kernel - Re: [PATCH v2 22/28] KVM: VMX: Clear controls obsoleted by EPT at runtime, not setup

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Date:   Fri, 08 Jul 2022 09:55:45 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 22/28] KVM: VMX: Clear controls obsoleted by EPT at
 runtime, not setup

Sean Christopherson <seanjc@...gle.com> writes:

> On Thu, Jul 07, 2022, Jim Mattson wrote:
>> On Thu, Jul 7, 2022 at 12:30 PM Sean Christopherson <seanjc@...gle.com> wrote:
>> >
>> > On Thu, Jul 07, 2022, Vitaly Kuznetsov wrote:
>> > > Jim Mattson <jmattson@...gle.com> writes:
>> > >
>> > > > On Wed, Jun 29, 2022 at 8:07 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>> > > >>
>> > > >> From: Sean Christopherson <seanjc@...gle.com>
>> > > >>
>> > > >> Clear the CR3 and INVLPG interception controls at runtime based on
>> > > >> whether or not EPT is being _used_, as opposed to clearing the bits at
>> > > >> setup if EPT is _supported_ in hardware, and then restoring them when EPT
>> > > >> is not used.  Not mucking with the base config will allow using the base
>> > > >> config as the starting point for emulating the VMX capability MSRs.
>> > > >>
>> > > >> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>> > > >> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> > > > Nit: These controls aren't "obsoleted" by EPT; they're just no longer
>> > > > required.
>
> Actually, they're still required if unrestricted guest isn't supported.
>
>> > Isn't that the definition of "obsolete"?  They're "no longer in use" when KVM
>> > enables EPT.
>> 
>> There are still reasons to use them aside from shadow page table
>> maintenance. For example, malware analysis may be interested in
>> intercepting CR3 changes to track process context (and to
>> enable/disable costly monitoring). EPT doesn't render these events
>> "obsolete," because you can't intercept these events using EPT.
>
> Fair enough, I was using "EPT" in the "KVM is using EPT" sense.  But even that's
> wrong as KVM intercepts CR3 accesses when EPT is enabled, but unrestricted guest
> is disabled and the guest disables paging.
>
> Vitaly, since the CR3 fields are still technically "needed", maybe just be
> explicit?
>
>   KVM: VMX: Adjust CR3/INVPLG interception for EPT=y at runtime, not setup
>

Sounds good, adjusted!

-- 
Vitaly