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From:   Aaron Lewis <aaronlewis@google.com>
Date:   Wed, 16 Sep 2020 13:13:00 -0700
Message-ID: <CAAAPnDEoQhtXuiqHwUtrsL7codcToAVwaR=+qVczZrz6RCWe0A@mail.gmail.com>
Subject: Re: [PATCH v6 5/7] KVM: x86: VMX: Prevent MSR passthrough when MSR
 access is denied
To:     Alexander Graf <graf@amazon.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        KarimAllah Raslan <karahmed@amazon.de>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        kvm list <kvm@vger.kernel.org>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
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> >
> >> +
> >>   /*
> >>    * These 2 parameters are used to config the controls for Pause-Loop Exiting:
> >>    * ple_gap:    upper bound on the amount of time between two successive
> >> @@ -622,6 +642,41 @@ static inline bool report_flexpriority(void)
> >>          return flexpriority_enabled;
> >>   }
> >
> > One thing that seems to be missing is removing MSRs from the
> > permission bitmap or resetting the permission bitmap to its original
> > state before adding changes on top of it.  This would be needed on
> > subsequent calls to kvm_vm_ioctl_set_msr_filter().  When that happens
> > the original changes made by KVM_REQ_MSR_FILTER_CHANGED need to be
> > backed out before applying the new set.
>
> I'm not sure I follow. Subsequent calls to set_msr_filter() will invoke
> the "please reset the whole MSR passthrough bitmap to a consistent
> state" which will then reapply the in-kvm desired state through the
> bitmap and filter state on top on each of those.
>

Yes, you're correct.  I discovered this after the fact by adding a
test to the selftest I wrote for the deny list system which I revamped
to work for your filter system.  It proved the permission bitmaps are
in fact set as expected on subsequent calls.  You can disregard this
comment.

As a side note, I'm happy to share the test if you'd like. I also used
it to uncover an issue in the first commit of this series.