linux-kernel - Re: [PATCH 1/2] KVM: x86/mmu: Drop RWX=0 SPTEs during ept_sync

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Date:   Sat, 14 May 2022 00:55:58 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 1/2] KVM: x86/mmu: Drop RWX=0 SPTEs during ept_sync_page()

On Fri, May 13, 2022, David Matlack wrote:
> On Fri, May 13, 2022 at 12:50 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Drop SPTEs whose new protections will yield a RWX=0 SPTE, i.e. a SPTE
> > that is marked shadow-present but is not-present in the page tables.  If
> > EPT with execute-only support is in use by L1, KVM can create a RWX=0
> > SPTE can be created for an EPTE if the upper level combined permissions
> > are R (or RW) and the leaf EPTE is changed from R (or RW) to X.
> 
> For some reason I found this sentence hard to read.

Heh, probably because "KVM can create a RWX=0 SPTE can be created" is nonsensical.
I botched a late edit to the changelog...

> What about this:
> 
>   When shadowing EPT and NX HugePages is enabled, if the guest changes

This doesn' thave anything to do with NX HugePages, it's an execute-only specific
bug where L1 can create a gPTE that is !READABLE but is considered PRESENT because
it is EXECUTABLE.  If the upper level protections are R or RW, the resulting
protections for the entire translation are RWX=0.  All of sync_page()'s existing
checks filter out only !PRESENT gPTE, because without execute-only, all upper
levels are guaranteed to be at least READABLE.