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Message-ID: <Yn7+HrYbXhror09V@google.com>
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.
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