Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220513195000.99371-1-seanjc@google.com> <20220513195000.99371-2-seanjc@google.com>
 <CALzav=d36gccJv345Phdr0AJx9=6=TP=iZ60dscgQr64Rq4Kew@mail.gmail.com> <Yn7+HrYbXhror09V@google.com>
In-Reply-To: <Yn7+HrYbXhror09V@google.com>
From:   David Matlack <dmatlack@google.com>
Date:   Mon, 16 May 2022 15:22:16 -0700
Message-ID: <CALzav=eogd=sNaPOSW3_9hLSZ6pXe5eakHLr5vm8bUiwQrmA6Q@mail.gmail.com>
Subject: Re: [PATCH 1/2] KVM: x86/mmu: Drop RWX=0 SPTEs during ept_sync_page()
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ben Gardon <bgardon@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, May 13, 2022 at 5:56 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, May 13, 2022, David Matlack wrote:
> > On Fri, May 13, 2022 at 12:50 PM Sean Christopherson <seanjc@google.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.

I see what you mean, thanks.

And I also recall now you mentioned (off-list) that the NX HugePage
scenario isn't possible because KVM does not let huge pages go unsync.