Re: [PATCH 1/2] KVM: x86/mmu: Drop RWX=0 SPTEs during ept_sync_page()

[Sean Christopherson <seanjc@google.com>]

On Mon, May 16, 2022, David Matlack wrote:
> 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.

Yep.  The other thing that's semi-relevant and I've mentioned off-list at least
once is that our (Google's) old kernel has a different NX HugePage implementation
that _can_ result in RWX=0 SPTEs.  Unlike upstream, the internal NX HugePage
implementation shatters a huge page _after_ installing said huge page, whereas
upstream demotes the huge page before it's installed.  If shattering fails on huge
page that L1 created a huge page with just X permissions, KVM is left with a RWX=0
huge page.