Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	David Matlack <dmatlack@google.com>,
	Mingwei Zhang <mizhang@google.com>,
	Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting SPTE
Date: Tue, 9 Aug 2022 15:05:04 +0000	[thread overview]
Message-ID: <YvJ3oOSMr37AciAT@google.com> (raw)
In-Reply-To: <c976a6e6-963e-d076-053b-15e381c3800a@redhat.com>

On Tue, Aug 09, 2022, Paolo Bonzini wrote:
> On 8/9/22 16:44, Sean Christopherson wrote:
> > On Tue, Aug 09, 2022, Paolo Bonzini wrote:
> > > and (4) is definitely ordered after (1) thanks to the READ_ONCE hidden
> > > within (3) and the data dependency from old_spte to sp.
> > 
> > Yes, I think that's correct.  Callers must verify the SPTE is present before getting
> > the associated child shadow page.  KVM does have instances where a shadow page is
> > retrieved from the SPTE _pointer_, but that's the parent shadow page, i.e. isn't
> > guarded by the SPTE being present.
> > 
> > 	struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(iter->sptep));
> > 
> > Something like this is as a separate patch?
> 
> Would you resubmit without the memory barriers then?

Ya.

> > diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> > index f0af385c56e0..9d982ccf4567 100644
> > --- a/arch/x86/kvm/mmu/tdp_iter.h
> > +++ b/arch/x86/kvm/mmu/tdp_iter.h
> > @@ -13,6 +13,12 @@
> >    * to be zapped while holding mmu_lock for read, and to allow TLB flushes to be
> >    * batched without having to collect the list of zapped SPs.  Flows that can
> >    * remove SPs must service pending TLB flushes prior to dropping RCU protection.
> > + *
> > + * The READ_ONCE() ensures that, if the SPTE points at a child shadow page, all
> > + * fields in struct kvm_mmu_page will be read after the caller observes the
> > + * present SPTE (KVM must check that the SPTE is present before following the
> > + * SPTE's pfn to its associated shadow page).  Pairs with the implicit memory
> 
> I guess you mean both the shadow page table itself and the struct
> kvm_mmu_page?

Yes.  It's a bug if KVM ever consumes a SPTE's pfn (read the next level of SPTEs
or pfn_to_page() to get kvm_mmu_page) without first checking that the SPTE is
MMU-present.  

> Or do you think to_shadow_page() should have a smp_rmb()?

I believe we're ok.  If any SP fields are toggled after the SP is marked present,
then we'd need separate barriers, e.g. similar to is_unsync_root() +
mmu_try_to_unsync_pages() (ignoring that we've since added a spinlock in the "try
to unsync" path).