[PATCH 5.18 0558/1095] KVM: x86/mmu: Drop RWX=0 SPTEs during ept_sync_page()

[Greg Kroah-Hartman]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

[ Upstream commit 9fb3565743d58352f00964bf47213b88aff4bb82 ]

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.  However, if EPT with execute-only support is in use by
L1, KVM can create an SPTE that is shadow-present but guest-inaccessible
(RWX=0) if the upper level combined permissions are R (or RW) and
the leaf EPTE is changed from R (or RW) to X.  Because the EPTE is
considered present when viewed in isolation, and no reserved bits are set,
FNAME(prefetch_invalid_gpte) will consider the GPTE valid, and cause a
not-present SPTE to be created.

The SPTE is "correct": the guest translation is inaccessible because
the combined protections of all levels yield RWX=0, and KVM will just
redirect any vmexits to the guest.  If EPT A/D bits are disabled, KVM
can mistake the SPTE for an access-tracked SPTE, but again such confusion
isn't fatal, as the "saved" protections are also RWX=0.  However,
creating a useless SPTE in general means that KVM messed up something,
even if this particular goof didn't manifest as a functional bug.
So, drop SPTEs whose new protections will yield a RWX=0 SPTE, and
add a WARN in make_spte() to detect creation of SPTEs that will
result in RWX=0 protections.

Fixes: d95c55687e11 ("kvm: mmu: track read permission explicitly for shadow EPT page tables")
Cc: David Matlack <dmatlack@xxxxxxxxxx>
Cc: Ben Gardon <bgardon@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-Id: <20220513195000.99371-2-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/paging_tmpl.h | 9 ++++++++-
 arch/x86/kvm/mmu/spte.c        | 2 ++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index beb3ce8d94eb..43f6a882615f 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -1044,7 +1044,14 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 		if (sync_mmio_spte(vcpu, &sp->spt[i], gfn, pte_access))
 			continue;
 
-		if (gfn != sp->gfns[i]) {
+		/*
+		 * Drop the SPTE if the new protections would result in a RWX=0
+		 * SPTE or if the gfn is changing.  The RWX=0 case only affects
+		 * EPT with execute-only support, i.e. EPT without an effective
+		 * "present" bit, as all other paging modes will create a
+		 * read-only SPTE if pte_access is zero.
+		 */
+		if ((!pte_access && !shadow_present_mask) || gfn != sp->gfns[i]) {
 			drop_spte(vcpu->kvm, &sp->spt[i]);
 			flush = true;
 			continue;
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index e5c0b6db6f2c..8223a80802e7 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -128,6 +128,8 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	u64 spte = SPTE_MMU_PRESENT_MASK;
 	bool wrprot = false;
 
+	WARN_ON_ONCE(!pte_access && !shadow_present_mask);
+
 	if (sp->role.ad_disabled)
 		spte |= SPTE_TDP_AD_DISABLED_MASK;
 	else if (kvm_mmu_page_ad_need_write_protect(sp))
-- 
2.35.1