[PATCH v2 4/5] KVM: x86/mmu: Warn if PFN changes on shadow-present SPTE in shadow MMU

[Yan Zhao]

Warn if PFN changes on shadow-present SPTE in mmu_set_spte().

KVM should _never_ change the PFN of a shadow-present SPTE. In
mmu_set_spte(), there is a WARN_ON_ONCE() on pfn changes on shadow-present
SPTE in mmu_spte_update() to detect this condition. However, that
WARN_ON_ONCE() is not hittable since mmu_set_spte() invokes drop_spte()
earlier before mmu_spte_update(), which clears SPTE to a !shadow-present
state. So, before invoking drop_spte(), add a WARN_ON_ONCE() in
mmu_set_spte() to warn PFN change of a shadow-present SPTE.

For the spurious prefetch fault, only return RET_PF_SPURIOUS directly when
PFN is not changed. When PFN changes, fall through to follow the sequence
of drop_spte(), warn of PFN change, make_spte(), flush tlb, rmap_add().

Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 94c677f8cc05..607cbb19ea96 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2846,7 +2846,8 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
 	}
 
 	if (is_shadow_present_pte(*sptep)) {
-		if (prefetch && is_last_spte(*sptep, level))
+		if (prefetch && is_last_spte(*sptep, level) &&
+		    pfn == spte_to_pfn(*sptep))
 			return RET_PF_SPURIOUS;
 
 		/*
@@ -2860,7 +2861,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
 			child = spte_to_child_sp(pte);
 			drop_parent_pte(vcpu->kvm, child, sptep);
 			flush = true;
-		} else if (pfn != spte_to_pfn(*sptep)) {
+		} else if (WARN_ON_ONCE(pfn != spte_to_pfn(*sptep))) {
 			drop_spte(vcpu->kvm, sptep);
 			flush = true;
 		} else
-- 
2.43.2