KVM: x86/tdp_mmu: WARN if PFN changes for spurious faults

Add a WARN() to assert that KVM does _not_ change the PFN of a
shadow-present SPTE during spurious fault handling.

KVM should _never_ change the PFN of a shadow-present SPTE and TDP MMU
already BUG()s on this. However, spurious faults just return early before
the existing BUG() could be hit.

Suggested-by: Sean Christopherson <[email protected]>
Signed-off-by: Yan Zhao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: yanzhao56

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1155,8 +1155,10 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
 
 	if (is_shadow_present_pte(iter->old_spte) &&
 	    (fault->prefetch || is_access_allowed(fault, iter->old_spte)) &&
-	    is_last_spte(iter->old_spte, iter->level))
+	    is_last_spte(iter->old_spte, iter->level)) {
+		WARN_ON_ONCE(fault->pfn != spte_to_pfn(iter->old_spte));
 		return RET_PF_SPURIOUS;
+	}
 
 	if (unlikely(!fault->slot))
 		new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL);