KVM: x86/mmu: WARN if old _or_ new SPTE is REMOVED in non-atomic path

WARN if the new_spte being set by __tdp_mmu_set_spte() is a REMOVED_SPTE,
which is called out by the comment as being disallowed but not actually
checked.  Keep the WARN on the old_spte as well, because overwriting a
REMOVED_SPTE in the non-atomic path is also disallowed (as evidence by
lack of splats with the existing WARN).

Fixes: 08f07c80 ("KVM: x86/mmu: Flush TLBs after zap in TDP MMU PF handler")
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220226001546.360188-12-seanjc@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -626,13 +626,13 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
 	lockdep_assert_held_write(&kvm->mmu_lock);
 
 	/*
-	 * No thread should be using this function to set SPTEs to the
+	 * No thread should be using this function to set SPTEs to or from the
 	 * temporary removed SPTE value.
 	 * If operating under the MMU lock in read mode, tdp_mmu_set_spte_atomic
 	 * should be used. If operating under the MMU lock in write mode, the
 	 * use of the removed SPTE should not be necessary.
 	 */
-	WARN_ON(is_removed_spte(iter->old_spte));
+	WARN_ON(is_removed_spte(iter->old_spte) || is_removed_spte(new_spte));
 
 	kvm_tdp_mmu_write_spte(iter->sptep, new_spte);