KVM: x86/mmu: Fix write-protection of PTs mapped by the TDP MMU

When the TDP MMU is write-protection GFNs for page table protection (as
opposed to for dirty logging, or due to the HVA not being writable), it
checks if the SPTE is already write-protected and if so skips modifying
the SPTE and the TLB flush.

This behavior is incorrect because it fails to check if the SPTE
is write-protected for page table protection, i.e. fails to check
that MMU-writable is '0'.  If the SPTE was write-protected for dirty
logging but not page table protection, the SPTE could locklessly be made
writable, and vCPUs could still be running with writable mappings cached
in their TLB.

Fix this by only skipping setting the SPTE if the SPTE is already
write-protected *and* MMU-writable is already clear.  Technically,
checking only MMU-writable would suffice; a SPTE cannot be writable
without MMU-writable being set.  But check both to be paranoid and
because it arguably yields more readable code.

Fixes: 46044f72 ("kvm: x86/mmu: Support write protection for nesting in tdp MMU")
Cc: stable@vger.kernel.org
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220113233020.3986005-2-dmatlack@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1442,12 +1442,12 @@ static bool write_protect_gfn(struct kvm *kvm, struct kvm_mmu_page *root,
 		    !is_last_spte(iter.old_spte, iter.level))
 			continue;
 
-		if (!is_writable_pte(iter.old_spte))
-			break;
-
 		new_spte = iter.old_spte &
 			~(PT_WRITABLE_MASK | shadow_mmu_writable_mask);
 
+		if (new_spte == iter.old_spte)
+			break;
+
 		tdp_mmu_set_spte(kvm, &iter, new_spte);
 		spte_set = true;
 	}