invoices: fail HTLCs that target the wrong type of invoice

invoices/resolution_result.go

@@ -110,6 +110,10 @@ const (
 	// payment.
 	ResultMppInProgress
 
+	// ResultHtlcInvoiceTypeMismatch is returned when an AMP HTLC targets a
+	// non-AMP invoice and vice versa.
+	ResultHtlcInvoiceTypeMismatch
+
 	// ResultAmpError is returned when we receive invalid AMP parameters.
 	ResultAmpError
 
@@ -176,6 +180,9 @@ func (f FailResolutionResult) FailureString() string {
 	case ResultMppInProgress:
 		return "mpp reception in progress"
 
+	case ResultHtlcInvoiceTypeMismatch:
+		return "htlc invoice type mismatch"
+
 	case ResultAmpError:
 		return "invalid amp parameters"
 

invoices/update.go

@@ -125,6 +125,21 @@ func updateMpp(ctx *invoiceUpdateCtx,
 	inv *channeldb.Invoice) (*channeldb.InvoiceUpdateDesc,
 	HtlcResolution, error) {
 
+	// Reject HTLCs to AMP invoices if they are missing an AMP payload, and
+	// HTLCs to MPP invoices if they have an AMP payload.
+	switch {
+
+	case inv.Terms.Features.RequiresFeature(lnwire.AMPRequired) &&
+		ctx.amp == nil:
+
+		return nil, ctx.failRes(ResultHtlcInvoiceTypeMismatch), nil
+
+	case !inv.Terms.Features.RequiresFeature(lnwire.AMPRequired) &&
+		ctx.amp != nil:
+
+		return nil, ctx.failRes(ResultHtlcInvoiceTypeMismatch), nil
+	}
+
 	setID := ctx.setID()
 
 	// Start building the accept descriptor.