lxd/instance/drivers: Check for unknown fields in migration header

Signed-off-by: Mark Bolton <mark.bolton@canonical.com>

lxd/instance/drivers/driver_qemu.go

@@ -6579,6 +6579,21 @@ func (d *qemu) MigrateSend(args instance.MigrateSendArgs) error {
 		return fmt.Errorf("Failed receiving migration offer response: %w", err)
 	}
 
+	// If we have unknown fields in the protobuf message, then we received
+	// unexpected data. This will only happen if we were sent a migration.MigrationControl
+	// instead of the expected migration.MigrationHeader. This implies that we have failed on
+	// the target before we were able to send the response header.
+	reflect := offerHeader.ProtoReflect()
+	unknownFields := reflect.GetUnknown()
+	if len(unknownFields) > 0 {
+		msg, err := shared.GetFirstUnknownField(unknownFields)
+		if err != nil {
+			return fmt.Errorf("Failed receiving migration offer response")
+		}
+
+		return fmt.Errorf(msg)
+	}
+
 	d.logger.Debug("Got migration offer response from target")
 
 	// Negotiated migration types.
@@ -7105,6 +7120,21 @@ func (d *qemu) MigrateReceive(args instance.MigrateReceiveArgs) error {
 		return fmt.Errorf("Failed receiving migration offer from source: %w", err)
 	}
 
+	// If we have unknown fields in the protobuf message, then we received
+	// unexpected data. This will only happen if we were sent a migration.MigrationControl
+	// instead of the expected migration.MigrationHeader. This implies that we have failed on
+	// the source before we were able to send the header.
+	reflect := offerHeader.ProtoReflect()
+	unknownFields := reflect.GetUnknown()
+	if len(unknownFields) > 0 {
+		msg, err := shared.GetFirstUnknownField(unknownFields)
+		if err != nil {
+			return fmt.Errorf("Failed receiving migration offer from source")
+		}
+
+		return fmt.Errorf(msg)
+	}
+
 	// When doing a cluster same-name move we cannot load the storage pool using the instance's volume DB
 	// record because it may be associated to the wrong cluster member. Instead we ascertain the pool to load
 	// using the instance's root disk device.