It may be legal to make a U-turn in a tunnel.

As long as traffic signs or road markings do not indicate otherwise, it is generally not prohibited to turn around in a tunnel.

In this tunnel, there is traffic in both directions separated by a warning line. Therefore, it's allowed to both overtake and make turns here.

But remember that you should always follow the basic rule of traffic: be cautious, attentive, and considerate. You must be completely sure that you can complete the turn without any risk to yourself or causing an obstacle for other drivers.