Time is of the essence! Don't keep running it or you could end up seizing the motor. I purchased just today at lunchtime a beautiful 2015 D105 with a seized engine. Owner indicated the problems started with the white smoke. I fully expect to find a blown head gasket right between the cylinder and the push rod chamber.
The good news for you is you can pull the head pretty easily and inspect/replace the head gasket now before a more catastrophic event. There are a number of YT videos to help. If you pull the head, be mindful of which pushrod goes where. One is aluminum(lighter colored) and one is steel(darker colored). If it warrants, it's a good time to lap the valves and adjust clearances when assembling(if you have a valve spring compressor). Otherwise a local shop could do a valve job for you hopefully for a reasonable price.