while driving on freeway a puff of white smoke comes out the exhaust

Is it pure white smoke or does it have a bluish tinge? Bluish white smoke is likely burning oil. Pure white smoke might be steam. Check the oil and see if there is any coolant in the crankcase. White smoke might be water/coolant getting into the combustion chamber, signs of a leaky head gasket or worse a cracked head. Bluish white smoke is oil being burned in the combustion chamber. Do you get any misfiring at the time? If - again if - you are getting a misfire it might be a valve not opening/closing at the proper time and oil from the valvetrain is getting into the combustion chamber and being burned off. That situation could from bad lifters, bad valve lash adjusters, worn valve guides, etc. especially if it's a high mileage motor or had poor maintenance/lack of regular oil changes. If it were something like bad piston rings you'd have burning oil continuously, but more noticeable under load.

I have purchased a brand new X5 in india and driven only 500kms the same thing happened to me I immediately pulled over and call the BMW dealer they say its normal and related to un burnt gases, I am still not convinced. Does anyone have a realistic reason