The Portuguese forward had not found the net since the middle of December but slammed two goals past the visitors in four minutes just before the break.
Real were cruising to victory in the second half when Ronaldo tried to spin away from the close attention of Malaga defender Patrick Mtiliga in his own half.
Real's win cut Barca's lead back to five points and moved them five clear of third-placed Valencia who were held 0-0 at Tenerife.
Real Mallorca are fourth with 34, ahead of Deportivo La Coruna on goal difference, after Mallorca's Borja Valero scored a disputed 80th-minute penalty to earn a 1-1 draw at Espanyol.
GUTI'S PASSES
At the Bernabeu, Hamburg-bound striker Ruud van Nistelrooy was given a warm send-off by the crowd before the start of Real's match.
Guti's return to favour greased the wheels in midfield and it was his pass that set in motion a flowing move which led to Kaka crossing low for Ronaldo to score in the 36th minute.
At the foot of the table, Xerez's change of coach failed to inspire them as they lost 2-1 to Osasuna and remain 20th, below the two clubs promoted with them from the second division last year, Real Zaragoza and Tenerife.
