Sociedad defender Aritz Elustondo headed in his first goal for the Basque club 36 seconds into the match.
But James Rodriguez headed the hosts level in the third minute, and Madrid never looked back at its Santiago Bernabeu Stadium.
Sergio Ramos scored in the 37th and Benzema made it 3-1 in the 53rd before he struck an exquisite second goal in the 76th.
Madrid opened a four-point gap over Barcelona before it hosts Villarreal on Sunday. Madrid will play its game in hand against Sevilla on Wednesday.