Isco halted Atletico early scare at the Vicente Calderon to ensure the Galacticos book a date with Juvetnus in a repeat of the 1998 UCL Final.
Saul Niguez scored a header in the 14th minutes before Griezman converted from the spot in the 16th minutes after Fernando Torres was brought down in the 18.
Vicente Calderon was roaring for a comeback of the 3-0 hauling at the Bernabeu until Benzema wriggled out of a seemingly impossible situation surrounded by three Atletico defenders and crossed for Toni Kroos, whose initial effort was blocked by Oblak before Isco shot home the rebound.
The goal took the sting out of Atletico who, from the cusp of an improbable leveller, suddenly found themselves needing to score five times on the night to give themselves a chance to advance.
The hosts had been indebted to goalkeeper Jan Oblak for keeping their slim hopes alive in the early stages when the Czech made a brilliant one-handed save to deny Casemiro.
Real Madrid will now face Italian Champions in a potentially lightening final at Cardiff on June 3.