Rust’s goal, his first of the season, put Pittsburgh ahead in the second period.
Hornqvist opened the scoring on a power play at 7:18 of the second with his ninth of the year, assisted by Phil Kessel and Evgeni Malkin.
Montreal defenseman P.K. Subban tied the score with 9:21 left in the second period with his second of the season — and first in 34 games — on a one-timer from the point. Andrei Markov and Tomas Plekanec earned assists.
Rust put the Penguins in front for good, beating Mike Condon top shelf on a breakaway with his second career goal less than four minutes later.
Fehr added a short-handed empty-netter with 10 seconds left in regulation for Pittsburgh, who is 3-1-2 in its last six games.
