Kramnik - Karjakin in the FIDE Candidates 2014, was 8...f5 correct? In the game between Kramnik and Karjakin shown below, the opening was a bit strange and the resulting position was odd. I am wondering, was 8...f5 correct? Was it a novelty of sorts or when did the game leave the established opening theory in this line? Thanks!

[Event "FIDE Candidates Tournament 2014"]
[Date "2014.03.14"]
[Round "2"]
[White "Kramnik, Vladimir"]
[Black "Karjakin, Sergey"]
[Result "1-0"]
[FEN ""]

1. d4 d5 2. c4 dxc4 3. e4 Nf6 4. e5 Nd5 5. Bxc4 Nb6 6. Bd3 Nc6 7. Be3 Nb4 8. Be4 f5 9. a3 fxe4 10. axb4 e6 11. Nc3 Bxb4 12. Qh5+ g6 13. Qg4 Bxc3+ 14. bxc3 Qd5 15. Ne2 Bd7 16. O-O Qc4 17. Ng3 Bc6 18. Ra5 O-O-O 19. Rc5 Qb3 20. c4 Kb8 21. Qxe6 Rde8 22. Qh3 Nxc4 23. Rxc6 bxc6 24. Nxe4 Nb6 25. Nc5 Qd5 26. Rc1 Ka8 27. Na6 Kb7 28. Nb4 Qf7 29. Qg4 Nd5 30. Nxc6 Re6 31. Na5+ Ka8 32. Qe4 Rb6 33. g4 h5 34. Rc5 Rd8 35. Nc6 Rxc6 36. Rxc6 hxg4 37. Rf6 Qh7 38. Bg5 Qg8 39. Rxg6 1-0
8...f5 is no novelty. It is a well known line and has been played successfully before by strong players like Adams, Salov and Karjakin himself. 

8...f5 is absolutely the correct move in the position, because it practically forces White to either surrender his strong central pawn on e5 or give up his strong light squared bishop for Black's knight on b4. I demonstrate a line below showing what happens if White tries to avoid both these possibilities with 9. Bf3. 

   [FEN "r1bqkb1r/ppp1p1pp/1n6/4Pp2/1n1PB3/4B3/PP3PPP/RN1QK1NR w KQkq f6 0 9"]

   1. Bf3 f4! 2. Bxf4 Qxd4 (2... Bf5!?) 3. Qxd4 Nc2+ 4. Kf1 Nxd4=


Thus, in the above position, White is practically forced to concede one of the two advantages. 

Normally, White players preferred to play 9.exf6 for "obvious" reasons like the light squared bishop is stronger than the knight on b4 and is White's most valuable piece in launching a kingside attack.

Thus, in this position the move 9. a3!? must have come as a complete shock to Karjakin. It has never been played before at Grandmaster level and was only played once before in Janczarski,M (2335)-Bartel,M (2273) Warsaw 2013 (source here). 

   [FEN "r1bqkb1r/ppp1p1pp/1n6/4Pp2/1n1PB3/4B3/PP3PPP/RN1QK1NR w KQkq f6 0 9"]

   9. a3!?


Whether 9.a3!? is good or bad is a different question. It seems to me that after 

   [FEN "r1bqkb1r/ppp1p1pp/1n6/4Pp2/1n1PB3/4B3/PP3PPP/RN1QK1NR w KQkq f6 0 9"]

   9. a3 N4d5 10. Bf3 Nxe3 11. fxe3 g6 12. Ne2 Bh6!


Black is perfectly fine. 

Conclusion: 8...f5 is correct!