Defending against a knife attack without a weapon of your own is a horrible prospect no matter what. It's extremely difficult when the attacker holds the knife in his lead hand, but even more so when the lead hand is used to shove you back while the rear hand repeatedly stabs. We call this the "jab and stab" attack. The repeated rear hand stabbing action is also called the "sewing machine" by some.
This attack is so difficult to deal with because the shoving hand makes it difficult to gain control of the knife. Additionally, if the attacker's head is covered by his shoving hand/arm it is almost impossible to hit him in the face without being stabbed. Here is a "good" solution: