Foward-fill dataframe based on mask. Fill with last valid value

Question

I have a dataframe like the following:

index,col1,col2
1,NaN,NaN
2,NaN,NaN
3,NaN,20
4,NaN,21
5,10,22
6,11,23
7,12,24
8,13,NaN
9,NaN,NaN

And a