How feasible is it to pursue PhD in mathematics for a computer science undergrad?
Any mathematics PhD program will require math coursework in abstract mathematics, which you likely have none of.

You will need enough real analysis to cover all of baby rudin, and a full year-long course in algebra. That's the bare basics for surviving first year graduate level courses for the first year exams.

It is completely feasible if you take another year or two and get a mathematics degree. You might want to consider getting courses in real analysis/algebra/complex analysis/logic and maybe a course in probability and then applying to masters programs for mathematics. There you could take graduate courses, do a math thesis, and then assuming your grades/prestige aren't already tanked, there's nothing in your current history that will prevent you from getting a good Mathematics PhD.