Hello,

The eligibility criteria for both UPSC and CDS is Bachelor's Degree in any discipline from a recognised university,(few of the posts have seperate eligibility like having maths and physics to opt for Air force  etc) and in both these exams people from diverse backgrounds appear and qualify, now coming from Maths Background will help you in CDS ( Quantitative aptitude )  and even in UPSC you can have it as optional subject in Mains which is the second stage of UPSC Civil Service Exam. Of course there are other subjects involve too, which you have to study, the syllabus of UPSC is comparatively extensive to that of CDS.

Hope this will help you !

All the best !