How to save existing permissions on schema and later on restore it?

Hi,

Could anyone please suggest me how to save the existing permissions on
schema and later on restore it? Basically, by default I have revoked all the
permissions from the mydb schema but there are some users who has all
permissions on mydb schema. I have one stored procedure which temporary
grant the ALL permissions on mydb schema and later on revoke it and just
grant USAGE permissions. Here I have problem if users have full permissions
on schema executes the stored procedure. His original schema permissions
will be altered by my stored procedure. I want to prevent this.

Thanks,
Dipti