David Seis locks down linked servers:
SQL Server Linked Servers provide a method to directly execute distributed queries on remote databases. However, they are not an ideal tool due to performance issues. If you decide to use them, it’s crucial to ensure they are secure. This post will outline some best practices for securing SQL Server Linked Servers.
There’s not an enormous amount you can do with security and linked servers. If you are looking for a bit more granularity in that regard, PolyBase could be a viable alternative…says the guy who wrote a book on PolyBase…