Which S3 buckets is Lambda allowed to read code from?
This represents a misunderstanding of when Lambda code is loaded.

AWS Lambda does not download a copy of the code from S3 every time a function is executed. Instead, Lambda has its own internal storage that is updated when invoking CreateFunction or UpdateFunctionCode either through the API or the Console.

Update with further clarification from jdog:


  Lambda is never given access to S3 buckets -- the user or role that runs CreateFunction is.