Return inside for loop breaks nested internal calls in Vyper

# VVE-2020-0002 Earlier today, we received a responsible disclosure of a potential issue from @michwill (developer of @curvefi) for Vyper users who use return statements inside for loops of nested internal calls. Returning inside a for loop causes an invalid jump dest, reverting the transaction unnecessarily. MWE: ```python @internal def _baz(): for i in range(1): return # Stack underflow happens here @internal def _bar(): self._baz() @external def foo(): self._bar() ``` ### Impact Impact is minor, it is unlikely a user would encounter this problem unless they were working with nested calls, and return statements inside calls. Even in that scenario, you would encounter a revert which should be noticeable with adequate testing. In limited circumstances, this could cause a DoS attack for public contracts under certain conditions. ### Patches Fixed in https://github.com/vyperlang/vyper/pull/2110. Please upgrade to [Vyper 0.2.3](https://pypi.org/project/vyper/) ### Workarounds Not returning inside a for loop nested 2+ internal calls deep works as is: ```python @internal def _baz(): for i in range(1): pass return # This works fine @internal def _bar(): self._baz() @external def foo(): self._bar() ``` ### For more information If you have any questions or comments about this advisory: * Chat with us in [our gitter ](https://gitter.im/vyperlang/community) * Open an issue in [https://github.com/vyperlang/vyper](https://github.com/vyperlang/vyper) * Email us at [[email protected]](mailto:[email protected])