[WP-M9] CEthInterface#repayBorrowBehalf() reading non-existing returns makes _repayAnyDebt() with CEther always revert

[code423n4]

Lines of code

https://github.com/code-423n4/2022-04-backd/blob/c856714a50437cb33240a5964b63687c9876275b/backd/interfaces/vendor/CTokenInterfaces.sol#L355-L358

Vulnerability details

https://github.com/code-423n4/2022-04-backd/blob/c856714a50437cb33240a5964b63687c9876275b/backd/interfaces/vendor/CTokenInterfaces.sol#L355-L358

function repayBorrowBehalf(address borrower, uint256 repayAmount)
        external
        payable
        returns (uint256);

repayBorrowBehalf() for native cToken (CEther) will return nothing, while the current CEthInterface interface defines the returns as (uint256).

As a result, ether.repayBorrowBehalf() will always revert

https://github.com/code-423n4/2022-04-backd/blob/c856714a50437cb33240a5964b63687c9876275b/backd/contracts/actions/topup/handlers/CompoundHandler.sol#L117-L118

    CEther cether = CEther(address(ctoken));
    err = cether.repayBorrowBehalf{value: debt}(account);

Ref:

method	CEther	CErc20
mint()	revert	error code
redeem()	error code	error code
repayBorrow()	revert	error code
repayBorrowBehalf()	revert	error code

• Compound cToken Repay Borrow Behalf doc: https://compound.finance/docs/ctokens#repay-borrow-behalf
• Compound CEther.repayBorrowBehalf() https://github.com/compound-finance/compound-protocol/blob/v2.8.1/contracts/CEther.sol#L92-L95
• Compound CErc20.repayBorrowBehalf() https://github.com/compound-finance/compound-protocol/blob/v2.8.1/contracts/CErc20.sol#L94-L97