[WP-M11] `CEthInterface#mint()` reading non-existing returns makes `topUp()` with native token alway revert

[code423n4]

Lines of code

https://github.com/code-423n4/2022-04-backd/blob/c856714a50437cb33240a5964b63687c9876275b/backd/interfaces/vendor/CTokenInterfaces.sol#L345

Vulnerability details

https://github.com/code-423n4/2022-04-backd/blob/c856714a50437cb33240a5964b63687c9876275b/backd/interfaces/vendor/CTokenInterfaces.sol#L345

 function mint() external payable returns (uint256);

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

In the current implementation, the interface for CToken is used for both CEther and CErc20.

As a result, the transaction will revert with the error: function returned an unexpected amount of data when topUp() with the native token (ETH).

https://github.com/code-423n4/2022-04-backd/blob/c856714a50437cb33240a5964b63687c9876275b/backd/contracts/actions/topup/handlers/CompoundHandler.sol#L57-L70

    CToken ctoken = cTokenRegistry.fetchCToken(underlying);
    uint256 initialTokens = ctoken.balanceOf(address(this));

    address addr = account.addr();

    if (repayDebt) {
        amount -= _repayAnyDebt(addr, underlying, amount, ctoken);
        if (amount == 0) return true;
    }

    uint256 err;
    if (underlying == address(0)) {
        err = ctoken.mint{value: amount}(amount);
    }

Ref:

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

• Compound's cToken mint doc: https://compound.finance/docs/ctokens#mint
• Compound CEther.mint() https://github.com/compound-finance/compound-protocol/blob/v2.8.1/contracts/CEther.sol#L46
• Compound CErc20.mint() https://github.com/compound-finance/compound-protocol/blob/v2.8.1/contracts/CErc20.sol#L46