Boost _ Folks Finance 34085 - [Smart Contract - Low] partial repayment with collaterals will revert due to underflow

Submitted on Mon Aug 05 2024 11:59:32 GMT-0400 (Atlantic Standard Time) by @A2Security for Boost | Folks Finance

Report ID: #34085

Report type: Smart Contract

Report severity: Low

Target: https://testnet.snowtrace.io/address/0x2cAa1315bd676FbecABFC3195000c642f503f1C9

Impacts:

• partial repayment reverts

Description

Title: repayWithCollateral will revert because of underflow

Impact

Repayments are always a healthy action on the protocol, and shouldn't revert. Due to this bug, repayment with collateral will revert, preventing users from repaying their debt in certain conditions and improving their loan health.

Description

contracts/hub/logic/HubPoolLogic.sol

    function updateWithRepayWithCollateral(HubPoolState.PoolData storage pool, uint256 principalPaid, uint256 interestPaid, uint256 loanStableRate)
        external
        returns (DataTypes.RepayWithCollateralPoolParams memory repayWithCollateralPoolParams)
    {
        if (loanStableRate > 0) {
            pool.stableBorrowData.averageInterestRate =
                MathUtils.calcDecreasingAverageStableBorrowInterestRate(principalPaid, loanStableRate, pool.stableBorrowData.totalAmount, pool.stableBorrowData.averageInterestRate);
            pool.stableBorrowData.totalAmount -= principalPaid;
        } else {
            pool.variableBorrowData.totalAmount -= principalPaid;
        }
        // @audit-issue : underflow when principalPaid < interestPaid
>>        pool.depositData.totalAmount -= principalPaid - interestPaid; // totalAmount - principal + interest
        repayWithCollateralPoolParams.fAmount = (principalPaid + interestPaid).toFAmount(pool.depositData.interestIndex);

        pool.updateInterestRates();
    }

The bugs simply concerns the line mentioned above, if principalPaid is less than interestPaid the transaction will revert due to underflow. Please also notice when reducing loan balance of a userLoan, it is the intended design to repay the interest off first before reducing the collateral

Recomendation

To fix this, and avoid the underflow, a possible fix would be like this

--    pool.depositData.totalAmount -= principalPaid - interestPaid; 
++    pool.depositData.totalAmount += interestPaid; 
++    pool.depositData.totalAmount -= principalPaid; 

Proof of concept

Proof of Concept

To showcase the problem we simply made a simple reimplementation of the vulenrable code to showcase the underflow: In this example:

• initial depositData.totalAmount = 1000

• interestPaid = 150

• principalPaid = 100

=> this will revert with an underflow Result:

└─$ forge test --mt test_poc_03 -vv
[⠊] Compiling...
No files changed, compilation skipped

Ran 1 test for test/pocs/test_poc.sol:Pocs_3
[FAIL. Reason: panic: arithmetic underflow or overflow (0x11)] test_poc_03() (gas: 407)
Suite result: FAILED. 0 passed; 1 failed; 0 skipped; finished in 706.78ms (143.02µs CPU time)

Ran 1 test suite in 711.03ms (706.78ms CPU time): 0 tests passed, 1 failed, 0 skipped (1 total tests)

Failing tests:
Encountered 1 failing test in test/pocs/test_poc.sol:Pocs_3
[FAIL. Reason: panic: arithmetic underflow or overflow (0x11)] test_poc_03() (gas: 407)

Encountered a total of 1 failing tests, 0 tests succeeded

contract Pocs_3 is Test {
    // Mock pool struct
    struct Pool {
        uint256 totalAmount;
    }
    function updatePool(Pool memory _pool, uint256 _principalPaid, uint256 _interestPaid) internal pure {
        _pool.totalAmount -= _principalPaid - _interestPaid;
    }
    function test_poc_03() public {
        // Setup
        uint256 initialTotalAmount = 1000;
        uint256 principalPaid = 100;
        uint256 interestPaid = 150;

        Pool memory pool = Pool(initialTotalAmount);

        // Test
 //       vm.expectRevert(stdError.arithmeticError);
        updatePool(pool, principalPaid, interestPaid);
    }
}