Minting stake tokens is not updating the pool's borrowing fee rate

Summary

When users mint new stake tokens, they provide liquidity to the pool, increasing the total amount and decreasing the borrowed utilization. However, this rate is not updated.

Vulnerability Detail

When users mint stake tokens, they add liquidity to the pool and increase the total amount held in the pool:

function _mintStakeToken(Mint.Request memory mintRequest) internal returns (uint256 stakeAmount) {
        //..
        -> pool.addBaseToken(cache.mintTokenAmount);
        .
    }

As we can see, the borrowing rate calculation will change accordingly. However, the rate is not updated:

function getLongBorrowingRatePerSecond(LpPool.Props storage pool) external view returns (uint256) {
        if (pool.baseTokenBalance.amount == 0 && pool.baseTokenBalance.unsettledAmount == 0) {
            return 0;
        }
        int256 totalAmount = pool.baseTokenBalance.amount.toInt256() + pool.baseTokenBalance.unsettledAmount;
        if (totalAmount <= 0) {
            return 0;
        }
        uint256 holdRate = CalUtils.divToPrecision(
            pool.baseTokenBalance.holdAmount,
            totalAmount.toUint256(),
            CalUtils.SMALL_RATE_PRECISION
        );
        return CalUtils.mulSmallRate(holdRate, AppPoolConfig.getLpPoolConfig(pool.stakeToken).baseInterestRate);
    }

Impact

Unfair accrual of borrowing fees. It can yield on lesser/higher fees for lps and position holders. It can also delay or cause unfair liquidations. Hence, high.

Code Snippet

https://github.com/sherlock-audit/2024-05-elfi-protocol/blob/8a1a01804a7de7f73a04d794bf6b8104528681ad/elfi-perp-contracts/contracts/process/MintProcess.sol#L45-L91

https://github.com/sherlock-audit/2024-05-elfi-protocol/blob/8a1a01804a7de7f73a04d794bf6b8104528681ad/elfi-perp-contracts/contracts/process/MintProcess.sol#L130-L213

https://github.com/sherlock-audit/2024-05-elfi-protocol/blob/8a1a01804a7de7f73a04d794bf6b8104528681ad/elfi-perp-contracts/contracts/process/MarketQueryProcess.sol#L82C5-L108

Tool used

Manual Review

Recommendation

Just like the opening orders update the rates after the pools base amounts changes.

sherlock-audit / 2024-05-elfi-protocol-judging

mstpr-brainbot - Minting stake tokens is not updating the pool's borrowing fee rate #136