Lines of code

Vulnerability details

Vulnerability details

A very important logic of update_market() is to update accCantoPerShare.
When updating, if it crosses the epoch boundary, it needs to use the corresponding epoch’s cantoPerBlock[epoch].
For example:
cantoPerBlock[100000] = 100
cantoPerBlock[200000] = 0 (No reward)
lastRewardBlock = 199999
block.number = 200002

At this time, accCantoPerShare needs to be increased:
= cantoPerBlock[100000] * 1 Delta + cantoPerBlock[200000] *2 Delta
= 100 * (200000 - 199999) + 0 * (200002 - 200000) = 100

The code is as follows:

    function update_market(address _market) public {
        require(lendingMarketWhitelist[_market], "Market not whitelisted");
        MarketInfo storage market = marketInfo[_market];
        if (block.number > market.lastRewardBlock) {
            uint256 marketSupply = lendingMarketTotalBalance[_market];
            if (marketSupply > 0) {
                uint256 i = market.lastRewardBlock;
                while (i < block.number) {
                    uint256 epoch = (i / BLOCK_EPOCH) * BLOCK_EPOCH; // Rewards and voting weights are aligned on a weekly basis
@>                  uint256 nextEpoch = i + BLOCK_EPOCH;
                    uint256 blockDelta = Math.min(nextEpoch, block.number) - i;
                    uint256 cantoReward = (blockDelta *
                        cantoPerBlock[epoch] *
                        gaugeController.gauge_relative_weight_write(_market, epoch)) / 1e18;
                    market.accCantoPerShare += uint128((cantoReward * 1e18) / marketSupply);
                    market.secRewardsPerShare += uint128((blockDelta * 1e18) / marketSupply); // TODO: Scaling
                    i += blockDelta;
                }
            }
            market.lastRewardBlock = uint64(block.number);
        }
    }

The above code, the calculation of nextEpoch is wrong

> uint256 nextEpoch = i + BLOCK_EPOCH;

The correct one should be

> uint256 nextEpoch = epoch + BLOCK_EPOCH;

As a result, the increase of accCantoPerShare becomes:
= cantoPerBlock[100000] * 3 Delta
= 100 * (200002 - 199999) = 300

Impact

The calculation of update_market() is incorrect when crossing the epoch boundary, causing the calculation of accCantoPerShare to be incorrect, which may increase the allocated rewards.

Proof of Concept

The following code demonstrates the above example

add to LendingLedgerTest.t.sol

    function test_ErrorNextEpoch() external {
        vm.roll(fromEpoch);        
        whiteListMarket();
        //only set first , second cantoPerBlock = 0
        vm.prank(goverance);
        ledger.setRewards(fromEpoch, fromEpoch, amountPerBlock);
        vm.prank(lendingMarket);
        ledger.sync_ledger(lender, 1e18);
        
        //set lastRewardBlock = nextEpoch - 10
        vm.roll(fromEpoch + BLOCK_EPOCH - 10);
        ledger.update_market(lendingMarket);

        //cross-border
        vm.roll(fromEpoch + BLOCK_EPOCH + 10);
        ledger.update_market(lendingMarket);

        //show more
        (uint128 accCantoPerShare,,) = ledger.marketInfo(lendingMarket);
        console.log("accCantoPerShare:",accCantoPerShare);
        console.log("more:",accCantoPerShare - amountPerEpoch);

    }


$ forge test -vvv --match-test test_ErrorNextEpoch

Running 1 test for src/test/LendingLedger.t.sol:LendingLedgerTest
[PASS] test_ErrorNextEpoch() (gas: 185201)
Logs:
  accCantoPerShare: 1000100000000000000
  more: 100000000000000

Recommended Mitigation

    function update_market(address _market) public {
        require(lendingMarketWhitelist[_market], "Market not whitelisted");
        MarketInfo storage market = marketInfo[_market];
        if (block.number > market.lastRewardBlock) {
            uint256 marketSupply = lendingMarketTotalBalance[_market];
            if (marketSupply > 0) {
                uint256 i = market.lastRewardBlock;
                while (i < block.number) {
                    uint256 epoch = (i / BLOCK_EPOCH) * BLOCK_EPOCH; // Rewards and voting weights are aligned on a weekly basis
-                   uint256 nextEpoch = i + BLOCK_EPOCH;
+                   uint256 nextEpoch = epoch + BLOCK_EPOCH;

Assessed type

Error

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