DoS for functions with invariant modifiers

Summary

There are some edge cases where the margin deposited total and stable collateral total are not adjusted to their real values to avoid a negative value. These adjustments will cause an imbalance in the collateral net balance that will revert the transaction when the function called uses the orderInvariantChecks or liquidationInvariantChecks modifiers.

Vulnerability Detail

In FlatcoinVault.settleFundingFees the funding fees are calculated, added to the margin deposited total, and subtracted from the stable collateral total. However, when the funding fees are greater than the margin deposited total, this value is set to 0 to avoid negative margin deposited values.

In the same way, FlatcoinVault._updateStableCollateralTotal adjusts the stable collateral total by adding or subtracting to it _stableCollateralAdjustment, and when this value is negative and greater than the stable collateral total in absolute value, stableCollateralTotal is set to 0.

Given that these edge cases do not adjust the margin deposited total and stable collateral total to their real values, this creates an imbalance in the accounting of the calculation of the collateral net balance and thus, will revert when the function called uses the orderInvariantChecks or liquidationInvariantChecks modifiers.

Impact

The functions LiquidationModule.liquidate, LimitOrder.executeLimitOrder, and DelayedOrder.executeOrder will not be executable when the margin deposited total and stable collateral total are not adjusted to their real values, as the orderInvariantChecks and liquidationInvariantChecks modifiers will revert the transaction.