fix: Removes max variable borrow rate check in GhoAaveSteward for rates updates (#436)

* feat(aave steward): rm max variable borrow rate check

* upd: sync change with certora

* doc: fix natspec of `updateGhoBorrowRate`

Author: DhairyaSethi

certora/steward/specs/GhoAaveSteward.spec

@@ -32,7 +32,6 @@ methods {
 
 
     function getGhoTimelocks() external returns (IGhoAaveSteward.GhoDebounce) envfree;
-    function GHO_BORROW_RATE_MAX() external returns uint32 envfree;
     function MINIMUM_DELAY() external returns uint256 envfree;
     function RISK_COUNCIL() external returns address envfree;
 
@@ -225,23 +224,6 @@ rule updateGhoSupplyCap__correctness() {
 }
 
 
-rule updateGhoBorrowRate__correctness() {
-    env e; uint16 optimalUsageRatio; uint32 baseVariableBorrowRate;
-    uint32 variableRateSlope1; uint32 variableRateSlope2;
-
-    uint16 optimalUsageRatio_before = OPTIMAL_USAGE_RATIO;
-    uint32 baseVariableBorrowRate_before = BASE_VARIABLE_BORROW_RATE;
-    uint32 variableRateSlope1_before = VARIABLE_RATE_SLOPE1;
-    uint32 variableRateSlope2_before = VARIABLE_RATE_SLOPE2;
-
-    updateGhoBorrowRate(e,optimalUsageRatio, baseVariableBorrowRate, variableRateSlope1, variableRateSlope2);
-
-    
-    assert baseVariableBorrowRate + variableRateSlope1 + variableRateSlope2 <= to_mathint(GHO_BORROW_RATE_MAX());
-}
-
-
-
 
 
 

src/contracts/misc/GhoAaveSteward.sol

@@ -21,9 +21,6 @@ import {RiskCouncilControlled} from './RiskCouncilControlled.sol';
 contract GhoAaveSteward is Ownable, RiskCouncilControlled, IGhoAaveSteward {
   using ReserveConfiguration for DataTypes.ReserveConfigurationMap;
 
-  /// @inheritdoc IGhoAaveSteward
-  uint32 public constant GHO_BORROW_RATE_MAX = 0.25e4; // 25.00%
-
   uint256 internal constant BPS_MAX = 100_00;
 
   /// @inheritdoc IGhoAaveSteward
@@ -227,14 +224,6 @@ contract GhoAaveSteward is Ownable, RiskCouncilControlled, IGhoAaveSteward {
       ),
       'INVALID_VARIABLE_RATE_SLOPE2'
     );
-
-    require(
-      uint256(newRates.baseVariableBorrowRate) +
-        uint256(newRates.variableRateSlope1) +
-        uint256(newRates.variableRateSlope2) <=
-        GHO_BORROW_RATE_MAX,
-      'BORROW_RATE_HIGHER_THAN_MAX'
-    );
   }
 
   /**

src/contracts/misc/interfaces/IGhoAaveSteward.sol

@@ -30,7 +30,6 @@ interface IGhoAaveSteward {
    * @notice Updates the borrow rate of GHO, only if:
    * - respects `MINIMUM_DELAY`, the minimum time delay between updates
    * - the update changes parameters up to the maximum allowed change according to risk config
-   * - the update is lower than `GHO_BORROW_RATE_MAX`
    * @dev Only callable by Risk Council
    * @dev Values are all expressed in BPS
    * @param optimalUsageRatio The new optimal usage ratio
@@ -90,12 +89,6 @@ interface IGhoAaveSteward {
    */
   function getGhoTimelocks() external view returns (GhoDebounce memory);
 
-  /**
-   * @notice Returns maximum value that can be assigned to GHO borrow rate.
-   * @return The maximum value that can be assigned to GHO borrow rate in ray (e.g. 0.01e27 results in 1.0%)
-   */
-  function GHO_BORROW_RATE_MAX() external view returns (uint32);
-
   /**
    * @notice The address of pool data provider of the POOL the steward controls
    */

src/test/TestGhoAaveSteward.t.sol

@@ -327,6 +327,21 @@ contract TestGhoAaveSteward is TestGhoBase {
     assertEq(currentBorrowRate, newBorrowRate);
   }
 
+  function testUpdateGhoBorrowRateUpwardsFromHigh() public {
+    // set a very high borrow rate of 80%
+    uint32 highBaseBorrowRate = 0.80e4;
+    _setGhoBorrowRateViaConfigurator(highBaseBorrowRate);
+    highBaseBorrowRate += 0.04e4;
+    vm.prank(RISK_COUNCIL);
+    GHO_AAVE_STEWARD.updateGhoBorrowRate(
+      defaultRateParams.optimalUsageRatio,
+      highBaseBorrowRate,
+      defaultRateParams.variableRateSlope1,
+      defaultRateParams.variableRateSlope2
+    );
+    assertEq(highBaseBorrowRate, _getGhoBorrowRate());
+  }
+
   function testUpdateGhoBorrowRateDownwards() public {
     uint32 oldBorrowRate = _getGhoBorrowRate();
     uint32 newBorrowRate = oldBorrowRate - 1;
@@ -341,18 +356,19 @@ contract TestGhoAaveSteward is TestGhoBase {
     assertEq(currentBorrowRate, newBorrowRate);
   }
 
-  function testUpdateGhoBorrowRateMaxValue() public {
-    uint32 ghoBorrowRateMax = GHO_AAVE_STEWARD.GHO_BORROW_RATE_MAX();
-    _setGhoBorrowRateViaConfigurator(ghoBorrowRateMax - 1);
+  function testUpdateGhoBorrowRateDownwardsFromHigh() public {
+    // set a very high borrow rate of 80%
+    uint32 highBaseBorrowRate = 0.80e4;
+    _setGhoBorrowRateViaConfigurator(highBaseBorrowRate);
+    highBaseBorrowRate -= 0.04e4;
     vm.prank(RISK_COUNCIL);
     GHO_AAVE_STEWARD.updateGhoBorrowRate(
       defaultRateParams.optimalUsageRatio,
-      ghoBorrowRateMax,
+      highBaseBorrowRate,
       defaultRateParams.variableRateSlope1,
       defaultRateParams.variableRateSlope2
     );
-    uint32 currentBorrowRate = _getGhoBorrowRate();
-    assertEq(currentBorrowRate, ghoBorrowRateMax);
+    assertEq(highBaseBorrowRate, _getGhoBorrowRate());
   }
 
   function testUpdateGhoBorrowRateMaxIncrement() public {
@@ -660,19 +676,6 @@ contract TestGhoAaveSteward is TestGhoBase {
     );
   }
 
-  function testRevertUpdateGhoBorrowRateIfValueMoreThanMax() public {
-    uint32 maxGhoBorrowRate = GHO_BORROW_RATE_MAX;
-    _setGhoBorrowRateViaConfigurator(maxGhoBorrowRate);
-    vm.prank(RISK_COUNCIL);
-    vm.expectRevert('BORROW_RATE_HIGHER_THAN_MAX');
-    GHO_AAVE_STEWARD.updateGhoBorrowRate(
-      defaultRateParams.optimalUsageRatio,
-      maxGhoBorrowRate + 1,
-      defaultRateParams.variableRateSlope1,
-      defaultRateParams.variableRateSlope2
-    );
-  }
-
   function testRevertUpdateGhoBorrowRateIfMaxExceededUpwards() public {
     uint32 oldBorrowRate = _getGhoBorrowRate();
     uint32 newBorrowRate = oldBorrowRate + GHO_BORROW_RATE_CHANGE_MAX + 1;

src/test/helpers/Constants.sol

@@ -52,7 +52,6 @@ contract Constants {
   // Gho Stewards
   uint32 constant GHO_BORROW_RATE_CHANGE_MAX = 0.05e4;
   uint256 constant GSM_FEE_RATE_CHANGE_MAX = 0.0050e4;
-  uint32 constant GHO_BORROW_RATE_MAX = 0.25e4;
   uint256 constant MINIMUM_DELAY_V2 = 1 days;
   uint256 constant FIXED_RATE_STRATEGY_FACTORY_REVISION = 1;