Merge pull request #1002 from 0xProject/feature/contracts/mutex

[contracts] Add mutexes and reentrancy guards

9 files changed, 384 insertions, 50 deletions

diff --git a/packages/contracts/src/2.0.0/protocol/Exchange/MixinExchangeCore.sol b/packages/contracts/src/2.0.0/protocol/Exchange/MixinExchangeCore.sol
index f3a0591b2..291af3792 100644
--- a/packages/contracts/src/2.0.0/protocol/Exchange/MixinExchangeCore.sol
+++ b/packages/contracts/src/2.0.0/protocol/Exchange/MixinExchangeCore.sol
@@ -19,6 +19,7 @@
 pragma solidity 0.4.24;
 pragma experimental ABIEncoderV2;
 
+import "../../utils/ReentrancyGuard/ReentrancyGuard.sol";
 import "./libs/LibConstants.sol";
 import "./libs/LibFillResults.sol";
 import "./libs/LibOrder.sol";
@@ -30,6 +31,7 @@ import "./mixins/MAssetProxyDispatcher.sol";
 
 
 contract MixinExchangeCore is
+    ReentrancyGuard,
     LibConstants,
     LibMath,
     LibOrder,
@@ -54,6 +56,7 @@ contract MixinExchangeCore is
     /// @param targetOrderEpoch Orders created with a salt less or equal to this value will be cancelled.
     function cancelOrdersUpTo(uint256 targetOrderEpoch)
         external
+        nonReentrant
     {
         address makerAddress = getCurrentContextAddress();
         // If this function is called via `executeTransaction`, we only update the orderEpoch for the makerAddress/msg.sender combination.
@@ -86,43 +89,14 @@ contract MixinExchangeCore is
         bytes memory signature
     )
         public
+        nonReentrant
         returns (FillResults memory fillResults)
     {
-        // Fetch order info
-        OrderInfo memory orderInfo = getOrderInfo(order);
-
-        // Fetch taker address
-        address takerAddress = getCurrentContextAddress();
-
-        // Get amount of takerAsset to fill
-        uint256 remainingTakerAssetAmount = safeSub(order.takerAssetAmount, orderInfo.orderTakerAssetFilledAmount);
-        uint256 takerAssetFilledAmount = min256(takerAssetFillAmount, remainingTakerAssetAmount);
-
-        // Validate context
-        assertValidFill(
+        fillResults = fillOrderInternal(
             order,
-            orderInfo,
-            takerAddress,
             takerAssetFillAmount,
-            takerAssetFilledAmount,
             signature
         );
-
-        // Compute proportional fill amounts
-        fillResults = calculateFillResults(order, takerAssetFilledAmount);
-
-        // Update exchange internal state
-        updateFilledState(
-            order,
-            takerAddress,
-            orderInfo.orderHash,
-            orderInfo.orderTakerAssetFilledAmount,
-            fillResults
-        );
-    
-        // Settle order
-        settleOrder(order, takerAddress, fillResults);
-
         return fillResults;
     }
 
@@ -131,6 +105,7 @@ contract MixinExchangeCore is
     /// @param order Order to cancel. Order must be OrderStatus.FILLABLE.
     function cancelOrder(Order memory order)
         public
+        nonReentrant
     {
         // Fetch current order status
         OrderInfo memory orderInfo = getOrderInfo(order);
@@ -203,6 +178,57 @@ contract MixinExchangeCore is
         return orderInfo;
     }
 
+    /// @dev Fills the input order.
+    /// @param order Order struct containing order specifications.
+    /// @param takerAssetFillAmount Desired amount of takerAsset to sell.
+    /// @param signature Proof that order has been created by maker.
+    /// @return Amounts filled and fees paid by maker and taker.
+    function fillOrderInternal(
+        Order memory order,
+        uint256 takerAssetFillAmount,
+        bytes memory signature
+    )
+        internal
+        returns (FillResults memory fillResults)
+    {
+        // Fetch order info
+        OrderInfo memory orderInfo = getOrderInfo(order);
+
+        // Fetch taker address
+        address takerAddress = getCurrentContextAddress();
+
+        // Get amount of takerAsset to fill
+        uint256 remainingTakerAssetAmount = safeSub(order.takerAssetAmount, orderInfo.orderTakerAssetFilledAmount);
+        uint256 takerAssetFilledAmount = min256(takerAssetFillAmount, remainingTakerAssetAmount);
+
+        // Validate context
+        assertValidFill(
+            order,
+            orderInfo,
+            takerAddress,
+            takerAssetFillAmount,
+            takerAssetFilledAmount,
+            signature
+        );
+
+        // Compute proportional fill amounts
+        fillResults = calculateFillResults(order, takerAssetFilledAmount);
+
+        // Update exchange internal state
+        updateFilledState(
+            order,
+            takerAddress,
+            orderInfo.orderHash,
+            orderInfo.orderTakerAssetFilledAmount,
+            fillResults
+        );
+    
+        // Settle order
+        settleOrder(order, takerAddress, fillResults);
+
+        return fillResults;
+    }
+
     /// @dev Updates state with results of a fill order.
     /// @param order that was filled.
     /// @param takerAddress Address of taker who filled the order.
diff --git a/packages/contracts/src/2.0.0/protocol/Exchange/MixinMatchOrders.sol b/packages/contracts/src/2.0.0/protocol/Exchange/MixinMatchOrders.sol
index 20f4b1c33..25220a673 100644
--- a/packages/contracts/src/2.0.0/protocol/Exchange/MixinMatchOrders.sol
+++ b/packages/contracts/src/2.0.0/protocol/Exchange/MixinMatchOrders.sol
@@ -14,6 +14,7 @@
 pragma solidity 0.4.24;
 pragma experimental ABIEncoderV2;
 
+import "../../utils/ReentrancyGuard/ReentrancyGuard.sol";
 import "./libs/LibConstants.sol";
 import "./libs/LibMath.sol";
 import "./libs/LibOrder.sol";
@@ -25,6 +26,7 @@ import "./mixins/MAssetProxyDispatcher.sol";
 
 
 contract MixinMatchOrders is
+    ReentrancyGuard,
     LibConstants,
     LibMath,
     MAssetProxyDispatcher,
@@ -48,6 +50,7 @@ contract MixinMatchOrders is
         bytes memory rightSignature
     )
         public
+        nonReentrant
         returns (LibFillResults.MatchedFillResults memory matchedFillResults)
     {
         // We assume that rightOrder.takerAssetData == leftOrder.makerAssetData and rightOrder.makerAssetData == leftOrder.takerAssetData.
diff --git a/packages/contracts/src/2.0.0/protocol/Exchange/MixinSignatureValidator.sol b/packages/contracts/src/2.0.0/protocol/Exchange/MixinSignatureValidator.sol
index f30adcdb8..4eb6a2fa6 100644
--- a/packages/contracts/src/2.0.0/protocol/Exchange/MixinSignatureValidator.sol
+++ b/packages/contracts/src/2.0.0/protocol/Exchange/MixinSignatureValidator.sol
@@ -19,6 +19,7 @@
 pragma solidity 0.4.24;
 
 import "../../utils/LibBytes/LibBytes.sol";
+import "../../utils/ReentrancyGuard/ReentrancyGuard.sol";
 import "./mixins/MSignatureValidator.sol";
 import "./mixins/MTransactions.sol";
 import "./interfaces/IWallet.sol";
@@ -26,6 +27,7 @@ import "./interfaces/IValidator.sol";
 
 
 contract MixinSignatureValidator is
+    ReentrancyGuard,
     MSignatureValidator,
     MTransactions
 {
@@ -69,6 +71,7 @@ contract MixinSignatureValidator is
         bool approval
     )
         external
+        nonReentrant
     {
         address signerAddress = getCurrentContextAddress();
         allowedValidators[signerAddress][validatorAddress] = approval;
diff --git a/packages/contracts/src/2.0.0/protocol/Exchange/MixinTransactions.sol b/packages/contracts/src/2.0.0/protocol/Exchange/MixinTransactions.sol
index 821d30279..4a59b6c0f 100644
--- a/packages/contracts/src/2.0.0/protocol/Exchange/MixinTransactions.sol
+++ b/packages/contracts/src/2.0.0/protocol/Exchange/MixinTransactions.sol
@@ -155,7 +155,8 @@ contract MixinTransactions is
         view
         returns (address)
     {
-        address contextAddress = currentContextAddress == address(0) ? msg.sender : currentContextAddress;
+        address currentContextAddress_ = currentContextAddress;
+        address contextAddress = currentContextAddress_ == address(0) ? msg.sender : currentContextAddress_;
         return contextAddress;
     }
 }
diff --git a/packages/contracts/src/2.0.0/protocol/Exchange/MixinWrapperFunctions.sol b/packages/contracts/src/2.0.0/protocol/Exchange/MixinWrapperFunctions.sol
index 1b77cfbcb..39fa724cc 100644
--- a/packages/contracts/src/2.0.0/protocol/Exchange/MixinWrapperFunctions.sol
+++ b/packages/contracts/src/2.0.0/protocol/Exchange/MixinWrapperFunctions.sol
@@ -19,18 +19,22 @@
 pragma solidity 0.4.24;
 pragma experimental ABIEncoderV2;
 
+import "../../utils/ReentrancyGuard/ReentrancyGuard.sol";
 import "./libs/LibMath.sol";
 import "./libs/LibOrder.sol";
 import "./libs/LibFillResults.sol";
 import "./libs/LibAbiEncoder.sol";
 import "./mixins/MExchangeCore.sol";
+import "./mixins/MWrapperFunctions.sol";
 
 
 contract MixinWrapperFunctions is
+    ReentrancyGuard,
     LibMath,
     LibFillResults,
     LibAbiEncoder,
-    MExchangeCore
+    MExchangeCore,
+    MWrapperFunctions
 {
 
     /// @dev Fills the input order. Reverts if exact takerAssetFillAmount not filled.
@@ -43,17 +47,14 @@ contract MixinWrapperFunctions is
         bytes memory signature
     )
         public
+        nonReentrant
         returns (FillResults memory fillResults)
     {
-        fillResults = fillOrder(
+        fillResults = fillOrKillOrderInternal(
             order,
             takerAssetFillAmount,
             signature
         );
-        require(
-            fillResults.takerAssetFilledAmount == takerAssetFillAmount,
-            "COMPLETE_FILL_FAILED"
-        );
         return fillResults;
     }
 
@@ -88,14 +89,7 @@ contract MixinWrapperFunctions is
                 fillOrderCalldata,                  // write output over input
                 128                                 // output size is 128 bytes
             )
-            switch success
-            case 0 {
-                mstore(fillResults, 0)
-                mstore(add(fillResults, 32), 0)
-                mstore(add(fillResults, 64), 0)
-                mstore(add(fillResults, 96), 0)
-            }
-            case 1 {
+            if success {
                 mstore(fillResults, mload(fillOrderCalldata))
                 mstore(add(fillResults, 32), mload(add(fillOrderCalldata, 32)))
                 mstore(add(fillResults, 64), mload(add(fillOrderCalldata, 64)))
@@ -117,11 +111,12 @@ contract MixinWrapperFunctions is
         bytes[] memory signatures
     )
         public
+        nonReentrant
         returns (FillResults memory totalFillResults)
     {
         uint256 ordersLength = orders.length;
         for (uint256 i = 0; i != ordersLength; i++) {
-            FillResults memory singleFillResults = fillOrder(
+            FillResults memory singleFillResults = fillOrderInternal(
                 orders[i],
                 takerAssetFillAmounts[i],
                 signatures[i]
@@ -143,11 +138,12 @@ contract MixinWrapperFunctions is
         bytes[] memory signatures
     )
         public
+        nonReentrant
         returns (FillResults memory totalFillResults)
     {
         uint256 ordersLength = orders.length;
         for (uint256 i = 0; i != ordersLength; i++) {
-            FillResults memory singleFillResults = fillOrKillOrder(
+            FillResults memory singleFillResults = fillOrKillOrderInternal(
                 orders[i],
                 takerAssetFillAmounts[i],
                 signatures[i]
@@ -195,6 +191,7 @@ contract MixinWrapperFunctions is
         bytes[] memory signatures
     )
         public
+        nonReentrant
         returns (FillResults memory totalFillResults)
     {
         bytes memory takerAssetData = orders[0].takerAssetData;
@@ -210,7 +207,7 @@ contract MixinWrapperFunctions is
             uint256 remainingTakerAssetFillAmount = safeSub(takerAssetFillAmount, totalFillResults.takerAssetFilledAmount);
 
             // Attempt to sell the remaining amount of takerAsset
-            FillResults memory singleFillResults = fillOrder(
+            FillResults memory singleFillResults = fillOrderInternal(
                 orders[i],
                 remainingTakerAssetFillAmount,
                 signatures[i]
@@ -282,6 +279,7 @@ contract MixinWrapperFunctions is
         bytes[] memory signatures
     )
         public
+        nonReentrant
         returns (FillResults memory totalFillResults)
     {
         bytes memory makerAssetData = orders[0].makerAssetData;
@@ -305,7 +303,7 @@ contract MixinWrapperFunctions is
             );
 
             // Attempt to sell the remaining amount of takerAsset
-            FillResults memory singleFillResults = fillOrder(
+            FillResults memory singleFillResults = fillOrderInternal(
                 orders[i],
                 remainingTakerAssetFillAmount,
                 signatures[i]
@@ -400,4 +398,28 @@ contract MixinWrapperFunctions is
         }
         return ordersInfo;
     }
+
+    /// @dev Fills the input order. Reverts if exact takerAssetFillAmount not filled.
+    /// @param order Order struct containing order specifications.
+    /// @param takerAssetFillAmount Desired amount of takerAsset to sell.
+    /// @param signature Proof that order has been created by maker.
+    function fillOrKillOrderInternal(
+        LibOrder.Order memory order,
+        uint256 takerAssetFillAmount,
+        bytes memory signature
+    )
+        internal
+        returns (FillResults memory fillResults)
+    {
+        fillResults = fillOrderInternal(
+            order,
+            takerAssetFillAmount,
+            signature
+        );
+        require(
+            fillResults.takerAssetFilledAmount == takerAssetFillAmount,
+            "COMPLETE_FILL_FAILED"
+        );
+        return fillResults;
+    }
 }
diff --git a/packages/contracts/src/2.0.0/protocol/Exchange/mixins/MExchangeCore.sol b/packages/contracts/src/2.0.0/protocol/Exchange/mixins/MExchangeCore.sol
index c165b647c..41832fe4b 100644
--- a/packages/contracts/src/2.0.0/protocol/Exchange/mixins/MExchangeCore.sol
+++ b/packages/contracts/src/2.0.0/protocol/Exchange/mixins/MExchangeCore.sol
@@ -59,6 +59,19 @@ contract MExchangeCore is
         uint256 orderEpoch                    // Orders with specified makerAddress and senderAddress with a salt less than this value are considered cancelled.
     );
 
+    /// @dev Fills the input order.
+    /// @param order Order struct containing order specifications.
+    /// @param takerAssetFillAmount Desired amount of takerAsset to sell.
+    /// @param signature Proof that order has been created by maker.
+    /// @return Amounts filled and fees paid by maker and taker.
+    function fillOrderInternal(
+        LibOrder.Order memory order,
+        uint256 takerAssetFillAmount,
+        bytes memory signature
+    )
+        internal
+        returns (LibFillResults.FillResults memory fillResults);
+
     /// @dev Updates state with results of a fill order.
     /// @param order that was filled.
     /// @param takerAddress Address of taker who filled the order.
diff --git a/packages/contracts/src/2.0.0/protocol/Exchange/mixins/MWrapperFunctions.sol b/packages/contracts/src/2.0.0/protocol/Exchange/mixins/MWrapperFunctions.sol
new file mode 100644
index 000000000..e04d4a429
--- /dev/null
+++ b/packages/contracts/src/2.0.0/protocol/Exchange/mixins/MWrapperFunctions.sol
@@ -0,0 +1,40 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity 0.4.24;
+pragma experimental ABIEncoderV2;
+
+import "../libs/LibOrder.sol";
+import "../libs/LibFillResults.sol";
+import "../interfaces/IWrapperFunctions.sol";
+
+
+contract MWrapperFunctions {
+
+    /// @dev Fills the input order. Reverts if exact takerAssetFillAmount not filled.
+    /// @param order LibOrder.Order struct containing order specifications.
+    /// @param takerAssetFillAmount Desired amount of takerAsset to sell.
+    /// @param signature Proof that order has been created by maker.
+    function fillOrKillOrderInternal(
+        LibOrder.Order memory order,
+        uint256 takerAssetFillAmount,
+        bytes memory signature
+    )
+        internal
+        returns (LibFillResults.FillResults memory fillResults);
+}
diff --git a/packages/contracts/src/2.0.0/test/ReentrantERC20Token/ReentrantERC20Token.sol b/packages/contracts/src/2.0.0/test/ReentrantERC20Token/ReentrantERC20Token.sol
new file mode 100644
index 000000000..8bfdd2e66
--- /dev/null
+++ b/packages/contracts/src/2.0.0/test/ReentrantERC20Token/ReentrantERC20Token.sol
@@ -0,0 +1,182 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity 0.4.24;
+pragma experimental ABIEncoderV2;
+
+import "../../utils/LibBytes/LibBytes.sol";
+import "../../tokens/ERC20Token/ERC20Token.sol";
+import "../../protocol/Exchange/interfaces/IExchange.sol";
+import "../../protocol/Exchange/libs/LibOrder.sol";
+
+
+contract ReentrantERC20Token is
+    ERC20Token
+{
+
+    using LibBytes for bytes;
+
+    // solhint-disable-next-line var-name-mixedcase
+    IExchange internal EXCHANGE;
+
+    bytes internal constant REENTRANCY_ILLEGAL_REVERT_REASON = abi.encodeWithSelector(
+        bytes4(keccak256("Error(string)")),
+        "REENTRANCY_ILLEGAL"
+    );
+
+    // All of these functions are potentially vulnerable to reentrancy
+    // We do not test any "noThrow" functions because `fillOrderNoThrow` makes a delegatecall to `fillOrder`
+    enum ExchangeFunction {
+        FILL_ORDER,
+        FILL_OR_KILL_ORDER,
+        BATCH_FILL_ORDERS,
+        BATCH_FILL_OR_KILL_ORDERS,
+        MARKET_BUY_ORDERS,
+        MARKET_SELL_ORDERS,
+        MATCH_ORDERS,
+        CANCEL_ORDER,
+        CANCEL_ORDERS_UP_TO,
+        SET_SIGNATURE_VALIDATOR_APPROVAL
+    }
+
+    uint8 internal currentFunctionId = 0;
+
+    constructor (address _exchange)
+        public
+    {
+        EXCHANGE = IExchange(_exchange);
+    }
+
+    /// @dev Set the current function that will be called when `transferFrom` is called.
+    /// @param _currentFunctionId Id that corresponds to function name.
+    function setCurrentFunction(uint8 _currentFunctionId)
+        external
+    {
+        currentFunctionId = _currentFunctionId;
+    }
+
+    /// @dev A version of `transferFrom` that attempts to reenter the Exchange contract.
+    /// @param _from The address of the sender
+    /// @param _to The address of the recipient
+    /// @param _value The amount of token to be transferred
+    function transferFrom(
+        address _from,
+        address _to,
+        uint256 _value
+    )
+        external
+        returns (bool)
+    {
+        // This order would normally be invalid, but it will be used strictly for testing reentrnacy.
+        // Any reentrancy checks will happen before any other checks that invalidate the order.
+        LibOrder.Order memory order;
+
+        // Initialize remaining null parameters
+        bytes memory signature;
+        LibOrder.Order[] memory orders;
+        uint256[] memory takerAssetFillAmounts;
+        bytes[] memory signatures;
+        bytes memory calldata;
+
+        // Create calldata for function that corresponds to currentFunctionId
+        if (currentFunctionId == uint8(ExchangeFunction.FILL_ORDER)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.fillOrder.selector,
+                order,
+                0,
+                signature
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.FILL_OR_KILL_ORDER)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.fillOrKillOrder.selector,
+                order,
+                0,
+                signature
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.BATCH_FILL_ORDERS)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.batchFillOrders.selector,
+                orders,
+                takerAssetFillAmounts,
+                signatures
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.BATCH_FILL_OR_KILL_ORDERS)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.batchFillOrKillOrders.selector,
+                orders,
+                takerAssetFillAmounts,
+                signatures
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.MARKET_BUY_ORDERS)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.marketBuyOrders.selector,
+                orders,
+                0,
+                signatures
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.MARKET_SELL_ORDERS)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.marketSellOrders.selector,
+                orders,
+                0,
+                signatures
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.MATCH_ORDERS)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.matchOrders.selector,
+                order,
+                order,
+                signature,
+                signature
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.CANCEL_ORDER)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.cancelOrder.selector,
+                order
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.CANCEL_ORDERS_UP_TO)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.cancelOrdersUpTo.selector,
+                0
+            );
+        } else if (currentFunctionId == uint8(ExchangeFunction.SET_SIGNATURE_VALIDATOR_APPROVAL)) {
+            calldata = abi.encodeWithSelector(
+                EXCHANGE.setSignatureValidatorApproval.selector,
+                address(0),
+                false
+            );
+        }
+
+        // Call Exchange function, swallow error
+        address(EXCHANGE).call(calldata);
+
+        // Revert reason is 100 bytes
+        bytes memory returnData = new bytes(100);
+
+        // Copy return data
+        assembly {
+            returndatacopy(add(returnData, 32), 0, 100)
+        }
+
+        // Revert if function reverted with REENTRANCY_ILLEGAL error
+        require(!REENTRANCY_ILLEGAL_REVERT_REASON.equals(returnData));
+
+        // Transfer will return true if function failed for any other reason
+        return true;
+    }
+}
\ No newline at end of file
diff --git a/packages/contracts/src/2.0.0/utils/ReentrancyGuard/ReentrancyGuard.sol b/packages/contracts/src/2.0.0/utils/ReentrancyGuard/ReentrancyGuard.sol
new file mode 100644
index 000000000..1dee512d4
--- /dev/null
+++ b/packages/contracts/src/2.0.0/utils/ReentrancyGuard/ReentrancyGuard.sol
@@ -0,0 +1,44 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+pragma solidity 0.4.24;
+
+
+contract ReentrancyGuard {
+
+    // Locked state of mutex
+    bool private locked = false;
+
+    /// @dev Functions with this modifer cannot be reentered. The mutex will be locked
+    ///      before function execution and unlocked after.
+    modifier nonReentrant() {
+        // Ensure mutex is unlocked
+        require(
+            !locked,
+            "REENTRANCY_ILLEGAL"
+        );
+
+        // Lock mutex before function call
+        locked = true;
+
+        // Perform function call
+        _;
+
+        // Unlock mutex after function call
+        locked = false;
+    }
+}