82 lines
2.6 KiB
Markdown
82 lines
2.6 KiB
Markdown
---
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title: "Checks-Effects-Interactions Pattern"
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type: concept
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tags: [blockchain, security, smart-contract, reentrancy, best-practices]
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sources: [blockchain-security-auditor]
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last_updated: 2026-05-30
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---
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## Aliases
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- Checks-Effects-Interactions
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- CEI Pattern
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- Checks-Effects-Interactions Pattern
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## Definition
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Checks-Effects-Interactions（CEI，检查-效果-交互）模式是智能合约安全编程的核心防御模式，用于防止重入攻击和其他逻辑漏洞。其核心原则是：**在执行任何外部调用之前，先完成所有的状态检查和状态更新**。
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## Pattern
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```
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function withdraw() external {
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// 1. CHECKS — 验证前置条件
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require(balances[msg.sender] > 0, "No balance");
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// 2. EFFECTS — 更新合约状态（在外部调用之前）
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balances[msg.sender] = 0;
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// 3. INTERACTIONS — 执行外部调用（最后一步）
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(bool success,) = msg.sender.call{value: amount}("");
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require(success, "Transfer failed");
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}
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```
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## Why It Works
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传统重入攻击利用的漏洞链：
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```
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外部调用（ETH转账）→ 攻击者receive()被触发
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→ 攻击者 re-enter withdraw()
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→ balances[msg.sender] 仍 > 0（未清零）
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→ 重复提取资金
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```
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CEI 模式切断漏洞链：**状态在外部调用之前清零**，即使攻击者 re-enter，条件检查阶段 `require(balances[msg.sender] > 0)` 已失败。
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## Common Mistakes
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### Wrong Order (Vulnerable)
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```solidity
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// VULNERABLE: 外部调用在状态更新之前
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(bool success,) = msg.sender.call{value: amount}("");
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require(success);
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balances[msg.sender] = 0; // 攻击者可在此之前 re-enter
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```
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### Missing Update (Vulnerable)
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```solidity
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// VULNERABLE: 状态根本没有更新
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require(balances[msg.sender] > 0);
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(bool success,) = msg.sender.call{value: amount}("");
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require(success);
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// balances[msg.sender] = 0; ← 忘记更新！
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```
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### ERC-20 Transfer Before Update
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```solidity
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// VULNERABLE: ERC-20 transfer 触发外部调用
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IERC20(token).transfer(msg.sender, amount); // 外部调用
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balances[msg.sender] -= amount; // 状态更新太晚
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```
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## Relationship to Other Defenses
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- **CEI is the primary defense** — should always be applied first
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- **ReentrancyGuard is additional defense** — use when CEI is difficult to apply (complex logic, callbacks)
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- CEI + ReentrancyGuard together provide defense in depth
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## Connections
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- [[Reentrancy]] ← prevents ← [[Checks-Effects-Interactions]]
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- [[Blockchain-Security-Auditor]] ← enforces ← [[Checks-Effects-Interactions]]
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- [[ReentrancyGuard]] ← supplements ← [[Checks-Effects-Interactions]]
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