WebAssembly (WASM) for High Performance Apps · レッスン

サンドボックス化と権限

WASMモジュールの機能をさらに制限し、ホスト環境で権限を管理するための戦略を実装します

「サンドボックス化と権限」はCoddyKit上の無料WebAssembly (WASM) for High Performance Appsレッスンです。これはレッスン2/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはWebAssembly (WASM) for High Performance Apps学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 WebAssembly (WASM) for High Performance Appsコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

WASM's Security Sandbox

WebAssembly (WASM) is designed with security at its core. It operates within a tightly controlled environment known as a sandbox.

This sandboxing mechanism isolates WASM code from the host system, preventing it from directly accessing sensitive resources or executing arbitrary operations outside its allocated space.

Inherent Restrictions

By default, a WASM module has no direct access to common system capabilities such as:

The host's file system
Network interfaces
Environment variables
Arbitrary memory outside its linear memory

This strict isolation is a fundamental security feature, making WASM a safe choice for executing untrusted code.

Granting Powers via Imports

If a WASM module needs to perform tasks like logging, accessing a file, or making a network request, it cannot do so directly. Instead, it must import functions from its host environment.

The host (e.g., a web browser's JavaScript engine or a WASI runtime) acts as a gatekeeper, explicitly providing these capabilities. This gives the host full control over what the WASM module can and cannot do.

WASM Requests Host Functions

Here's how a Rust-compiled WASM module might declare its need for a host function. It expects the host to provide a function called host_log within the env module.

/* src/lib.rs */
// Link to an imported function from the 'env' module
#[link(wasm_import_module = "env")]
extern "C" {
    fn host_log(ptr: *const u8, len: usize);
}

#[no_mangle]
pub extern "C" fn log_message_from_wasm() {
    let message = "Hello from WASM!";
    let ptr = message.as_ptr();
    let len = message.len();
    unsafe {
        // Call the imported host function
        host_log(ptr, len);
    }
}

Host Grants or Denies Permission

The JavaScript host decides whether to provide the host_log function. If it's provided, WASM can call it. If not, the WASM module will fail to instantiate, enforcing the permission.

Try running this example:

// host.js
async function loadWasmWithPermission() {
  const wasmBytes = await fetch("log_module.wasm").then(res => res.arrayBuffer());

  // Define the imports object for WASM
  const imports = {
    env: {
      host_log: (ptr, len) => {
        // This function is provided by JS to WASM
        const memory = instance.exports.memory;
        const data = new Uint8Array(memory.buffer, ptr, len);
        const text = new TextDecoder("utf-8").decode(data);
        console.log(`WASM says: ${text}`);
      }
    }
  };

  try {
    const { instance } = await WebAssembly.instantiate(wasmBytes, imports);
    instance.exports.log_message_from_from_wasm(); // Call the WASM function
  } catch (e) {
    console.error("WASM instantiation failed:", e);
  }
}
loadWasmWithPermission();

Controlling WASM Memory

Beyond functions, the host environment also controls the WASM module's linear memory. When instantiating a module, the host can specify the initial and maximum memory size (in 64KB pages).

This prevents a malicious or buggy WASM module from consuming excessive memory and impacting the host application.

// host.js
async function loadWasmWithMemory() {
  // Create a WebAssembly.Memory object
  const memory = new WebAssembly.Memory({ initial: 1, maximum: 2 }); 
  // 1 page (64KB) initial, max 2 pages (128KB)

  const wasmBytes = await fetch("memory_user.wasm").then(res => res.arrayBuffer());

  const imports = {
    env: {
      memory: memory // Pass the controlled memory object to WASM
    }
  };

  const { instance } = await WebAssembly.instantiate(wasmBytes, imports);
  console.log("WASM module loaded with controlled memory.");
  // WASM can now access and grow this memory up to 2 pages
}
loadWasmWithMemory();

Principle of Least Privilege

A critical security practice when working with WASM is the Principle of Least Privilege.

This means you should grant a WASM module only the permissions and resources it absolutely needs to perform its intended function, and no more. Minimizing access reduces the attack surface and potential impact of vulnerabilities.

Beyond Browser: WASI & Host Capabilities

For environments outside the web browser (like servers or IoT devices), the WebAssembly System Interface (WASI) standardizes system-level access.

Even with WASI, the underlying WASI runtime (e.g., Wasmtime, Wasmer) is the host. It still enforces permissions, deciding which file paths, network sockets, or environment variables a WASI module can access.

Secure Interaction Tips

To build secure WASM applications:

Minimize Imports: Only provide host functions that are strictly necessary.
Validate Inputs: Always validate data passed from WASM to JavaScript (and vice-versa) to prevent injection or buffer overflow issues.
Memory Boundaries: Set appropriate initial and maximum memory limits.
Trusted Modules: Only load WebAssembly modules from trusted and verified sources.

Permission Check

Consider a WebAssembly module that needs to read from a specific file. Which entity is primarily responsible for granting or denying this capability?

Recap: Secure WASM

We've learned that WebAssembly modules operate within a secure sandbox, relying entirely on the host environment to provide any external capabilities. Managing imports and controlling memory are crucial for maintaining security and adhering to the principle of least privilege.

Understanding these mechanisms helps you build robust, high-performance, and secure WebAssembly applications.

よくある質問

「サンドボックス化と権限」レッスンは無料ですか？

はい。「サンドボックス化と権限」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、WebAssembly (WASM) for High Performance Appsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 WebAssembly (WASM) for High Performance Appsコースには全4レッスンが含まれています。

「サンドボックス化と権限」で何を学びますか？

WASMモジュールの機能をさらに制限し、ホスト環境で権限を管理するための戦略を実装しますブラウザで直接実行するハンズオンコードでWebAssembly (WASM) for High Performance Appsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

WebAssembly (WASM) for High Performance Appsを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのWebAssembly (WASM) for High Performance Appsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン2/4です。

「サンドボックス化と権限」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このWebAssembly (WASM) for High Performance Appsレッスンでコードを書いて実行できますか？

はい。すべてのWebAssembly (WASM) for High Performance Appsレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

← WebAssembly (WASM) for High Performance Appsに戻る