Harden Bubblewrap version probing

docs/bwrap-support/bubblewrap-backend.md

@@ -26,7 +26,14 @@ requiring root privileges or a container runtime.
   environment is built with `--clearenv` (bwrap 0.5.0+), so **bwrap 0.5.0 or
   newer** is required. Platform detection probes `bwrap --version` and reports
   the backend as unavailable — with the detected version — when the host is
-  below that floor.
+  below that floor. The probe has a 5-second deadline and retains at most 64 KB
+  from each output stream. On timeout, its process group is terminated with
+  `SIGKILL` so wrappers and descendants cannot keep the probe alive. Successful
+  Rust-executor advisory results are cached for the process lifetime; Rust
+  failures are not cached, and execution validation probes again before launch
+  so a changed PATH target cannot reuse an advisory result. The Node SDK caches
+  the complete `getPlatformSupport()` result, including failures, for the module
+  lifetime; restart the Node process after remediating the host.
 - User namespaces must be enabled:
   ```bash
   # Check: should print "1"

sdk/node/README.md

@@ -79,7 +79,7 @@ The default `processcontainer`, `bubblewrap`, `lxc`, and `seatbelt` backends wor
 
 > **Hyperlight** is an opt-in build flavor (Linux x64 and Windows x64) gated by the `--with-hyperlight` cargo feature. Default shipped binaries do not include it; build from source with `build.bat --with-hyperlight` (Windows) or the equivalent cargo invocation on Linux.
 
-`getPlatformSupport()` reports backend availability and, when the native probe can determine it, `uiCapabilities`: a platform-neutral view of which UI restrictions the host can enforce. This is currently populated only by the Windows native probe, where it is derived from `JOB_OBJECT_UILIMIT_*` support; Linux and macOS omit the field until their probes expose equivalent data.
+`getPlatformSupport()` reports backend availability and, when the native probe can determine it, `uiCapabilities`: a platform-neutral view of which UI restrictions the host can enforce. This is currently populated only by the Windows native probe, where it is derived from `JOB_OBJECT_UILIMIT_*` support; Linux and macOS omit the field until their probes expose equivalent data. On Linux, `unavailableReasons` provides a diagnostic for each unavailable LXC or Bubblewrap backend even when the other backend keeps the platform supported.
 
 **Node.js:** ≥ 18.
 
@@ -340,7 +340,7 @@ Setting `cwd` (or the `workingDirectory` argument) does **not** add that path to
 
 | Error | Cause | Fix |
 | --- | --- | --- |
-| `MXC is not supported on this platform` | `getPlatformSupport()` returned `isSupported: false`. On Linux: neither LXC nor Bubblewrap on PATH. On macOS: schema version < `0.6.0-alpha`. | Install LXC/Bubblewrap, or switch to schema `0.6.0-alpha` (or `0.7.0-alpha` if you need state-aware lifecycle). |
+| `MXC is not supported on this platform` | `getPlatformSupport()` returned `isSupported: false`. On Linux, neither LXC nor a usable Bubblewrap 0.5.0+ installation is available. On macOS, the Seatbelt platform probe could not find `/usr/bin/sandbox-exec`. | Inspect `support.reason`. On Linux, also inspect `support.unavailableReasons` and install LXC or Bubblewrap 0.5.0+. On macOS, verify that `/usr/bin/sandbox-exec` exists; its absence indicates an incomplete or unsupported macOS installation. |
 | `wxc-exec.exe not found` / `lxc-exec not found` | The SDK couldn't locate the native binary. | Set `MXC_BIN_DIR=<dir>` so `<dir>/<arch>/wxc-exec.exe` (or `lxc-exec`) exists, or pass `options.executablePath` explicitly. |
 | `Invalid containment value '<x>'` | `containment` field doesn't match the parser's accepted values. | Use one of the abstract intents (`process`, `vm`, `microvm`) or a concrete backend listed in [Choosing a Backend](#choosing-a-backend). |
 | `'<x>' containment requires experimental mode` | A `windows_sandbox` / `wslc` / `microvm` / `isolation_session` / `hyperlight` backend was selected without the flag. | Pass `{ experimental: true }` in `SandboxSpawnOptions`. |

sdk/node/src/bwrap-probe-anchor.ts

@@ -0,0 +1,93 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+import { spawn } from 'node:child_process';
+
+// Strip NODE_OPTIONS so a host's loader/inspector/require flags cannot
+// interfere with the helper's minimal, trusted `bwrap --version` probe.
+const spawnEnv = { ...process.env };
+delete spawnEnv.NODE_OPTIONS;
+
+const helperPath = process.argv[2];
+const timeoutMs = Number(process.argv[3]);
+const outputLimit = process.argv[4];
+let resultWritten = false;
+let resultFlushed = false;
+let shuttingDown = false;
+let helperClosed = false;
+let output = '';
+const hold = setInterval(() => {}, 0x3fffffff);
+
+function terminateHelper(): void {
+  if (shuttingDown) return;
+  shuttingDown = true;
+  const pid = helperProcess.pid;
+  if (pid && !helperClosed) {
+    try {
+      process.kill(pid, 'SIGKILL');
+    } catch {
+      // The helper may already have exited.
+    }
+  }
+}
+
+function terminateOwnedGroup(): void {
+  try {
+    process.kill(-process.pid, 'SIGKILL');
+  } catch {
+    process.exit(1);
+  }
+}
+
+function finishIfReady(): void {
+  if (!helperClosed || !resultFlushed) return;
+  clearTimeout(watchdog);
+  clearInterval(hold);
+  // The anchor is still the unreaped group leader. The helper has already
+  // been reaped, so terminating the owned group cannot target a recycled ID.
+  terminateOwnedGroup();
+}
+
+const watchdog = setTimeout(() => {
+  terminateHelper();
+  setTimeout(terminateOwnedGroup, 500).unref();
+}, timeoutMs + 1000);
+watchdog.unref();
+
+process.on('SIGTERM', () => {
+  terminateHelper();
+});
+
+function emitFailure(detail: string): void {
+  if (resultWritten) return;
+  resultWritten = true;
+  process.stdout.write(`${JSON.stringify({ kind: 'spawnError', detail })}\n`, () => {
+    resultFlushed = true;
+    finishIfReady();
+  });
+}
+
+const helperProcess = spawn(
+  process.execPath,
+  [helperPath, String(timeoutMs), outputLimit],
+  { detached: false, stdio: ['ignore', 'pipe', 'ignore'], env: spawnEnv },
+);
+helperProcess.stdout.setEncoding('utf8');
+helperProcess.stdout.on('data', (chunk: string) => {
+  output += chunk;
+  const newline = output.indexOf('\n');
+  if (!resultWritten && newline !== -1) {
+    resultWritten = true;
+    process.stdout.write(output.slice(0, newline + 1), () => {
+      resultFlushed = true;
+      finishIfReady();
+    });
+    terminateHelper();
+  }
+});
+helperProcess.on('error', (error) => emitFailure(error.message));
+helperProcess.on('close', () => {
+  helperClosed = true;
+  emitFailure('probe helper exited without a result');
+  finishIfReady();
+});

sdk/node/src/bwrap-probe-helper.ts

@@ -0,0 +1,113 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+import { spawn, ChildProcessByStdio } from 'node:child_process';
+import * as fs from 'node:fs/promises';
+import * as path from 'node:path';
+import { Readable } from 'node:stream';
+
+type HelperResult =
+  | { kind: 'completed'; status: number | null; signal: NodeJS.Signals | null; stdout: string; stderr: string }
+  | { kind: 'notFound' }
+  | { kind: 'timeout' }
+  | { kind: 'overflow' }
+  | { kind: 'spawnError'; detail: string };
+
+const timeoutMs = Number(process.argv[2]);
+const outputLimit = Number(process.argv[3]);
+let child: ChildProcessByStdio<null, Readable, Readable> | undefined;
+let finished = false;
+let classifyingSpawnError = false;
+let timer: NodeJS.Timeout | undefined;
+let stdoutLength = 0;
+let stderrLength = 0;
+const stdoutChunks: Buffer[] = [];
+const stderrChunks: Buffer[] = [];
+const hold = setInterval(() => {}, 0x3fffffff);
+
+setTimeout(() => {
+  if (child) {
+    try {
+      child.kill('SIGKILL');
+    } catch {
+      // The child may already have exited.
+    }
+  }
+  process.exit(1);
+}, timeoutMs + 1000).unref();
+
+function capture(chunks: Buffer[], chunk: Buffer, currentLength: number): number {
+  const remaining = Math.max(0, outputLimit - currentLength);
+  if (remaining > 0) chunks.push(chunk.subarray(0, remaining));
+  return currentLength + chunk.length;
+}
+
+function emit(result: HelperResult): void {
+  if (finished) return;
+  finished = true;
+  if (timer) clearTimeout(timer);
+  process.stdout.write(`${JSON.stringify(result)}\n`);
+}
+
+async function handleSpawnError(error: NodeJS.ErrnoException): Promise<void> {
+  if (error.code !== 'ENOENT') {
+    emit({ kind: 'spawnError', detail: error.message });
+    return;
+  }
+  for (const entry of (process.env.PATH ?? '').split(path.delimiter)) {
+    const candidate = path.join(entry, 'bwrap');
+    try {
+      if ((await fs.stat(candidate)).isFile()) {
+        emit({
+          kind: 'spawnError',
+          detail: `${candidate} was found but could not be executed; check for a missing interpreter or loader`,
+        });
+        return;
+      }
+    } catch (statError) {
+      const error = statError as NodeJS.ErrnoException;
+      if (error.code !== 'ENOENT' && error.code !== 'ENOTDIR') {
+        emit({ kind: 'spawnError', detail: `failed to inspect ${candidate}: ${error.message}` });
+        return;
+      }
+    }
+  }
+  emit({ kind: 'notFound' });
+}
+
+try {
+  child = spawn('bwrap', ['--version'], {
+    detached: false,
+    stdio: ['ignore', 'pipe', 'pipe'],
+  });
+} catch (error) {
+  void handleSpawnError(error as NodeJS.ErrnoException);
+}
+
+if (child) {
+  child.stdout.on('data', (chunk: Buffer) => {
+    stdoutLength = capture(stdoutChunks, chunk, stdoutLength);
+    if (stdoutLength > outputLimit) emit({ kind: 'overflow' });
+  });
+  child.stderr.on('data', (chunk: Buffer) => {
+    stderrLength = capture(stderrChunks, chunk, stderrLength);
+    if (stderrLength > outputLimit) emit({ kind: 'overflow' });
+  });
+  child.on('error', (error: NodeJS.ErrnoException) => {
+    classifyingSpawnError = true;
+    void handleSpawnError(error);
+  });
+  child.on('close', (status, signal) => {
+    if (classifyingSpawnError) return;
+    emit({
+      kind: 'completed',
+      status,
+      signal,
+      stdout: Buffer.concat(stdoutChunks).toString('utf8'),
+      stderr: Buffer.concat(stderrChunks).toString('utf8'),
+    });
+  });
+  timer = setTimeout(() => emit({ kind: 'timeout' }), timeoutMs);
+}
+
+void hold;

sdk/node/src/bwrap-probe-worker.ts

@@ -0,0 +1,135 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+import { spawn, ChildProcessByStdio } from 'node:child_process';
+import { Readable } from 'node:stream';
+import { workerData } from 'node:worker_threads';
+
+// Strip NODE_OPTIONS so a host's loader/inspector/require flags cannot
+// interfere with the anchor's minimal, trusted probe supervision.
+const spawnEnv = { ...process.env };
+delete spawnEnv.NODE_OPTIONS;
+
+interface ProbeWorkerData {
+  shared: SharedArrayBuffer;
+  anchorPath: string;
+  helperPath: string;
+  probeTimeoutMs: number;
+  publishTimeoutMs: number;
+  outputLimit: number;
+}
+
+const data = workerData as ProbeWorkerData;
+const header = new Int32Array(data.shared, 0, 3);
+const payload = new Uint8Array(data.shared, 12);
+let anchor: ChildProcessByStdio<null, Readable, Readable> | undefined;
+let output = '';
+let finished = false;
+let anchorExited = false;
+let anchorClosed = false;
+let completionRequested = false;
+let pendingResult: unknown;
+let timeout: NodeJS.Timeout | undefined;
+
+function stopAnchor(): void {
+  const pid = anchor?.pid;
+  if (!pid || anchorExited) return;
+  try {
+    process.kill(pid, 'SIGTERM');
+  } catch {
+    // The process may have exited between the ownership check and the signal.
+  }
+}
+
+function publish(result: unknown): void {
+  if (finished) return;
+  finished = true;
+  if (timeout) clearTimeout(timeout);
+  let encoded = Buffer.from(JSON.stringify(result));
+  if (encoded.length > payload.length) {
+    encoded = Buffer.from(JSON.stringify({
+      kind: 'spawnError',
+      detail: 'probe helper result exceeded its bound',
+    }));
+  }
+  payload.set(encoded);
+  Atomics.store(header, 1, encoded.length);
+  if (Atomics.compareExchange(header, 0, 0, 1) === 0) {
+    Atomics.notify(header, 0);
+  }
+}
+
+function completeAfterCleanup(result: unknown, stop = false): void {
+  if (finished || completionRequested) return;
+  completionRequested = true;
+  pendingResult = result;
+  if (stop) stopAnchor();
+  if (anchorClosed) publish(pendingResult);
+}
+
+if (Atomics.load(header, 0) === 0) {
+  try {
+    const spawnedAnchor = spawn(
+      process.execPath,
+      [
+        data.anchorPath,
+        data.helperPath,
+        String(data.probeTimeoutMs),
+        String(data.outputLimit),
+      ],
+      { detached: true, stdio: ['ignore', 'pipe', 'pipe'], env: spawnEnv },
+    );
+    anchor = spawnedAnchor;
+    const anchorPid = spawnedAnchor.pid;
+    if (anchorPid === undefined) {
+      completeAfterCleanup({
+        kind: 'spawnError',
+        detail: 'probe anchor did not receive a process id',
+      });
+    } else {
+      Atomics.store(header, 2, anchorPid);
+    }
+    if (Atomics.load(header, 0) !== 0) {
+      stopAnchor();
+    }
+    spawnedAnchor.stdout.setEncoding('utf8');
+    spawnedAnchor.stdout.on('data', (chunk: string) => {
+      output += chunk;
+      const newline = output.indexOf('\n');
+      if (newline !== -1) {
+        try {
+          completeAfterCleanup(JSON.parse(output.slice(0, newline)));
+        } catch (error) {
+          const detail = error instanceof Error ? error.message : String(error);
+          completeAfterCleanup(
+            { kind: 'spawnError', detail: `invalid probe helper result: ${detail}` },
+            true,
+          );
+        }
+      }
+    });
+    spawnedAnchor.on('error', (error) => {
+      completeAfterCleanup({ kind: 'spawnError', detail: error.message }, true);
+    });
+    spawnedAnchor.on('exit', () => {
+      anchorExited = true;
+    });
+    spawnedAnchor.on('close', () => {
+      anchorClosed = true;
+      publish(
+        completionRequested
+          ? pendingResult
+          : { kind: 'spawnError', detail: 'probe helper exited without a result' },
+      );
+    });
+    // Ask the anchor to stop inside the caller's budget. Publication waits for
+    // close so the result cannot escape before group teardown and reaping.
+    timeout = setTimeout(
+      () => completeAfterCleanup({ kind: 'timeout' }, true),
+      data.publishTimeoutMs,
+    );
+  } catch (error) {
+    const detail = error instanceof Error ? error.message : String(error);
+    publish({ kind: 'spawnError', detail });
+  }
+}

sdk/node/src/helper.ts

@@ -260,8 +260,11 @@ export function resolveExecutableAndArgs(
       effectiveContainment as ContainmentBackend
     );
     if (!isIntent && !isExperimental && !isAvailable) {
+      const unavailableReason =
+        platformSupport.unavailableReasons?.[effectiveContainment as ContainmentBackend];
       throw new Error(
         `Containment backend '${rawContainment}' is not available on this platform. ` +
+        (unavailableReason ? `${unavailableReason} ` : '') +
         `Available methods: ${platformSupport.availableMethods.join(', ')}`
       );
     }

sdk/node/src/index.ts

@@ -9,6 +9,8 @@
  * `processContainer.learningMode: true` to enable deny-and-record learning
  * mode. Learning-mode capability names are reserved and must not be supplied
  * directly in `processContainer.capabilities`.
+ * On Linux, `getPlatformSupport()` reports failures for individual backends
+ * through `PlatformSupport.unavailableReasons`, including when none is usable.
  *
  * @example
  * ```typescript