doc, test: document and test vm timeout escapes

Using `process.nextTick()` or `Promise`, it
is possible to escape the `timeout` set when running code with
`vm.runInContext()`, `vm.runInThisContext()`, and
`vm.runInNewContext()`.

This documents the issue and adds two known_issues tests.

Refs: #3020
PR-URL: #23743
Refs: #3020
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>

doc/api/vm.md

@@ -944,6 +944,38 @@ within which it can operate. The process of creating the V8 Context and
 associating it with the `sandbox` object is what this document refers to as
 "contextifying" the `sandbox`.
 
+## Timeout limitations when using process.nextTick(), and Promises
+
+Because of the internal mechanics of how the `process.nextTick()` queue and
+the microtask queue that underlies Promises are implemented within V8 and
+Node.js, it is possible for code running within a context to "escape" the
+`timeout` set using `vm.runInContext()`, `vm.runInNewContext()`, and
+`vm.runInThisContext()`.
+
+For example, the following code executed by `vm.runInNewContext()` with a
+timeout of 5 milliseconds schedules an infinite loop to run after a promise
+resolves. The scheduled loop is never interrupted by the timeout:
+
+```js
+const vm = require('vm');
+
+function loop() {
+  while (1) console.log(Date.now());
+}
+
+vm.runInNewContext(
+  'Promise.resolve().then(loop);',
+  { loop, console },
+  { timeout: 5 }
+);
+```
+
+This issue also occurs when the `loop()` call is scheduled using
+the `process.nextTick()` function.
+
+This issue occurs because all contexts share the same microtask and nextTick
+queues.
+
 [`Error`]: errors.html#errors_class_error
 [`URL`]: url.html#url_class_url
 [`eval()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval

test/known_issues/test-vm-timeout-escape-nexttick.js

@@ -0,0 +1,41 @@
+'use strict';
+
+// https://github.com/nodejs/node/issues/3020
+// Promises, nextTick, and queueMicrotask allow code to escape the timeout
+// set for runInContext, runInNewContext, and runInThisContext
+
+require('../common');
+const assert = require('assert');
+const vm = require('vm');
+
+const NS_PER_MS = 1000000n;
+
+const hrtime = process.hrtime.bigint;
+const nextTick = process.nextTick;
+
+function loop() {
+  const start = hrtime();
+  while (1) {
+    const current = hrtime();
+    const span = (current - start) / NS_PER_MS;
+    if (span >= 100n) {
+      throw new Error(
+        `escaped timeout at ${span} milliseconds!`);
+    }
+  }
+}
+
+assert.throws(() => {
+  vm.runInNewContext(
+    'nextTick(loop); loop();',
+    {
+      hrtime,
+      nextTick,
+      loop
+    },
+    { timeout: 5 }
+  );
+}, {
+  code: 'ERR_SCRIPT_EXECUTION_TIMEOUT',
+  message: 'Script execution timed out after 5ms'
+});

test/known_issues/test-vm-timeout-escape-promise.js

@@ -0,0 +1,39 @@
+'use strict';
+
+// https://github.com/nodejs/node/issues/3020
+// Promises, nextTick, and queueMicrotask allow code to escape the timeout
+// set for runInContext, runInNewContext, and runInThisContext
+
+require('../common');
+const assert = require('assert');
+const vm = require('vm');
+
+const NS_PER_MS = 1000000n;
+
+const hrtime = process.hrtime.bigint;
+
+function loop() {
+  const start = hrtime();
+  while (1) {
+    const current = hrtime();
+    const span = (current - start) / NS_PER_MS;
+    if (span >= 100n) {
+      throw new Error(
+        `escaped timeout at ${span} milliseconds!`);
+    }
+  }
+}
+
+assert.throws(() => {
+  vm.runInNewContext(
+    'Promise.resolve().then(loop); loop();',
+    {
+      hrtime,
+      loop
+    },
+    { timeout: 5 }
+  );
+}, {
+  code: 'ERR_SCRIPT_EXECUTION_TIMEOUT',
+  message: 'Script execution timed out after 5ms'
+});