This is fixed by making the "about to be notified rejected promises
list" use JS::Handle instead of JS::NonnullGCPtr. This UAF happens
because notify_about_rejected_promises makes a local copy of this list,
empties the member variable list and then moves the local copy into a
JS::SafeFunction lambda. JS::SafeFunction can only see GC pointers that
are in its storage, not external storage.
Example exploit (requires fixed microtask timing by removing the dummy
execution context):
```html
<script>
Promise.reject(new Error);
// Exit the script block, causing a microtask checkpoint and thus
// queuing of a task to fire the unhandled rejection event for the
// above promise.
// During the time after being queued but before being ran, these
// promises are not kept alive. This is because JS::SafeFunction cannot
// see into a Vector, meaning it can't visit the stored NonnullGCPtrs.
</script>
<script defer>
// Cause a garbage collection, destroying the above promise.
const b = [];
for (var i = 0; i < 200000; i++)
b.push({});
// Some time after this script block, the queued unhandled rejection
// event task will fire, with the event object containing the dead
// promise.
window.onunhandledrejection = (event) => {
let value = event.promise;
console.log(value);
}
</script>
```
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