Bug 297874 – [Site Isolation] Crash when accessing cross origin iframe contentWindow from WebInspector

• Component: WebKit Process Model
• Status: RESOLVED FIXED

[Anthony Tarbinian]

rdar://159134034 InRadar

[Anthony Tarbinian]

Summary:
Crash when accessing the contenWindow attribute of a cross origin iframe with site isolation turned on. Only reproducible when running JS from the WebInspector. 

Steps To Reproduce:
1. Open MiniBrowser
2. Ensure that site isolation is enabled (Settings -> Internal Features -> Site Isolation)
3. Navigate to about:blank
4. Open the Web Inspector Console
5. Run the following JS in the console:
```
let i = document.createElement('iframe'); document.body.appendChild(i);
i.src = 'https://example.org';
console.log(i.contentWindow);
```

Results:
Observe a crash when trying to access the contentWindow attribute.

Crash occurs because a downcast from RemoteDOMWindow to LocalDOMWindow fails (specifically fails this assert https://searchfox.org/wubkat/source/Source/WTF/wtf/TypeCasts.h#96).

The downcast is invoked from `WebCore::activeDOMWindow` which was originally called from `JSDOMBindingSecurity::remoteFrameAccessError` https://searchfox.org/wubkat/source/Source/WebCore/bindings/js/JSDOMBindingSecurity.cpp#49.


Regression:
Issue only occurs from running the JS inside the WebInspector. Adding a cross origin iframe and accessing its contentWindow works fine in layout tests, just note in the WebInspector UI. Reproducing on WebKit commit 9892235 from 8/20.

[Anthony Tarbinian]

Pull request: https://github.com/WebKit/WebKit/pull/49861

[EWS]

Committed 299777@main (8fb7be5683f4): <https://commits.webkit.org/299777@main>

Reviewed commits have been landed. Closing PR #49861 and removing active labels.