Bug 314037 – [Site Isolation] subtest 4 of imported/w3c/web-platform-tests/html/infrastructure/safe-passing-of-structured-data/shared-array-buffers/window-iframe-messagechannel.https.html is failing

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[Anthony Tarbinian]

This bug pertains to subtest 4 of imported/w3c/web-platform-tests/html/infrastructure/safe-passing-of-structured-data/shared-array-buffers/window-iframe-messagechannel.https.html
which fails with site isolation enabled.

This subtest is the one titled:
"postMessaging with a MessageChannel that's been cross-site should succeed	"

Here's the order of events in the test:

1. cross-origin iframe sends port1 to parent
2.parent sends SAB across port1 (port2 is currently in cross-origin iframe). WebKit drops the SharedArrayBuffer since it's trying to be sent across processes (this is because SharedArrayBuffers are not serialized over IPC. I'll explain this below)
3. cross-origin iframe sends port2 to parent
4. parent starts listening for onmessage and tries to read the SAB. I think WebKit should wait until this point to send the message

WebKit drops SharedArrayBuffers that are attempted to be sent cross-process:
1. Start off in MessagePort::postMessage. Note how we create a SerializedScriptValue with a SerializationContext of SerializationContext::WorkerPostMessage, this will be relevant in the next step.
    https://github.com/WebKit/WebKit/blob/013da9aa8cc1a0e26f5f34408efa118956499193/Source/WebCore/dom/MessagePort.cpp#L158
2. When the data is serialized in SerializedScriptValue::dumpIfTerminal, there's a case for SerializationContext::WorkerPostMessage where it copies over the contents of the shared array buffer.
    https://github.com/WebKit/WebKit/blob/013da9aa8cc1a0e26f5f34408efa118956499193/Source/WebCore/bindings/js/SerializedScriptValue.cpp#L2190-L2203
3. The message is actually sent over IPC in WebMessagePortChannelProvider::postMessageToRemote
    https://github.com/WebKit/WebKit/blob/013da9aa8cc1a0e26f5f34408efa118956499193/Source/WebKit/WebProcess/WebCoreSupport/WebMessagePortChannelProvider.cpp#L127
4. However, this IPC message which is defined in MessageWithMessagePorts and contains SerialiazedScriptValue which contains the sharedBufferContentsArray which is defined as [NotSerialized] :
  [NotSerialized] std::unique_ptr<Vector<JSC::ArrayBufferContents>> sharedBufferContentsArray;
  https://github.com/WebKit/WebKit/blob/013da9aa8cc1a0e26f5f34408efa118956499193/Source/WebKit/Shared/WebCoreArgumentCoders.serialization.in#L8011

[Radar WebKit Bug Importer]

<rdar://problem/176222943>

[Anthony Tarbinian]

Pull request: https://github.com/WebKit/WebKit/pull/64231

[EWS]

Committed 313226@main (49d2e914a4cc): <https://commits.webkit.org/313226@main>

Reviewed commits have been landed. Closing PR #64231 and removing active labels.