Re: [PATCH] nfc: virtual_ncidev: Add variable to check if ndev is running

Phi Nguyen
2023-11-20T18:24:00Z

On 11/20/2023 6:45 PM, Krzysztof Kozlowski wrote:
> On 20/11/2023 11:39, Nguyen Dinh Phi wrote:
>>>>> mutex_lock(&vdev->mtx);
>>>>> kfree_skb(vdev->send_buff);
>>>>> vdev->send_buff = NULL;
>>>>> + vdev->running = false;
>>>>> mutex_unlock(&vdev->mtx);
>>>>>
>>>>> return 0;
>>>>> @@ -50,7 +55,7 @@ static int virtual_nci_send(struct nci_dev *ndev, struct sk_buff *skb)
>>>>> struct virtual_nci_dev *vdev = nci_get_drvdata(ndev);
>>>>>
>>>>> mutex_lock(&vdev->mtx);
>>>>> - if (vdev->send_buff) {
>>>>> + if (vdev->send_buff || !vdev->running) {
>>>>
>>>> Dear Krzysztof,
>>>>
>>>> I agree this defensive code.
>>>> But i think NFC submodule has to avoid this situation.(calling send function of closed nci_dev)
>>>> Could you check this?
>>>
>>> This code looks not effective. At this point vdev->send_buff is always
>>> false, so the additional check would not bring any value.
>>>
>>> I don't see this fixing anything. Syzbot also does not seem to agree.
>>>
>>> Nguyen, please test your patches against syzbot *before* sending them.
>>> If you claim this fixes the report, please provide me the link to syzbot
>>> test results confirming it is fixed.
>>>
>>> I looked at syzbot dashboard and do not see this issue fixed with this
>>> patch.
>>>
>>> Best regards,
>>> Krzysztof
>>>
>>
>> Hi Krzysztof,
>>
>> I've submitted it to syzbot, it is the test request that created at
>> [2023/11/20 09:39] in dashboard link
>> https://syzkaller.appspot.com/bug?extid=6eb09d75211863f15e3e
>
> ...and I see there two errors.
>
These are because I sent email wrongly and syzbot truncates the patch
and can not compile

> I don't know, maybe I miss something obvious (our brains like to do it
> sometimes), but please explain me how this could fix anything?
>
> Best regards,
> Krzysztof
>

The issue arises when an skb is added to the send_buff after invoking
ndev->ops->close() but before unregistering the device. In such cases,
the virtual device will generate a copy of skb, but with no consumer
thereafter. Consequently, this object persists indefinitely.

This problem seems to stem from the existence of time gaps between
ops->close() and the destruction of the workqueue. During this interval,
incoming requests continue to trigger the send function.

best regards,
Phi