Re: [PATCH] nbd: defer config put in recv_work

[Zheng Qixing <zhengqixing@huawei.com>]

Hi,


在 2025/11/10 8:54, Lizhi Xu 写道:
> On Sat,  8 Nov 2025 15:02:02 +0800, Zheng Qixing wrote:
>> Reported-by: syzbot+56fbf4c7ddf65e95c7cc@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/all/6907edce.a70a0220.37351b.0014.GAE@google.com/T/
>> Fixes: 87aac3a80af5 ("nbd: make the config put is called before the notifying the waiter")
>> Depends-on: e2daec488c57 ("nbd: Fix hungtask when nbd_config_put")
>> Signed-off-by: Zheng Qixing <zhengqixing@huawei.com>
>> ---
>>   drivers/block/nbd.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index a853c65ac65d..215fc18115b7 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -1024,9 +1024,9 @@ static void recv_work(struct work_struct *work)
>>   	nbd_mark_nsock_dead(nbd, nsock, 1);
>>   	mutex_unlock(&nsock->tx_lock);
>>
>> -	nbd_config_put(nbd);
>>   	atomic_dec(&config->recv_threads);
>>   	wake_up(&config->recv_wq);
>> +	nbd_config_put(nbd);
>>   	kfree(args);
>>   }
> This only makes the problem more hidden, and that's far from enough.
> I tested the same patch on syzbot on October 3rd before you did; you
> can check it out here [1].
>
> [1] https://syzkaller.appspot.com/bug?extid=56fbf4c7ddf65e95c7cc


The same patch was triggered by eslam.medhat1993 on Nov 5 via syzbot,

but it didn't produce the same stack trace. Is this stack trace necessarily

related to the UAF issue in nbd? It seems more likely to be a memory

corruption problem, but I'm not certain.


In addition, this issue arises from the mixed use of netlink and ioctl. 
Since

user-space tools generally do not mix these two interfaces, I believe a 
simple

solution along these lines could effectively avoid the UAF problem.


Regards,

Qixing