netdev - Re: [PATCH net V2] tuntap: synchronize through tfiles array instead of tun->numqueues

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Date:   Thu, 9 May 2019 20:55:28 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        YueHaibing <yuehaibing@...wei.com>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net V2] tuntap: synchronize through tfiles array instead
 of tun->numqueues


On 2019/5/9 下午1:34, Cong Wang wrote:
> On Tue, May 7, 2019 at 7:54 PM Jason Wang <jasowang@...hat.com> wrote:
>> This is only true if you can make sure tfile[tun->numqueues] is not
>> freed. Either my patch or SOCK_RCU_FREE can solve this, but for
>> SOCK_RCU_FREE we need do extra careful audit to make sure it doesn't
>> break someting. So synchronize through pointers in tfiles[] which is
>> already protected by RCU is much more easier. It can make sure no
>> dereference from xmit path after synchornize_net(). And this matches the
>> assumptions of the codes after synchronize_net().
>>
> It is hard to tell which sock_put() matches with this synchronize_net()
> given the call path is complicated.
>
> With SOCK_RCU_FREE, no such a problem, all sock_put() will be safe.
> So to me SOCK_RCU_FREE is much easier to understand and audit.


The problem is not tfile itself but the data structure associated. As I 
mentioned earlier, the xdp_rxq_info_unreg() looks racy if tun_net_xmit() 
can read stale value of numqueues. It's just one example, we may meet 
similar issues in the future when adding more features.

Thanks


>
> Thanks.