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

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Date:   Wed, 8 May 2019 10:54:18 +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 2019/5/7 下午10:41, Cong Wang wrote:
> On Mon, May 6, 2019 at 11:19 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> On 2019/5/7 下午12:54, Cong Wang wrote:
>>> On Mon, May 6, 2019 at 9:03 PM Jason Wang <jasowang@...hat.com> wrote:
>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>>> index e9ca1c0..32a0b23 100644
>>>> --- a/drivers/net/tun.c
>>>> +++ b/drivers/net/tun.c
>>>> @@ -700,6 +700,8 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>>>>                                      tun->tfiles[tun->numqueues - 1]);
>>>>                   ntfile = rtnl_dereference(tun->tfiles[index]);
>>>>                   ntfile->queue_index = index;
>>>> +               rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
>>>> +                                  NULL);
>>>>
>>> How does this work? Existing readers could still read this
>>> tun->tfiles[tun->numqueues - 1] before you NULL it. And,
>>> _if_ the following sock_put() is the one frees it, you still miss
>>> a RCU grace period.
>>>
>>>                   if (clean) {
>>>                           RCU_INIT_POINTER(tfile->tun, NULL);
>>>                           sock_put(&tfile->sk);
>>>
>>>
>>> Thanks.
>>
>> My understanding is the socket will never be freed for this sock_put().
>> We just drop an extra reference count we held when the socket was
>> attached to the netdevice (there's a sock_hold() in tun_attach()). The
>> real free should happen at another sock_put() in the end of this function.
> So you are saying readers will never read this sock after free, then
> what are you fixing with this patch? Nothing, right?


It's the issue of the second sock_put() in tun_detach() not the first 
one. Without proper synchronization for tun->numqueues, tun_net_xmit() 
can actually dereference a tfile which has been freed by second 
sock_put(). The synchornize_net() doesn't help since we're trying to 
synchronize through tun->numqueues.


>
> As I said, reading a stale tun->numqueues is fine, you just keep
> believing it is a problem.


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().

Thanks