netdev - Re: 答复: [PATCH] ipv6: addrconf: fix dev refcont leak when DAD failed

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Date:   Mon, 5 Sep 2016 14:03:38 +0200
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     "weiyongjun (A)" <weiyongjun1@...wei.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Patrick McHardy <kaber@...sh.net>,
        Dingtianhong <dingtianhong@...wei.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: 答复: [PATCH] ipv6: addrconf: fix dev refcont leak when DAD failed

On 05.09.2016 13:54, weiyongjun (A) wrote:
> On 05.09.2016 10:06, Wei Yongjun wrote:
>>>> In general, when DAD detected IPv6 duplicate address, ifp->state will 
>>>> be set to INET6_IFADDR_STATE_ERRDAD and DAD is stopped by a delayed 
>>>> work, the call tree should be like this:
>>>>
>>>> ndisc_recv_ns
>>>>   -> addrconf_dad_failure        <- missing ifp put
>>>>      -> addrconf_mod_dad_work
>>>>        -> schedule addrconf_dad_work()
>>>>          -> addrconf_dad_stop()  <- missing ifp hold before call it
>>>>
>>>> addrconf_dad_failure() called with ifp refcont holding but not put.
>>>> addrconf_dad_work() call addrconf_dad_stop() without extra holding 
>>>> refcount. This will not cause any issue normally.
>>>>
>>>> But the race between addrconf_dad_failure() and addrconf_dad_work() 
>>>> may cause ifp refcount leak and netdevice can not be unregister, 
>>>> dmesg show the following messages:
>>>>
>>>> IPv6: eth0: IPv6 duplicate address fe80::XX:XXXX:XXXX:XX detected!
>>>> ...
>>>> unregister_netdevice: waiting for eth0 to become free. Usage count = 
>>>> 1
>>>>
>>>> Cc: stable@...r.kernel.org
>>>> Fixes: c15b1ccadb32 ("ipv6: move DAD and addrconf_verify processing 
>>>> to
>>>> workqueue")
>>>> Signed-off-by: Wei Yongjun <weiyongjun1@...wei.com>
>>>>
>>>> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c index
>>>> bdf368e..2f1f5d4 100644
>>>> --- a/net/ipv6/addrconf.c
>>>> +++ b/net/ipv6/addrconf.c
>>>> @@ -1948,6 +1948,7 @@ errdad:
>>>>  	spin_unlock_bh(&ifp->lock);
>>>>  
>>>>  	addrconf_mod_dad_work(ifp, 0);
>>>> +	in6_ifa_put(ifp);
>>>>  }
> 
>>> This in6_ifa_put makes sense.
> 
>>>>  
>>>>  /* Join to solicited addr multicast group.
>>>> @@ -3857,6 +3858,7 @@ static void addrconf_dad_work(struct work_struct *w)
>>>>  		addrconf_dad_begin(ifp);
>>>>  		goto out;
>>>>  	} else if (action == DAD_ABORT) {
>>>> +		in6_ifa_hold(ifp);
>>>>  		addrconf_dad_stop(ifp, 1);
>>>>  		if (disable_ipv6)
>>>>  			addrconf_ifdown(idev->dev, 0);
>>>>
> 
>>> But why you add a in6_ifa_hold here isn't clear to me. Could you 
>>> explain why this is necessary? I don't see any async stuff being done 
>>> in addrconf_dad_stop, thus the reference we already have should be sufficient for the lifetime of addrconf_dad_stop.
> 
>> I think it that link local is added with flag IFA_F_PERMANENT, which we real need it is to remove in6_ifa_put() in addrconf_dad_stop.
> 
>> static void addrconf_dad_stop(...)
>> {
>> 	if (ifp->flags&IFA_F_PERMANENT) {
>> 		...
>> 		in6_ifa_put(ifp);   <== remove this line since caller hold refcount
>> 	} else if (ifp->flags&IFA_F_TEMPORARY) {
>> 		...
>> 		ipv6_del_addr(ifp);
>> 	} else {
>> 		ipv6_del_addr(ifp);
>> 	}
>> }
> 
> I see the comment of ipv6_del_addr:
> 
> /* This function wants to get referenced ifp and releases it before return */
> static void ipv6_del_addr(struct inet6_ifaddr *ifp)
> {
>    ...
> }
> 
> Both in6_ifa_put() and ipv6_del_addr() need a refcount holding, so we should
> use a extra in6_ifa_hold() call before call addrconf_dad_stop() as the origin patch.

You are correct!

We always call in6_ifa_put() at the end of addrconf_dad_work. In this
case it is not correct. Either we add a in6_ifa_hold or we suppress or
jump over the last in6_ifa_put.

Good catch!

Hannes