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

["weiyongjun (A)" <weiyongjun1@xxxxxxxxxx>]

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@xxxxxxxxxxxxxxx
>> Fixes: c15b1ccadb32 ("ipv6: move DAD and addrconf_verify processing to 
>> workqueue")
>> Signed-off-by: Wei Yongjun <weiyongjun1@xxxxxxxxxx>
>> 
>> 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);
	}
}

If so, the addrconf_dad_begin() also need to fix because if hold a ref before
addrconf_dad_stop():

static void addrconf_dad_begin(struct inet6_ifaddr *ifp)
{
 ...
  	in6_ifa_hold(ifp);  <-- remove this line 
	addrconf_dad_stop(ifp, 0);
 ...
}

Also inet6_addr_del which called ipv6_del_addr with refcount hold:
inet6_addr_del(...)
{
  ...
  list_for_each_entry(...) {
    ...
    in6_ifa_hold(ifp);   <-- remove this line
	...
	ipv6_del_addr(ifp);
	...
  }
  ...
}

Regards,
Yongjun Wei
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