Re: [PATCH net] net/sched: fix netdevice reference leaks in attach_one_default_qdisc()

["wanghai (M)" <wanghai38@huawei.com>]

在 2022/8/19 1:56, Jakub Kicinski 写道:
> On Wed, 17 Aug 2022 18:46:46 +0800 Wang Hai wrote:
>> In attach_default_qdiscs(), when attach default qdisc (fq_codel) fails
>> and fallback to noqueue, if the original attached qdisc is not released
>> and a new one is directly attached, this will cause netdevice reference
>> leaks.
> Could you provide more details on the failure path? My preference would
> be to try to clean up properly there, if possible.
Hi Jakub.

Here are the details of the failure. Do I need to do cleanup under the 
failed path?

If a dev has multiple queues and queue 0 fails to attach qdisc
because there is no memory in attach_one_default_qdisc(). Then
dev->qdisc will be noop_qdisc by default. But the other queues
may be able to successfully attach to default qdisc.

In this case, the fallback to noqueue process will be triggered

static void attach_default_qdiscs(struct net_device *dev)
{
     ...
     if (!netif_is_multiqueue(dev) ||
         dev->priv_flags & IFF_NO_QUEUE) {
             ...
             netdev_for_each_tx_queue(dev, attach_one_default_qdisc, 
NULL); // queue 0 attach failed because -ENOBUFS, but the other queues 
attach successfully
             qdisc = txq->qdisc_sleeping;
             rcu_assign_pointer(dev->qdisc, qdisc); // dev->qdisc = 
&noop_qdisc
             ...
     }
     ...
     if (qdisc == &noop_qdisc) {
         ...
         netdev_for_each_tx_queue(dev, attach_one_default_qdisc, NULL); 
// Re-attach, but not release the previously created qdisc
         ...
     }
}

>> The following is the bug log:
>>
>> veth0: default qdisc (fq_codel) fail, fallback to noqueue
>> unregister_netdevice: waiting for veth0 to become free. Usage count = 32
>> leaked reference.
>>   qdisc_alloc+0x12e/0x210
>>   qdisc_create_dflt+0x62/0x140
>>   attach_one_default_qdisc.constprop.41+0x44/0x70
>>   dev_activate+0x128/0x290
>>   __dev_open+0x12a/0x190
>>   __dev_change_flags+0x1a2/0x1f0
>>   dev_change_flags+0x23/0x60
>>   do_setlink+0x332/0x1150
>>   __rtnl_newlink+0x52f/0x8e0
>>   rtnl_newlink+0x43/0x70
>>   rtnetlink_rcv_msg+0x140/0x3b0
>>   netlink_rcv_skb+0x50/0x100
>>   netlink_unicast+0x1bb/0x290
>>   netlink_sendmsg+0x37c/0x4e0
>>   sock_sendmsg+0x5f/0x70
>>   ____sys_sendmsg+0x208/0x280
>>
>> In attach_one_default_qdisc(), release the old one before attaching
>> a new qdisc to fix this bug.
>>
>> Fixes: bf6dba76d278 ("net: sched: fallback to qdisc noqueue if default qdisc setup fail")
>> Signed-off-by: Wang Hai <wanghai38@huawei.com>
>> ---
>>   net/sched/sch_generic.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>> index d47b9689eba6..87b61ef14497 100644
>> --- a/net/sched/sch_generic.c
>> +++ b/net/sched/sch_generic.c
>> @@ -1140,6 +1140,11 @@ static void attach_one_default_qdisc(struct net_device *dev,
>>   
>>   	if (!netif_is_multiqueue(dev))
>>   		qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
>> +
>> +	if (dev_queue->qdisc_sleeping &&
>> +	    dev_queue->qdisc_sleeping != &noop_qdisc)
>> +		qdisc_put(dev_queue->qdisc_sleeping);
>> +
>>   	dev_queue->qdisc_sleeping = qdisc;
>>   }
>>   
> .

-- 
Wang Hai