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

[wanghai (M)]

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@xxxxxxxxxx>
> > ---
> >   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