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

From: Jakub Kicinski <[email-masked]>
Date: Thu, 18 Aug 2022 10:56:42 -0700

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.

> 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 <[email-masked]>
> ---
>  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;
>  }
>