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

[Wang Hai <wanghai38@huawei.com>]

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.

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;
 }
 
-- 
2.17.1