[PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

[Yunsheng Lin <linyunsheng@huawei.com>]

Currently there is concurrent reset and enqueue operation for the
same lockless qdisc when there is no lock to synchronize the
q->enqueue() in __dev_xmit_skb() with the qdisc reset operation in
qdisc_deactivate() called by dev_deactivate_queue(), which may cause
out-of-bounds access for priv->ring[] in hns3 driver if user has
requested a smaller queue num when __dev_xmit_skb() still enqueue a
skb with a larger queue_mapping after the corresponding qdisc is
reset, and call hns3_nic_net_xmit() with that skb later.

Avoid the above concurrent op by calling synchronize_rcu_tasks()
after assigning new qdisc to dev_queue->qdisc and before calling
qdisc_deactivate() to make sure skb with larger queue_mapping
enqueued to old qdisc will always be reset when qdisc_deactivate()
is called.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
---
 net/sched/sch_generic.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 265a61d..6e42237 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1160,8 +1160,13 @@ static void dev_deactivate_queue(struct net_device *dev,
 
 	qdisc = rtnl_dereference(dev_queue->qdisc);
 	if (qdisc) {
-		qdisc_deactivate(qdisc);
 		rcu_assign_pointer(dev_queue->qdisc, qdisc_default);
+
+		/* Make sure lockless qdisc enqueuing is done with the
+		 * old qdisc in __dev_xmit_skb().
+		 */
+		synchronize_rcu_tasks();
+		qdisc_deactivate(qdisc);
 	}
 }
 
-- 
2.8.1