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

[Eric Dumazet]

On 8/31/20 5:55 PM, Yunsheng Lin wrote:
> 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.
> 

We request Fixes: tag for fixes in networking land.

> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> ---
>  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();

This seems quite wrong, there is not a single use of synchronize_rcu_tasks() in net/,
we probably do not want this.

I bet that synchronize_net() is appropriate, if not please explain/comment why we want this instead.

Adding one synchronize_net() per TX queue is a killer for devices with 128 or 256 TX queues.

I would rather find a way of not calling qdisc_reset() from qdisc_deactivate().

This lockless pfifo_fast is a mess really.