Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

[Cong Wang]

On Tue, Sep 8, 2020 at 4:06 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> 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.
>
> Reused the existing synchronize_net() in dev_deactivate_many() to
> make sure skb with larger queue_mapping enqueued to old qdisc(which
> is saved in dev_queue->qdisc_sleeping) will always be reset when
> dev_reset_queue() is called.
>
> Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> ---
> ChangeLog V2:
>         Reuse existing synchronize_net().
> ---
>  net/sched/sch_generic.c | 48 +++++++++++++++++++++++++++++++++---------------
>  1 file changed, 33 insertions(+), 15 deletions(-)
>
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 265a61d..54c4172 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -1131,24 +1131,10 @@ EXPORT_SYMBOL(dev_activate);
>
>  static void qdisc_deactivate(struct Qdisc *qdisc)
>  {
> -       bool nolock = qdisc->flags & TCQ_F_NOLOCK;
> -
>         if (qdisc->flags & TCQ_F_BUILTIN)
>                 return;
> -       if (test_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state))
> -               return;
> -
> -       if (nolock)
> -               spin_lock_bh(&qdisc->seqlock);
> -       spin_lock_bh(qdisc_lock(qdisc));
>
>         set_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state);
> -
> -       qdisc_reset(qdisc);
> -
> -       spin_unlock_bh(qdisc_lock(qdisc));
> -       if (nolock)
> -               spin_unlock_bh(&qdisc->seqlock);
>  }
>
>  static void dev_deactivate_queue(struct net_device *dev,
> @@ -1165,6 +1151,30 @@ static void dev_deactivate_queue(struct net_device *dev,
>         }
>  }
>
> +static void dev_reset_queue(struct net_device *dev,
> +                           struct netdev_queue *dev_queue,
> +                           void *_unused)
> +{
> +       struct Qdisc *qdisc;
> +       bool nolock;
> +
> +       qdisc = dev_queue->qdisc_sleeping;
> +       if (!qdisc)
> +               return;
> +
> +       nolock = qdisc->flags & TCQ_F_NOLOCK;
> +
> +       if (nolock)
> +               spin_lock_bh(&qdisc->seqlock);
> +       spin_lock_bh(qdisc_lock(qdisc));


I think you do not need this lock for lockless one.

> +
> +       qdisc_reset(qdisc);
> +
> +       spin_unlock_bh(qdisc_lock(qdisc));
> +       if (nolock)
> +               spin_unlock_bh(&qdisc->seqlock);
> +}
> +
>  static bool some_qdisc_is_busy(struct net_device *dev)
>  {
>         unsigned int i;
> @@ -1213,12 +1223,20 @@ void dev_deactivate_many(struct list_head *head)
>                 dev_watchdog_down(dev);
>         }
>
> -       /* Wait for outstanding qdisc-less dev_queue_xmit calls.
> +       /* Wait for outstanding qdisc-less dev_queue_xmit calls or
> +        * outstanding qdisc enqueuing calls.
>          * This is avoided if all devices are in dismantle phase :
>          * Caller will call synchronize_net() for us
>          */
>         synchronize_net();
>
> +       list_for_each_entry(dev, head, close_list) {
> +               netdev_for_each_tx_queue(dev, dev_reset_queue, NULL);
> +
> +               if (dev_ingress_queue(dev))
> +                       dev_reset_queue(dev, dev_ingress_queue(dev), NULL);
> +       }
> +
>         /* Wait for outstanding qdisc_run calls. */
>         list_for_each_entry(dev, head, close_list) {
>                 while (some_qdisc_is_busy(dev)) {

Do you want to reset before waiting for TX action?

I think it is safer to do it after, at least prior to commit 759ae57f1b
we did after.

Thanks.