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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2020/9/11 4:07, Cong Wang wrote:
> On Tue, Sep 8, 2020 at 4:06 AM Yunsheng Lin <linyunsheng@huawei.com> 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@huawei.com>
>> ---
>> 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.

It seems so.
Maybe another patch to remove qdisc_lock(qdisc) for lockless
qdisc?


> 
>> +
>> +       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.

The reference to the txq->qdisc is always protected by RCU, so the synchronize_net()
should be enought to ensure there is no skb enqueued to the old qdisc that is saved
in the dev_queue->qdisc_sleeping, because __dev_queue_xmit can only see the new qdisc
after synchronize_net(), which is noop_qdisc, and noop_qdisc will make sure any skb
enqueued to it will be dropped and freed, right?

If we do any additional reset that is not related to qdisc in dev_reset_queue(), we
can move it after some_qdisc_is_busy() checking.

Also, it seems the __QDISC_STATE_DEACTIVATED checking in qdisc_run() is unnecessary
after this patch, because after synchronize_net() qdisc_run() will now see the old
qdisc.

static inline void qdisc_run(struct Qdisc *q)
{
	if (qdisc_run_begin(q)) {
		/* NOLOCK qdisc must check 'state' under the qdisc seqlock
		 * to avoid racing with dev_qdisc_reset()
		 */
		if (!(q->flags & TCQ_F_NOLOCK) ||
		    likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
			__qdisc_run(q);
		qdisc_run_end(q);
	}
}

> 
> Thanks.
> .
>