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 16:13, Yunsheng Lin wrote:
> 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.

now -> not
sorry for the typo.

> 
> 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.
>> .
>>
> .
>