Date: Thu, 7 Dec 2023 13:31:59 +0800
From: Jason Wang <jasowang@...hat.com>
To: Heng Qi <hengqi@...ux.alibaba.com>
Cc: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, mst@...hat.com, kuba@...nel.org, 
	yinjun.zhang@...igine.com, edumazet@...gle.com, davem@...emloft.net, 
	hawk@...nel.org, john.fastabend@...il.com, ast@...nel.org, horms@...nel.org, 
	xuanzhuo@...ux.alibaba.com
Subject: Re: [PATCH net-next v6 4/5] virtio-net: add spin lock for ctrl cmd access

On Thu, Dec 7, 2023 at 12:47 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>
>
>
> 在 2023/12/7 下午12:19, Jason Wang 写道:
> > On Wed, Dec 6, 2023 at 9:03 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> >>
> >>
> >> 在 2023/12/6 下午8:27, Paolo Abeni 写道:
> >>> On Tue, 2023-12-05 at 19:05 +0800, Heng Qi wrote:
> >>>> 在 2023/12/5 下午4:35, Jason Wang 写道:
> >>>>> On Tue, Dec 5, 2023 at 4:02 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> >>>>>> Currently access to ctrl cmd is globally protected via rtnl_lock and works
> >>>>>> fine. But if dim work's access to ctrl cmd also holds rtnl_lock, deadlock
> >>>>>> may occur due to cancel_work_sync for dim work.
> >>>>> Can you explain why?
> >>>> For example, during the bus unbind operation, the following call stack
> >>>> occurs:
> >>>> virtnet_remove -> unregister_netdev -> rtnl_lock[1] -> virtnet_close ->
> >>>> cancel_work_sync -> virtnet_rx_dim_work -> rtnl_lock[2] (deadlock occurs).
> >>>>
> >>>>>> Therefore, treating
> >>>>>> ctrl cmd as a separate protection object of the lock is the solution and
> >>>>>> the basis for the next patch.
> >>>>> Let's don't do that. Reasons are:
> >>>>>
> >>>>> 1) virtnet_send_command() may wait for cvq commands for an indefinite time
> >>>> Yes, I took that into consideration. But ndo_set_rx_mode's need for an
> >>>> atomic
> >>>> environment rules out the mutex lock.
> >>>>
> >>>>> 2) hold locks may complicate the future hardening works around cvq
> >>>> Agree, but I don't seem to have thought of a better way besides passing
> >>>> the lock.
> >>>> Do you have any other better ideas or suggestions?
> >>> What about:
> >>>
> >>> - using the rtnl lock only
> >>> - virtionet_close() invokes cancel_work(), without flushing the work
> >>> - virtnet_remove() calls flush_work() after unregister_netdev(),
> >>> outside the rtnl lock
> >>>
> >>> Should prevent both the deadlock and the UaF.
> >>
> >> Hi, Paolo and Jason!
> >>
> >> Thank you very much for your effective suggestions, but I found another
> >> solution[1],
> >> based on the ideas of rtnl_trylock and refill_work, which works very well:
> >>
> >> [1]
> >> +static void virtnet_rx_dim_work(struct work_struct *work)
> >> +{
> >> +    struct dim *dim = container_of(work, struct dim, work);
> >> +    struct receive_queue *rq = container_of(dim,
> >> +            struct receive_queue, dim);
> >> +    struct virtnet_info *vi = rq->vq->vdev->priv;
> >> +    struct net_device *dev = vi->dev;
> >> +    struct dim_cq_moder update_moder;
> >> +    int i, qnum, err;
> >> +
> >> +    if (!rtnl_trylock())
> >> +        return;
> > Don't we need to reschedule here?
> >
> > like
> >
> > if (rq->dim_enabled)
> >         sechedule_work()
> >
> > ?
>
> I think no, we don't need this.
>
> The work of each queue will be called by "net_dim()->schedule_work()"
> when napi traffic changes (before schedule_work(), the dim->profile_ix
> of the corresponding rxq has been updated).
> So we only need to traverse and update the profiles of all rxqs in the
> work which is obtaining the rtnl_lock.

Ok, let's have a comment here then.

Thanks

>
> Thanks!
>
> >
> > Thanks
> >
> >> +
> >> +    for (i = 0; i < vi->curr_queue_pairs; i++) {
> >> +        rq = &vi->rq[i];
> >> +        dim = &rq->dim;
> >> +        qnum = rq - vi->rq;
> >> +
> >> +        if (!rq->dim_enabled)
> >> +            continue;
> >> +
> >> +        update_moder = net_dim_get_rx_moderation(dim->mode,
> >> dim->profile_ix);
> >> +        if (update_moder.usec != rq->intr_coal.max_usecs ||
> >> +            update_moder.pkts != rq->intr_coal.max_packets) {
> >> +            err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, qnum,
> >> +                                   update_moder.usec,
> >> +                                   update_moder.pkts);
> >> +            if (err)
> >> +                pr_debug("%s: Failed to send dim parameters on rxq%d\n",
> >> +                     dev->name, qnum);
> >> +            dim->state = DIM_START_MEASURE;
> >> +        }
> >> +    }
> >> +
> >> +    rtnl_unlock();
> >> +}
> >>
> >>
> >> In addition, other optimizations[2] have been tried, but it may be due
> >> to the sparsely
> >> scheduled work that the retry condition is always satisfied, affecting
> >> performance,
> >> so [1] is the final solution:
> >>
> >> [2]
> >>
> >> +static void virtnet_rx_dim_work(struct work_struct *work)
> >> +{
> >> +    struct dim *dim = container_of(work, struct dim, work);
> >> +    struct receive_queue *rq = container_of(dim,
> >> +            struct receive_queue, dim);
> >> +    struct virtnet_info *vi = rq->vq->vdev->priv;
> >> +    struct net_device *dev = vi->dev;
> >> +    struct dim_cq_moder update_moder;
> >> +    int i, qnum, err, count;
> >> +
> >> +    if (!rtnl_trylock())
> >> +        return;
> >> +retry:
> >> +    count = vi->curr_queue_pairs;
> >> +    for (i = 0; i < vi->curr_queue_pairs; i++) {
> >> +        rq = &vi->rq[i];
> >> +        dim = &rq->dim;
> >> +        qnum = rq - vi->rq;
> >> +        update_moder = net_dim_get_rx_moderation(dim->mode,
> >> dim->profile_ix);
> >> +        if (update_moder.usec != rq->intr_coal.max_usecs ||
> >> +            update_moder.pkts != rq->intr_coal.max_packets) {
> >> +            --count;
> >> +            err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, qnum,
> >> +                                   update_moder.usec,
> >> +                                   update_moder.pkts);
> >> +            if (err)
> >> +                pr_debug("%s: Failed to send dim parameters on rxq%d\n",
> >> +                     dev->name, qnum);
> >> +            dim->state = DIM_START_MEASURE;
> >> +        }
> >> +    }
> >> +
> >> +    if (need_resched()) {
> >> +        rtnl_unlock();
> >> +        schedule();
> >> +    }
> >> +
> >> +    if (count)
> >> +        goto retry;
> >> +
> >> +    rtnl_unlock();
> >> +}
> >>
> >> Thanks a lot!
> >>
> >>> Side note: for this specific case any functional test with a
> >>> CONFIG_LOCKDEP enabled build should suffice to catch the deadlock
> >>> scenario above.
> >>>
> >>> Cheers,
> >>>
> >>> Paolo
>

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