Re: [PATCH] virtio-net: fix race between ndo_open() and virtio_device_ready()

[Jason Wang]

On Fri, Jun 17, 2022 at 6:13 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Fri, Jun 17, 2022 at 03:29:49PM +0800, Jason Wang wrote:
> > We used to call virtio_device_ready() after netdev registration. This
> > cause a race between ndo_open() and virtio_device_ready(): if
> > ndo_open() is called before virtio_device_ready(), the driver may
> > start to use the device before DRIVER_OK which violates the spec.
> >
> > Fixing this by switching to use register_netdevice() and protect the
> > virtio_device_ready() with rtnl_lock() to make sure ndo_open() can
> > only be called after virtio_device_ready().
> >
> > Fixes: 4baf1e33d0842 ("virtio_net: enable VQs early")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >  drivers/net/virtio_net.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index db05b5e930be..8a5810bcb839 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -3655,14 +3655,20 @@ static int virtnet_probe(struct virtio_device *vdev)
> >       if (vi->has_rss || vi->has_rss_hash_report)
> >               virtnet_init_default_rss(vi);
> >
> > -     err = register_netdev(dev);
> > +     /* serialize netdev register + virtio_device_ready() with ndo_open() */
> > +     rtnl_lock();
> > +
> > +     err = register_netdevice(dev);
> >       if (err) {
> >               pr_debug("virtio_net: registering device failed\n");
> > +             rtnl_unlock();
> >               goto free_failover;
> >       }
> >
> >       virtio_device_ready(vdev);
> >
> > +     rtnl_unlock();
> > +
> >       err = virtnet_cpu_notif_add(vi);
> >       if (err) {
> >               pr_debug("virtio_net: registering cpu notifier failed\n");
>
>
> Looks good but then don't we have the same issue when removing the
> device?
>
> Actually I looked at  virtnet_remove and I see
>         unregister_netdev(vi->dev);
>
>         net_failover_destroy(vi->failover);
>
>         remove_vq_common(vi); <- this will reset the device
>
> a window here?

Probably. For safety, we probably need to reset before unregistering.

>
>
> Really, I think what we had originally was a better idea -
> instead of dropping interrupts they were delayed and
> when driver is ready to accept them it just enables them.

The problem is that it works only on some specific setup:

- doesn't work on shared IRQ
- doesn't work on some specific driver e.g virtio-blk

> We just need to make sure driver does not wait for
> interrupts before enabling them.
>
> And I suspect we need to make this opt-in on a per driver
> basis.

Exactly.

Thanks

>
>
>
> > --
> > 2.25.1
>