Re: [PATCH 3/4] vsock/virtio: fix flush of works during the .remove()

[Stefano Garzarella]

On Thu, May 30, 2019 at 05:46:18PM +0800, Jason Wang wrote:
> 
> On 2019/5/29 äå6:58, Stefano Garzarella wrote:
> > On Wed, May 29, 2019 at 11:22:40AM +0800, Jason Wang wrote:
> > > On 2019/5/28 äå6:56, Stefano Garzarella wrote:
> > > > We flush all pending works before to call vdev->config->reset(vdev),
> > > > but other works can be queued before the vdev->config->del_vqs(vdev),
> > > > so we add another flush after it, to avoid use after free.
> > > > 
> > > > Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > > > ---
> > > >    net/vmw_vsock/virtio_transport.c | 23 +++++++++++++++++------
> > > >    1 file changed, 17 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> > > > index e694df10ab61..ad093ce96693 100644
> > > > --- a/net/vmw_vsock/virtio_transport.c
> > > > +++ b/net/vmw_vsock/virtio_transport.c
> > > > @@ -660,6 +660,15 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
> > > >    	return ret;
> > > >    }
> > > > +static void virtio_vsock_flush_works(struct virtio_vsock *vsock)
> > > > +{
> > > > +	flush_work(&vsock->loopback_work);
> > > > +	flush_work(&vsock->rx_work);
> > > > +	flush_work(&vsock->tx_work);
> > > > +	flush_work(&vsock->event_work);
> > > > +	flush_work(&vsock->send_pkt_work);
> > > > +}
> > > > +
> > > >    static void virtio_vsock_remove(struct virtio_device *vdev)
> > > >    {
> > > >    	struct virtio_vsock *vsock = vdev->priv;
> > > > @@ -668,12 +677,6 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
> > > >    	mutex_lock(&the_virtio_vsock_mutex);
> > > >    	the_virtio_vsock = NULL;
> > > > -	flush_work(&vsock->loopback_work);
> > > > -	flush_work(&vsock->rx_work);
> > > > -	flush_work(&vsock->tx_work);
> > > > -	flush_work(&vsock->event_work);
> > > > -	flush_work(&vsock->send_pkt_work);
> > > > -
> > > >    	/* Reset all connected sockets when the device disappear */
> > > >    	vsock_for_each_connected_socket(virtio_vsock_reset_sock);
> > > > @@ -690,6 +693,9 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
> > > >    	vsock->event_run = false;
> > > >    	mutex_unlock(&vsock->event_lock);
> > > > +	/* Flush all pending works */
> > > > +	virtio_vsock_flush_works(vsock);
> > > > +
> > > >    	/* Flush all device writes and interrupts, device will not use any
> > > >    	 * more buffers.
> > > >    	 */
> > > > @@ -726,6 +732,11 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
> > > >    	/* Delete virtqueues and flush outstanding callbacks if any */
> > > >    	vdev->config->del_vqs(vdev);
> > > > +	/* Other works can be queued before 'config->del_vqs()', so we flush
> > > > +	 * all works before to free the vsock object to avoid use after free.
> > > > +	 */
> > > > +	virtio_vsock_flush_works(vsock);
> > > 
> > > Some questions after a quick glance:
> > > 
> > > 1) It looks to me that the work could be queued from the path of
> > > vsock_transport_cancel_pkt() . Is that synchronized here?
> > > 
> > Both virtio_transport_send_pkt() and vsock_transport_cancel_pkt() can
> > queue work from the upper layer (socket).
> > 
> > Setting the_virtio_vsock to NULL, should synchronize, but after a careful look
> > a rare issue could happen:
> > we are setting the_virtio_vsock to NULL at the start of .remove() and we
> > are freeing the object pointed by it at the end of .remove(), so
> > virtio_transport_send_pkt() or vsock_transport_cancel_pkt() may still be
> > running, accessing the object that we are freed.
> 
> 
> Yes, that's my point.
> 
> 
> > 
> > Should I use something like RCU to prevent this issue?
> > 
> >      virtio_transport_send_pkt() and vsock_transport_cancel_pkt()
> >      {
> >          rcu_read_lock();
> >          vsock = rcu_dereference(the_virtio_vsock_mutex);
> 
> 
> RCU is probably a way to go. (Like what vhost_transport_send_pkt() did).
> 

Okay, I'm going this way.

> 
> >          ...
> >          rcu_read_unlock();
> >      }
> > 
> >      virtio_vsock_remove()
> >      {
> >          rcu_assign_pointer(the_virtio_vsock_mutex, NULL);
> >          synchronize_rcu();
> > 
> >          ...
> > 
> >          free(vsock);
> >      }
> > 
> > Could there be a better approach?
> > 
> > 
> > > 2) If we decide to flush after dev_vqs(), is tx_run/rx_run/event_run still
> > > needed? It looks to me we've already done except that we need flush rx_work
> > > in the end since send_pkt_work can requeue rx_work.
> > The main reason of tx_run/rx_run/event_run is to prevent that a worker
> > function is running while we are calling config->reset().
> > 
> > E.g. if an interrupt comes between virtio_vsock_flush_works() and
> > config->reset(), it can queue new works that can access the device while
> > we are in config->reset().
> > 
> > IMHO they are still needed.
> > 
> > What do you think?
> 
> 
> I mean could we simply do flush after reset once and without tx_rx/rx_run
> tricks?
> 
> rest();
> 
> virtio_vsock_flush_work();
> 
> virtio_vsock_free_buf();

My only doubt is:
is it safe to call config->reset() while a worker function could access
the device?

I had this doubt reading the Michael's advice[1] and looking at
virtnet_remove() where there are these lines before the config->reset():

	/* Make sure no work handler is accessing the device. */
	flush_work(&vi->config_work);

Thanks,
Stefano

[1] https://lore.kernel.org/netdev/20190521055650-mutt-send-email-mst@xxxxxxxxxx