Messages in this thread | | | Subject | Re: [PATCH 3/4] vsock/virtio: fix flush of works during the .remove() | From | Jason Wang <> | Date | Thu, 30 May 2019 19:59:14 +0800 |
| |
On 2019/5/30 下午6:10, Stefano Garzarella wrote: > On Thu, May 30, 2019 at 05:46:18PM +0800, Jason Wang wrote: >> On 2019/5/29 下午6:58, Stefano Garzarella wrote: >>> On Wed, May 29, 2019 at 11:22:40AM +0800, Jason Wang wrote: >>>> On 2019/5/28 下午6:56, 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@redhat.com> >>>>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> >>>>> --- >>>>> 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@kernel.org
Good point. Then I agree with you. But if we can use the RCU to detect the detach of device from socket for these, it would be even better.
Thanks
| |