[PATCH net] virtio_net: adjust the execution order of function `virtnet_close` during freeze

[Junnan Wu <junnan01.wu@samsung.com>]

"Use after free" issue appears in suspend once race occurs when
napi poll scheduls after `netif_device_detach` and before napi disables.

For details, during suspend flow of virtio-net,
the tx queue state is set to "__QUEUE_STATE_DRV_XOFF" by CPU-A.

And at some coincidental times, if a TCP connection is still working,
CPU-B does `virtnet_poll` before napi disable.
In this flow, the state "__QUEUE_STATE_DRV_XOFF"
of tx queue will be cleared. This is not the normal process it expects.

After that, CPU-A continues to close driver then virtqueue is removed.

Sequence likes below:
--------------------------------------------------------------------------
              CPU-A                            CPU-B
              -----                            -----
         suspend is called                  A TCP based on
                                        virtio-net still work
 virtnet_freeze
 |- virtnet_freeze_down
 | |- netif_device_detach
 | | |- netif_tx_stop_all_queues
 | |  |- netif_tx_stop_queue
 | |   |- set_bit
 | |     (__QUEUE_STATE_DRV_XOFF,...)
 | |                                     softirq rasied
 | |                                    |- net_rx_action
 | |                                     |- napi_poll
 | |                                      |- virtnet_poll
 | |                                       |- virtnet_poll_cleantx
 | |                                        |- netif_tx_wake_queue
 | |                                         |- test_and_clear_bit
 | |                                          (__QUEUE_STATE_DRV_XOFF,...)
 | |- virtnet_close
 |  |- virtnet_disable_queue_pair
 |   |- virtnet_napi_tx_disable
 |- remove_vq_common
--------------------------------------------------------------------------

When TCP delayack timer is up, a cpu gets softirq and irq handler
`tcp_delack_timer_handler` will be called, which will finally call
`start_xmit` in virtio net driver.
Then the access to tx virtq will cause panic.

The root cause of this issue is that napi tx
is not disable before `netif_tx_stop_queue`,
once `virnet_poll` schedules in such coincidental time,
the tx queue state will be cleared.

To solve this issue, adjusts the order of
function `virtnet_close` in `virtnet_freeze_down`.

Co-developed-by: Ying Xu <ying123.xu@samsung.com>
Signed-off-by: Ying Xu <ying123.xu@samsung.com>
Signed-off-by: Junnan Wu <junnan01.wu@samsung.com>
---
 drivers/net/virtio_net.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index d14e6d602273..975bdc5dab84 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -5758,14 +5758,15 @@ static void virtnet_freeze_down(struct virtio_device *vdev)
 	disable_rx_mode_work(vi);
 	flush_work(&vi->rx_mode_work);
 
-	netif_tx_lock_bh(vi->dev);
-	netif_device_detach(vi->dev);
-	netif_tx_unlock_bh(vi->dev);
 	if (netif_running(vi->dev)) {
 		rtnl_lock();
 		virtnet_close(vi->dev);
 		rtnl_unlock();
 	}
+
+	netif_tx_lock_bh(vi->dev);
+	netif_device_detach(vi->dev);
+	netif_tx_unlock_bh(vi->dev);
 }
 
 static int init_vqs(struct virtnet_info *vi);
-- 
2.34.1