virtio-net: execute xdp_do_flush() before napi_complete_done()

Make sure that xdp_do_flush() is always executed before
napi_complete_done(). This is important for two reasons. First, a
redirect to an XSKMAP assumes that a call to xdp_do_redirect() from
napi context X on CPU Y will be followed by a xdp_do_flush() from the
same napi context and CPU. This is not guaranteed if the
napi_complete_done() is executed before xdp_do_flush(), as it tells
the napi logic that it is fine to schedule napi context X on another
CPU. Details from a production system triggering this bug using the
veth driver can be found following the first link below.

The second reason is that the XDP_REDIRECT logic in itself relies on
being inside a single NAPI instance through to the xdp_do_flush() call
for RCU protection of all in-kernel data structures. Details can be
found in the second link below.

Fixes: 186b3c9 ("virtio-net: support XDP_REDIRECT")
Signed-off-by: Magnus Karlsson <[email protected]>
Acked-by: Toke Høiland-Jørgensen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/all/[email protected]/
Acked-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/net/virtio_net.c

@@ -1677,13 +1677,13 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 
 	received = virtnet_receive(rq, budget, &xdp_xmit);
 
+	if (xdp_xmit & VIRTIO_XDP_REDIR)
+		xdp_do_flush();
+
 	/* Out of packets? */
 	if (received < budget)
 		virtqueue_napi_complete(napi, rq->vq, received);
 
-	if (xdp_xmit & VIRTIO_XDP_REDIR)
-		xdp_do_flush();
-
 	if (xdp_xmit & VIRTIO_XDP_TX) {
 		sq = virtnet_xdp_get_sq(vi);
 		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {