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: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
Link: https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 18b3de854aeb9b50b5dc2d7acece66ab55dce824..6df14dd5bf4608a509ad30f119948e0a75d7739b 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/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)) {