netdev - [PATCH net 3/5] virtio-net: execute xdp_do_flush() before napi_complete

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Message-Id: <20230117092533.5804-4-magnus.karlsson@gmail.com>
Date:   Tue, 17 Jan 2023 10:25:31 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     magnus.karlsson@...el.com, bjorn@...nel.org, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, toke@...hat.com, pabeni@...hat.com,
        davem@...emloft.net, aelior@...vell.com, manishc@...vell.com,
        horatiu.vultur@...rochip.com, UNGLinuxDriver@...rochip.com,
        mst@...hat.com, jasowang@...hat.com, ioana.ciornei@....com,
        madalin.bucur@....com
Cc:     bpf@...r.kernel.org
Subject: [PATCH net 3/5] virtio-net: execute xdp_do_flush() before napi_complete_done()

From: Magnus Karlsson <magnus.karlsson@...el.com>

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 follwed 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@...el.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/
---
 drivers/net/virtio_net.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 7723b2a49d8e..bc4d79fe3c83 100644
--- 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)) {
-- 
2.34.1