[SRU][B:linux-azure-4.15][PATCH 11/40] virtio_net: fix ndo_xdp_xmit crash towards dev not ready for XDP

[William Breathitt Gray]

From: Jesper Dangaard Brouer <brouer at redhat.com>

BugLink: https://bugs.launchpad.net/bugs/1877654

When a driver implements the ndo_xdp_xmit() function, there is
(currently) no generic way to determine whether it is safe to call.

It is e.g. unsafe to call the drivers ndo_xdp_xmit, if it have not
allocated the needed XDP TX queues yet.  This is the case for
virtio_net, which first allocates the XDP TX queues once an XDP/bpf
prog is attached (in virtnet_xdp_set()).

Thus, a crash will occur for virtio_net when redirecting to another
virtio_net device's ndo_xdp_xmit, which have not attached a XDP prog.
The sample xdp_redirect_map tries to attach a dummy XDP prog to take
this into account, but it can also easily fail if the virtio_net (or
actually underlying vhost driver) have not allocated enough extra
queues for the device.

Allocating more queue this is currently a manual config.
Hint for libvirt XML add:

  <driver name='vhost' queues='16'>
    <host mrg_rxbuf='off'/>
    <guest tso4='off' tso6='off' ecn='off' ufo='off'/>
  </driver>

The solution in this patch is to check that the device have loaded an
XDP/bpf prog before proceeding.  This is similar to the check
performed in driver ixgbe.

Signed-off-by: Jesper Dangaard Brouer <brouer at redhat.com>
Acked-by: John Fastabend <john.fastabend at gmail.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 8dcc5b0ab0ec9a2efb3362d380272546b8b2ee26)
Signed-off-by: William Breathitt Gray <william.gray at canonical.com>
---
 drivers/net/virtio_net.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 76a1eb622a04..80aad47c8c97 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -435,8 +435,18 @@ static bool __virtnet_xdp_xmit(struct virtnet_info *vi,
 static int virtnet_xdp_xmit(struct net_device *dev, struct xdp_buff *xdp)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
-	bool sent = __virtnet_xdp_xmit(vi, xdp);
+	struct receive_queue *rq = vi->rq;
+	struct bpf_prog *xdp_prog;
+	bool sent;
+
+	/* Only allow ndo_xdp_xmit if XDP is loaded on dev, as this
+	 * indicate XDP resources have been successfully allocated.
+	 */
+	xdp_prog = rcu_dereference(rq->xdp_prog);
+	if (!xdp_prog)
+		return -ENXIO;
 
+	sent = __virtnet_xdp_xmit(vi, xdp);
 	if (!sent)
 		return -ENOSPC;
 	return 0;
-- 
2.25.1