virtio_net: fix ndo_xdp_xmit crash towards dev not ready for XDP

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@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 1 deletions

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 1e0e0fce3ab2..9bb9e562b893 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -452,8 +452,18 @@ 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;