IPoIB: Fix possible NULL dereference in ipoib_start_xmit()

Fix a bug introduced in 69cce1d14049 ("net: Abstract dst->neighbour
accesses behind helpers.") where we might dereference skb_dst(skb)
even if it is NULL, which causes:

    [  240.944030] BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
    [  240.948007] IP: [<ffffffffa0366ce9>] ipoib_start_xmit+0x39/0x280 [ib_ipoib]
    [...]
    [  240.948007] Call Trace:
    [  240.948007]  <IRQ>
    [  240.948007]  [<ffffffff812cd5e0>] dev_hard_start_xmit+0x2a0/0x590
    [  240.948007]  [<ffffffff8131f680>] ? arp_create+0x70/0x200
    [  240.948007]  [<ffffffff812e8e1f>] sch_direct_xmit+0xef/0x1c0

Addresses: https://bugzilla.kernel.org/show_bug.cgi?id=41212
Signed-off-by: Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>
Signed-off-by: Roland Dreier <roland@purestorage.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/drivers/infiniband/ulp/ipoib/ipoib_main.c b/drivers/infiniband/ulp/ipoib/ipoib_main.c
index 43f89ba0a908..fe89c4660d55 100644
--- a/drivers/infiniband/ulp/ipoib/ipoib_main.c
+++ b/drivers/infiniband/ulp/ipoib/ipoib_main.c
@@ -717,11 +717,13 @@ static int ipoib_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
         struct ipoib_dev_priv *priv = netdev_priv(dev);
         struct ipoib_neigh *neigh;
-        struct neighbour *n;
+        struct neighbour *n = NULL;
         unsigned long flags;
 
-        n = dst_get_neighbour(skb_dst(skb));
-        if (likely(skb_dst(skb) && n)) {
+        if (likely(skb_dst(skb)))
+                n = dst_get_neighbour(skb_dst(skb));
+
+        if (likely(n)) {
                 if (unlikely(!*to_ipoib_neigh(n))) {
                         ipoib_path_lookup(skb, dev);
                         return NETDEV_TX_OK;