Fix dst_neigh_lookup/dst_neigh_lookup_skb return value handling bug

When neighbour table is full, dst_neigh_lookup/dst_neigh_lookup_skb will return
-ENOBUFS which is absolutely non zero, while all the code in kernel which use
above functions assume failure only on zero return which will cause panic. (for
example: : https://bugzilla.kernel.org/show_bug.cgi?id=54731).

This patch corrects above error with smallest changes to kernel source code and
also correct two return value check missing bugs in drivers/infiniband/hw/cxgb4/cm.c

Tested on my x86_64 SMP machine

Reported-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Tested-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/infiniband/hw/cxgb4/cm.c

@@ -1575,6 +1575,12 @@ static int c4iw_reconnect(struct c4iw_ep *ep)
 
  neigh = dst_neigh_lookup(ep->dst,
  &ep->com.cm_id->remote_addr.sin_addr.s_addr);
+ if (!neigh) {
+ pr_err("%s - cannot alloc neigh.\n", __func__);
+ err = -ENOMEM;
+ goto fail4;
+ }
+
  /* get a l2t entry */
  if (neigh->dev->flags & IFF_LOOPBACK) {
  PDBG("%s LOOPBACK\n", __func__);
@@ -3053,6 +3059,12 @@ static int rx_pkt(struct c4iw_dev *dev, struct sk_buff *skb)
  dst = &rt->dst;
  neigh = dst_neigh_lookup_skb(dst, skb);
 
+ if (!neigh) {
+ pr_err("%s - failed to allocate neigh!\n",
+ __func__);
+ goto free_dst;
+ }
+
  if (neigh->dev->flags & IFF_LOOPBACK) {
  pdev = ip_dev_find(&init_net, iph->daddr);
  e = cxgb4_l2t_get(dev->rdev.lldi.l2t, neigh,

include/net/dst.h

@@ -413,13 +413,15 @@ static inline int dst_neigh_output(struct dst_entry *dst, struct neighbour *n,
 
 static inline struct neighbour *dst_neigh_lookup(const struct dst_entry *dst, const void *daddr)
 {
- return dst->ops->neigh_lookup(dst, NULL, daddr);
+ struct neighbour *n = dst->ops->neigh_lookup(dst, NULL, daddr);
+ return IS_ERR(n) ? NULL : n;
 }
 
 static inline struct neighbour *dst_neigh_lookup_skb(const struct dst_entry *dst,
  struct sk_buff *skb)
 {
- return dst->ops->neigh_lookup(dst, skb, NULL);
+ struct neighbour *n = dst->ops->neigh_lookup(dst, skb, NULL);
+ return IS_ERR(n) ? NULL : n;
 }
 
 static inline void dst_link_failure(struct sk_buff *skb)