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)