net: make skb_gso_segment error handling more robust

skb_gso_segment has three possible return values:
1. a pointer to the first segmented skb
2. an errno value (IS_ERR())
3. NULL.  This can happen when GSO is used for header verification.

However, several callers currently test IS_ERR instead of IS_ERR_OR_NULL
and would oops when NULL is returned.

Note that these call sites should never actually see such a NULL return
value; all callers mask out the GSO bits in the feature argument.

However, there have been issues with some protocol handlers erronously not
respecting the specified feature mask in some cases.

It is preferable to get 'have to turn off hw offloading, else slow' reports
rather than 'kernel crashes'.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -231,7 +231,7 @@ static int ip_finish_output_gso(struct sk_buff *skb)
 	 */
 	features = netif_skb_features(skb);
 	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
-	if (IS_ERR(segs)) {
+	if (IS_ERR_OR_NULL(segs)) {
 		kfree_skb(skb);
 		return -ENOMEM;
 	}

net/netfilter/nfnetlink_queue_core.c

@@ -665,7 +665,7 @@ nfqnl_enqueue_packet(struct nf_queue_entry *entry, unsigned int queuenum)
 	 * returned by nf_queue.  For instance, callers rely on -ECANCELED to
 	 * mean 'ignore this hook'.
 	 */
-	if (IS_ERR(segs))
+	if (IS_ERR_OR_NULL(segs))
 		goto out_err;
 	queued = 0;
 	err = 0;

net/openvswitch/datapath.c

@@ -324,6 +324,8 @@ static int queue_gso_packets(struct datapath *dp, struct sk_buff *skb,
 	segs = __skb_gso_segment(skb, NETIF_F_SG, false);
 	if (IS_ERR(segs))
 		return PTR_ERR(segs);
+	if (segs == NULL)
+		return -EINVAL;
 
 	/* Queue all of the segments. */
 	skb = segs;

net/xfrm/xfrm_output.c

@@ -157,6 +157,8 @@ static int xfrm_output_gso(struct sk_buff *skb)
 	kfree_skb(skb);
 	if (IS_ERR(segs))
 		return PTR_ERR(segs);
+	if (segs == NULL)
+		return -EINVAL;
 
 	do {
 		struct sk_buff *nskb = segs->next;