net: preserve MSG_ZEROCOPY with forwarding

MSG_ZEROCOPY data must be copied before data is queued to local
sockets, to avoid indefinite timeout until memory release.

This test is performed by skb_orphan_frags_rx, which is called when
looping an egress skb to packet sockets, error queue or ingress path.

To preserve zerocopy for skbs that are looped to ingress but are then
forwarded to an egress device rather than delivered locally, defer
this last check until an skb enters the local IP receive path.

This is analogous to existing behavior of skb_clear_delivery_time.

Signed-off-by: Willem de Bruijn <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/dev.c

···
 	}
 
 	if (pt_prev) {
-		if (unlikely(skb_orphan_frags_rx(skb, GFP_ATOMIC)))
-			goto drop;
 		*ppt_prev = pt_prev;
 	} else {
 drop:

net/ipv4/ip_input.c

···
 
 static int ip_local_deliver_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
+	if (unlikely(skb_orphan_frags_rx(skb, GFP_ATOMIC))) {
+		__IP_INC_STATS(net, IPSTATS_MIB_INDISCARDS);
+		kfree_skb_reason(skb, SKB_DROP_REASON_NOMEM);
+		return 0;
+	}
+
 	skb_clear_delivery_time(skb);
 	__skb_pull(skb, skb_network_header_len(skb));
 

net/ipv6/ip6_input.c

···
 
 static int ip6_input_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
+	if (unlikely(skb_orphan_frags_rx(skb, GFP_ATOMIC))) {
+		__IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
+				IPSTATS_MIB_INDISCARDS);
+		kfree_skb_reason(skb, SKB_DROP_REASON_NOMEM);
+		return 0;
+	}
+
 	skb_clear_delivery_time(skb);
 	ip6_protocol_deliver_rcu(net, skb, 0, false);