tcp: make sure skb is not shared before using skb_get()

IPv6 can keep a copy of SYN message using skb_get() in
tcp_v6_conn_request() so that caller wont free the skb when calling
kfree_skb() later.

Therefore TCP fast open has to clone the skb it is queuing in
child->sk_receive_queue, as all skbs consumed from receive_queue are
freed using __kfree_skb() (ie assuming skb->users == 1)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Fixes: 5b7ed0892f2af ("tcp: move fastopen functions to tcp_fastopen.c")
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_fastopen.c

@@ -134,6 +134,7 @@ static bool tcp_fastopen_create_child(struct sock *sk,
 	struct tcp_sock *tp;
 	struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
 	struct sock *child;
+	u32 end_seq;
 
 	req->num_retrans = 0;
 	req->num_timeout = 0;
@@ -185,20 +186,35 @@ static bool tcp_fastopen_create_child(struct sock *sk,
 
 	/* Queue the data carried in the SYN packet. We need to first
 	 * bump skb's refcnt because the caller will attempt to free it.
+	 * Note that IPv6 might also have used skb_get() trick
+	 * in tcp_v6_conn_request() to keep this SYN around (treq->pktopts)
+	 * So we need to eventually get a clone of the packet,
+	 * before inserting it in sk_receive_queue.
 	 *
 	 * XXX (TFO) - we honor a zero-payload TFO request for now,
 	 * (any reason not to?) but no need to queue the skb since
 	 * there is no data. How about SYN+FIN?
 	 */
-	if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq + 1) {
-		skb = skb_get(skb);
-		skb_dst_drop(skb);
-		__skb_pull(skb, tcp_hdr(skb)->doff * 4);
-		skb_set_owner_r(skb, child);
-		__skb_queue_tail(&child->sk_receive_queue, skb);
-		tp->syn_data_acked = 1;
+	end_seq = TCP_SKB_CB(skb)->end_seq;
+	if (end_seq != TCP_SKB_CB(skb)->seq + 1) {
+		struct sk_buff *skb2;
+
+		if (unlikely(skb_shared(skb)))
+			skb2 = skb_clone(skb, GFP_ATOMIC);
+		else
+			skb2 = skb_get(skb);
+
+		if (likely(skb2)) {
+			skb_dst_drop(skb2);
+			__skb_pull(skb2, tcp_hdrlen(skb));
+			skb_set_owner_r(skb2, child);
+			__skb_queue_tail(&child->sk_receive_queue, skb2);
+			tp->syn_data_acked = 1;
+		} else {
+			end_seq = TCP_SKB_CB(skb)->seq + 1;
+		}
 	}
-	tcp_rsk(req)->rcv_nxt = tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
+	tcp_rsk(req)->rcv_nxt = tp->rcv_nxt = end_seq;
 	sk->sk_data_ready(sk);
 	bh_unlock_sock(child);
 	sock_put(child);