packet: only test po->has_vnet_hdr once in packet_snd

BugLink: http://bugs.launchpad.net/bugs/1729105


[ Upstream commit da7c9561 ]

Packet socket option po->has_vnet_hdr can be updated concurrently with
other operations if no ring is attached.

Do not test the option twice in packet_snd, as the value may change in
between calls. A race on setsockopt disable may cause a packet > mtu
to be sent without having GSO options set.

Fixes: bfd5f4a3 ("packet: Add GSO/csum offload support.")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/packet/af_packet.c

@@ -1717,7 +1717,7 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 	}
 	spin_unlock(&po->bind_lock);
 
-	if (err && !refcount_read(&match->sk_ref)) {
+	if (err && !atomic_read(&match->sk_ref)) {
 		list_del(&match->list);
 		kfree(match);
 	}
@@ -2656,6 +2656,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 	int vnet_hdr_len;
 	struct packet_sock *po = pkt_sk(sk);
 	unsigned short gso_type = 0;
+	bool has_vnet_hdr = false;
 	int hlen, tlen, linear;
 	int extra_len = 0;
 	ssize_t n;
@@ -2743,6 +2744,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 				goto out_unlock;
 
 		}
+		has_vnet_hdr = true;
 	}
 
 	if (unlikely(sock_flag(sk, SOCK_NOFCS))) {
@@ -2802,7 +2804,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 
 	packet_pick_tx_queue(dev, skb);
 
-	if (po->has_vnet_hdr) {
+	if (has_vnet_hdr) {
 		if (vnet_hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
 			u16 s = __virtio16_to_cpu(vio_le(), vnet_hdr.csum_start);
 			u16 o = __virtio16_to_cpu(vio_le(), vnet_hdr.csum_offset);