packet: in packet_do_bind, test fanout with bind_lock held

From: Willem de Bruijn <[email protected]>

Once a socket has po->fanout set, it remains a member of the group until it is destroyed. The prot_hook must be constant and identical across sockets in the group.

If fanout_add races with packet_do_bind between the test of po->fanout and taking the lock, the bind call may make type or dev inconsistent with that of the fanout group.

Hold po->bind_lock when testing po->fanout to avoid this race.

I had to introduce artificial delay (local_bh_enable) to actually observe the race.

---
 net/packet/af_packet.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

index 1da0851f51f2..bec01a3daf5b 100644
@@ -3071,13 +3071,15 @@ static int packet_do_bind(struct sock *sk, const char *name, int ifindex,
 	int ret = 0;
 	bool unlisted = false;
 
-	if (po->fanout)
-		return -EINVAL;
-
 	lock_sock(sk);
 	spin_lock(&po->bind_lock);
 	rcu_read_lock();
 
+	if (po->fanout) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
+
 	if (name) {
 		dev = dev_get_by_name_rcu(sock_net(sk), name);
 		if (!dev) {

2.14.1.821.g8fa685d3b7-goog

From: Willem de Bruijn <[email protected]>
Date: Tue, 26 Sep 2017 12:19:37 -0400

Applied and queued up for -stable.