packet: fix races in fanout_add()

commit d199fab6 upstream.

Multiple threads can call fanout_add() at the same time.

We need to grab fanout_mutex earlier to avoid races that could
lead to one thread freeing po->rollover that was set by another thread.

Do the same in fanout_release(), for peace of mind, and to help us
finding lockdep issues earlier.

Fixes: dc99f600 ("packet: Add fanout support.")
Fixes: 0648ab70 ("packet: rollover prepare: per-socket state")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16:
 - No rollover queue stats
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/packet/af_packet.c

@@ -1451,13 +1451,16 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 		return -EINVAL;
 	}
 
+	mutex_lock(&fanout_mutex);
+
+	err = -EINVAL;
 	if (!po->running)
-		return -EINVAL;
+		goto out;
 
+	err = -EALREADY;
 	if (po->fanout)
-		return -EALREADY;
+		goto out;
 
-	mutex_lock(&fanout_mutex);
 	match = NULL;
 	list_for_each_entry(f, &fanout_list, list) {
 		if (f->id == id &&
@@ -1513,17 +1516,16 @@ static void fanout_release(struct sock *sk)
 	struct packet_sock *po = pkt_sk(sk);
 	struct packet_fanout *f;
 
-	f = po->fanout;
-	if (!f)
-		return;
-
 	mutex_lock(&fanout_mutex);
-	po->fanout = NULL;
+	f = po->fanout;
+	if (f) {
+		po->fanout = NULL;
 
-	if (atomic_dec_and_test(&f->sk_ref)) {
-		list_del(&f->list);
-		dev_remove_pack(&f->prot_hook);
-		kfree(f);
+		if (atomic_dec_and_test(&f->sk_ref)) {
+			list_del(&f->list);
+			dev_remove_pack(&f->prot_hook);
+			kfree(f);
+		}
 	}
 	mutex_unlock(&fanout_mutex);
 }