sctp: hold transport before accessing its asoc in sctp_transport_get_next

As Marcelo noticed, in sctp_transport_get_next, it is iterating over
transports but then also accessing the association directly, without
checking any refcnts before that, which can cause an use-after-free
Read.

So fix it by holding transport before accessing the association. With
that, sctp_transport_hold calls can be removed in the later places.

Fixes: 626d16f5 ("sctp: export some apis or variables for sctp_diag and reuse some for proc")
Reported-by: syzbot+fe62a0c9aa6a85c6de16@syzkaller.appspotmail.com
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Acked-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/proc.c

@@ -264,8 +264,6 @@ static int sctp_assocs_seq_show(struct seq_file *seq, void *v)
 	}
 
 	transport = (struct sctp_transport *)v;
-	if (!sctp_transport_hold(transport))
-		return 0;
 	assoc = transport->asoc;
 	epb = &assoc->base;
 	sk = epb->sk;
@@ -322,8 +320,6 @@ static int sctp_remaddr_seq_show(struct seq_file *seq, void *v)
 	}
 
 	transport = (struct sctp_transport *)v;
-	if (!sctp_transport_hold(transport))
-		return 0;
 	assoc = transport->asoc;
 
 	list_for_each_entry_rcu(tsp, &assoc->peer.transport_addr_list,

net/sctp/socket.c

@@ -5005,9 +5005,14 @@ struct sctp_transport *sctp_transport_get_next(struct net *net,
 			break;
 		}
 
+		if (!sctp_transport_hold(t))
+			continue;
+
 		if (net_eq(sock_net(t->asoc->base.sk), net) &&
 		    t->asoc->peer.primary_path == t)
 			break;
+
+		sctp_transport_put(t);
 	}
 
 	return t;
@@ -5017,13 +5022,18 @@ struct sctp_transport *sctp_transport_get_idx(struct net *net,
 					      struct rhashtable_iter *iter,
 					      int pos)
 {
-	void *obj = SEQ_START_TOKEN;
+	struct sctp_transport *t;
 
-	while (pos && (obj = sctp_transport_get_next(net, iter)) &&
-	       !IS_ERR(obj))
-		pos--;
+	if (!pos)
+		return SEQ_START_TOKEN;
 
-	return obj;
+	while ((t = sctp_transport_get_next(net, iter)) && !IS_ERR(t)) {
+		if (!--pos)
+			break;
+		sctp_transport_put(t);
+	}
+
+	return t;
 }
 
 int sctp_for_each_endpoint(int (*cb)(struct sctp_endpoint *, void *),
@@ -5082,8 +5092,6 @@ int sctp_for_each_transport(int (*cb)(struct sctp_transport *, void *),
 
 	tsp = sctp_transport_get_idx(net, &hti, *pos + 1);
 	for (; !IS_ERR_OR_NULL(tsp); tsp = sctp_transport_get_next(net, &hti)) {
-		if (!sctp_transport_hold(tsp))
-			continue;
 		ret = cb(tsp, p);
 		if (ret)
 			break;