net: fix iteration for sctp transport seq_files

The sctp transport seq_file iterators take a reference to the transport
in the ->start and ->next functions and releases the reference in the
->show function.  The preferred handling for such resources is to
release them in the subsequent ->next or ->stop function call.

Since Commit 1f4aace ("fs/seq_file.c: simplify seq_file iteration
code and interface") there is no guarantee that ->show will be called
after ->next, so this function can now leak references.

So move the sctp_transport_put() call to ->next and ->stop.

Fixes: 1f4aace ("fs/seq_file.c: simplify seq_file iteration code and interface")
Reported-by: Xin Long <[email protected]>
Signed-off-by: NeilBrown <[email protected]>
Acked-by: Marcelo Ricardo Leitner <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

net/sctp/proc.c

@@ -215,13 +215,25 @@ static void sctp_transport_seq_stop(struct seq_file *seq, void *v)
 {
 	struct sctp_ht_iter *iter = seq->private;
 
+	if (v && v != SEQ_START_TOKEN) {
+		struct sctp_transport *transport = v;
+
+		sctp_transport_put(transport);
+	}
+
 	sctp_transport_walk_stop(&iter->hti);
 }
 
 static void *sctp_transport_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
 	struct sctp_ht_iter *iter = seq->private;
 
+	if (v && v != SEQ_START_TOKEN) {
+		struct sctp_transport *transport = v;
+
+		sctp_transport_put(transport);
+	}
+
 	++*pos;
 
 	return sctp_transport_get_next(seq_file_net(seq), &iter->hti);
@@ -277,8 +289,6 @@ static int sctp_assocs_seq_show(struct seq_file *seq, void *v)
 		sk->sk_rcvbuf);
 	seq_printf(seq, "\n");
 
-	sctp_transport_put(transport);
-
 	return 0;
 }
 
@@ -354,8 +364,6 @@ static int sctp_remaddr_seq_show(struct seq_file *seq, void *v)
 		seq_printf(seq, "\n");
 	}
 
-	sctp_transport_put(transport);
-
 	return 0;
 }