team: check return value of team_get_port_by_index_rcu() for NULL

[ Upstream commit 76c455de ]

team_get_port_by_index_rcu() might return NULL due to race between port
removal and skb tx path. Panic is easily triggeable when txing packets
and adding/removing port in a loop.

introduced by commit 3d249d4c "net: introduce ethernet teaming device"
and commit 753f9939 "team: introduce random mode" (for random mode)
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/team/team_mode_roundrobin.c

@@ -52,6 +52,8 @@ static bool rr_transmit(struct team *team, struct sk_buff *skb)
 
 	port_index = rr_priv(team)->sent_packets++ % team->port_count;
 	port = team_get_port_by_index_rcu(team, port_index);
+	if (unlikely(!port))
+		goto drop;
 	port = __get_first_port_up(team, port);
 	if (unlikely(!port))
 		goto drop;