net: cgroup: fix out of bounds accesses

dev->priomap is allocated by extend_netdev_table() called from
update_netdev_tables().
And this is only called if write_priomap() is called.

But if write_priomap() is not called, it seems we can have out of bounds
accesses in cgrp_destroy(), read_priomap() & skb_update_prio()

With help from Gao Feng

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2444,8 +2444,12 @@ static void skb_update_prio(struct sk_buff *skb)
 {
 	struct netprio_map *map = rcu_dereference_bh(skb->dev->priomap);
 
-	if ((!skb->priority) && (skb->sk) && map)
-		skb->priority = map->priomap[skb->sk->sk_cgrp_prioidx];
+	if (!skb->priority && skb->sk && map) {
+		unsigned int prioidx = skb->sk->sk_cgrp_prioidx;
+
+		if (prioidx < map->priomap_len)
+			skb->priority = map->priomap[prioidx];
+	}
 }
 #else
 #define skb_update_prio(skb)

net/core/netprio_cgroup.c

@@ -142,7 +142,7 @@ static void cgrp_destroy(struct cgroup *cgrp)
 	rtnl_lock();
 	for_each_netdev(&init_net, dev) {
 		map = rtnl_dereference(dev->priomap);
-		if (map)
+		if (map && cs->prioidx < map->priomap_len)
 			map->priomap[cs->prioidx] = 0;
 	}
 	rtnl_unlock();
@@ -166,7 +166,7 @@ static int read_priomap(struct cgroup *cont, struct cftype *cft,
 	rcu_read_lock();
 	for_each_netdev_rcu(&init_net, dev) {
 		map = rcu_dereference(dev->priomap);
-		priority = map ? map->priomap[prioidx] : 0;
+		priority = (map && prioidx < map->priomap_len) ? map->priomap[prioidx] : 0;
 		cb->fill(cb, dev->name, priority);
 	}
 	rcu_read_unlock();