net: bridge: fix a bug on using a neighbour cache entry without checking its state

When handling DNAT'ed packets on a bridge device, the neighbour cache entry
from lookup was used without checking its state. It means that a cache entry
in the NUD_STALE state will be used directly instead of entering the NUD_DELAY
state to confirm the reachability of the neighbor.

This problem becomes worse after commit 2724680b ("neigh: Keep neighbour
cache entries if number of them is small enough."), since all neighbour cache
entries in the NUD_STALE state will be kept in the neighbour table as long as
the number of cache entries does not exceed the value specified in gc_thresh1.

This commit validates the state of a neighbour cache entry before using
the entry.
Signed-off-by: JianJhen Chen <kchen@synology.com>
Reviewed-by: JinLin Chen <jlchen@synology.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_netfilter_hooks.c

@@ -265,7 +265,7 @@ int br_nf_pre_routing_finish_bridge(struct net *net, struct sock *sk, struct sk_
 		struct nf_bridge_info *nf_bridge = nf_bridge_info_get(skb);
 		int ret;
 
-		if (neigh->hh.hh_len) {
+		if ((neigh->nud_state & NUD_CONNECTED) && neigh->hh.hh_len) {
 			neigh_hh_bridge(&neigh->hh, skb);
 			skb->dev = nf_bridge->physindev;
 			ret = br_handle_frame_finish(net, sk, skb);