ipv4: not do cache for local delivery if bc_forwarding is enabled

With the topo:

    h1 ---| rp1            |
          |     route  rp3 |--- h3 (192.168.200.1)
    h2 ---| rp2            |

If rp1 bc_forwarding is set while rp2 bc_forwarding is not, after
doing "ping 192.168.200.255" on h1, then ping 192.168.200.255 on
h2, and the packets can still be forwared.

This issue was caused by the input route cache. It should only do
the cache for either bc forwarding or local delivery. Otherwise,
local delivery can use the route cache for bc forwarding of other
interfaces.

This patch is to fix it by not doing cache for local delivery if
all.bc_forwarding is enabled.

Note that we don't fix it by checking route cache local flag after
rt_cache_valid() in "local_input:" and "ip_mkroute_input", as the
common route code shouldn't be touched for bc_forwarding.

Fixes: 5cbf777cfd ("route: add support for directed broadcast forwarding")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1985,7 +1985,7 @@ static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 	u32		itag = 0;
 	struct rtable	*rth;
 	struct flowi4	fl4;
-	bool do_cache;
+	bool do_cache = true;
 
 	/* IP on this device is disabled. */
 
@@ -2062,6 +2062,9 @@ static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 	if (res->type == RTN_BROADCAST) {
 		if (IN_DEV_BFORWARD(in_dev))
 			goto make_route;
+		/* not do cache if bc_forwarding is enabled */
+		if (IPV4_DEVCONF_ALL(net, BC_FORWARDING))
+			do_cache = false;
 		goto brd_input;
 	}
 
@@ -2099,18 +2102,15 @@ brd_input:
 	RT_CACHE_STAT_INC(in_brd);
 
 local_input:
-	do_cache = false;
-	if (res->fi) {
-		if (!itag) {
-			struct fib_nh_common *nhc = FIB_RES_NHC(*res);
+	do_cache &= res->fi && !itag;
+	if (do_cache) {
+		struct fib_nh_common *nhc = FIB_RES_NHC(*res);
 
-			rth = rcu_dereference(nhc->nhc_rth_input);
-			if (rt_cache_valid(rth)) {
-				skb_dst_set_noref(skb, &rth->dst);
-				err = 0;
-				goto out;
-			}
-			do_cache = true;
+		rth = rcu_dereference(nhc->nhc_rth_input);
+		if (rt_cache_valid(rth)) {
+			skb_dst_set_noref(skb, &rth->dst);
+			err = 0;
+			goto out;
 		}
 	}