ipv4: Invalidate neighbour for broadcast address upon address addition

In case user space sends a packet destined to a broadcast address when a
matching broadcast route is not configured, the kernel will create a
unicast neighbour entry that will never be resolved [1].

When the broadcast route is configured, the unicast neighbour entry will
not be invalidated and continue to linger, resulting in packets being
dropped.

Solve this by invalidating unresolved neighbour entries for broadcast
addresses after routes for these addresses are internally configured by
the kernel. This allows the kernel to create a broadcast neighbour entry
following the next route lookup.

Another possible solution that is more generic but also more complex is
to have the ARP code register a listener to the FIB notification chain
and invalidate matching neighbour entries upon the addition of broadcast
routes.

It is also possible to wave off the issue as a user space problem, but
it seems a bit excessive to expect user space to be that intimately
familiar with the inner workings of the FIB/neighbour kernel code.

[1] https://lore.kernel.org/netdev/55a04a8f-56f3-f73c-2aea-2195923f09d1@huawei.com/Reported-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/arp.h

@@ -65,6 +65,7 @@ void arp_send(int type, int ptype, __be32 dest_ip,
 	      const unsigned char *src_hw, const unsigned char *th);
 int arp_mc_map(__be32 addr, u8 *haddr, struct net_device *dev, int dir);
 void arp_ifdown(struct net_device *dev);
+int arp_invalidate(struct net_device *dev, __be32 ip, bool force);
 
 struct sk_buff *arp_create(int type, int ptype, __be32 dest_ip,
 			   struct net_device *dev, __be32 src_ip,

net/ipv4/arp.c

@@ -1116,13 +1116,18 @@ static int arp_req_get(struct arpreq *r, struct net_device *dev)
 	return err;
 }
 
-static int arp_invalidate(struct net_device *dev, __be32 ip)
+int arp_invalidate(struct net_device *dev, __be32 ip, bool force)
 {
 	struct neighbour *neigh = neigh_lookup(&arp_tbl, &ip, dev);
 	int err = -ENXIO;
 	struct neigh_table *tbl = &arp_tbl;
 
 	if (neigh) {
+		if ((neigh->nud_state & NUD_VALID) && !force) {
+			neigh_release(neigh);
+			return 0;
+		}
+
 		if (neigh->nud_state & ~NUD_NOARP)
 			err = neigh_update(neigh, NULL, NUD_FAILED,
 					   NEIGH_UPDATE_F_OVERRIDE|
@@ -1169,7 +1174,7 @@ static int arp_req_delete(struct net *net, struct arpreq *r,
 		if (!dev)
 			return -EINVAL;
 	}
-	return arp_invalidate(dev, ip);
+	return arp_invalidate(dev, ip, true);
 }
 
 /*

net/ipv4/fib_frontend.c

@@ -1124,9 +1124,11 @@ void fib_add_ifaddr(struct in_ifaddr *ifa)
 		return;
 
 	/* Add broadcast address, if it is explicitly assigned. */
-	if (ifa->ifa_broadcast && ifa->ifa_broadcast != htonl(0xFFFFFFFF))
+	if (ifa->ifa_broadcast && ifa->ifa_broadcast != htonl(0xFFFFFFFF)) {
 		fib_magic(RTM_NEWROUTE, RTN_BROADCAST, ifa->ifa_broadcast, 32,
 			  prim, 0);
+		arp_invalidate(dev, ifa->ifa_broadcast, false);
+	}
 
 	if (!ipv4_is_zeronet(prefix) && !(ifa->ifa_flags & IFA_F_SECONDARY) &&
 	    (prefix != addr || ifa->ifa_prefixlen < 32)) {
@@ -1140,6 +1142,7 @@ void fib_add_ifaddr(struct in_ifaddr *ifa)
 		if (ifa->ifa_prefixlen < 31) {
 			fib_magic(RTM_NEWROUTE, RTN_BROADCAST, prefix | ~mask,
 				  32, prim, 0);
+			arp_invalidate(dev, prefix | ~mask, false);
 		}
 	}
 }