batman-adv: fix packet loss for broadcasted DHCP packets to a server

DHCP connectivity issues can currently occur if the following conditions
are met:

1) A DHCP packet from a client to a server
2) This packet has a multicast destination
3) This destination has a matching entry in the translation table
   (FF:FF:FF:FF:FF:FF for IPv4, 33:33:00:01:00:02/33:33:00:01:00:03
    for IPv6)
4) The orig-node determined by TT for the multicast destination
   does not match the orig-node determined by best-gateway-selection

In this case the DHCP packet will be dropped.

The "gateway-out-of-range" check is supposed to only be applied to
unicasted DHCP packets to a specific DHCP server.

In that case dropping the the unicasted frame forces the client to
retry via a broadcasted one, but now directed to the new best
gateway.

A DHCP packet with broadcast/multicast destination is already ensured to
always be delivered to the best gateway. Dropping a multicasted
DHCP packet here will only prevent completing DHCP as there is no
other fallback.

So far, it seems the unicast check was implicitly performed by
expecting the batadv_transtable_search() to return NULL for multicast
destinations. However, a multicast address could have always ended up in
the translation table and in fact is now common.

To fix this potential loss of a DHCP client-to-server packet to a
multicast address this patch adds an explicit multicast destination
check to reliably bail out of the gateway-out-of-range check for such
destinations.

The issue and fix were tested in the following three node setup:

- Line topology, A-B-C
- A: gateway client, DHCP client
- B: gateway server, hop-penalty increased: 30->60, DHCP server
- C: gateway server, code modifications to announce FF:FF:FF:FF:FF:FF

Without this patch, A would never transmit its DHCP Discover packet
due to an always "out-of-range" condition. With this patch,
a full DHCP handshake between A and B was possible again.

Fixes: be7af5cf ("batman-adv: refactoring gateway handling code")
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/gateway_client.c

@@ -746,7 +746,7 @@ bool batadv_gw_out_of_range(struct batadv_priv *bat_priv,
 {
 	struct batadv_neigh_node *neigh_curr = NULL;
 	struct batadv_neigh_node *neigh_old = NULL;
-	struct batadv_orig_node *orig_dst_node;
+	struct batadv_orig_node *orig_dst_node = NULL;
 	struct batadv_gw_node *gw_node = NULL;
 	struct batadv_gw_node *curr_gw = NULL;
 	struct batadv_neigh_ifinfo *curr_ifinfo, *old_ifinfo;
@@ -757,6 +757,9 @@ bool batadv_gw_out_of_range(struct batadv_priv *bat_priv,
 
 	vid = batadv_get_vid(skb, 0);
 
+	if (is_multicast_ether_addr(ethhdr->h_dest))
+		goto out;
+
 	orig_dst_node = batadv_transtable_search(bat_priv, ethhdr->h_source,
 						 ethhdr->h_dest, vid);
 	if (!orig_dst_node)