net: bridge: Prevent unicast ARP/NS packets from being suppressed by bridge

When Proxy ARP or ARP/ND suppression are enabled, ARP/NS packets can be
handled by bridge in br_do_proxy_suppress_arp()/br_do_suppress_nd().
For broadcast packets, they are replied by bridge, but later they are not
flooded. Currently, unicast packets are replied by bridge when suppression
is enabled, and they are also forwarded, which results two replicas of
ARP reply/NA - one from the bridge and second from the target.

RFC 1122 describes use case for unicat ARP packets - "unicast poll" -
actively poll the remote host by periodically sending a point-to-point ARP
request to it, and delete the entry if no ARP reply is received from N
successive polls.

The purpose of ARP/ND suppression is to reduce flooding in the broadcast
domain. If a host is sending a unicast ARP/NS, then it means it already
knows the address and the switches probably know it as well and there
will not be any flooding.

In addition, the use case of unicast ARP/NS is to poll a specific host,
so it does not make sense to have the switch answer on behalf of the host.

According to RFC 9161:
"A PE SHOULD reply to broadcast/multicast address resolution messages,
i.e., ARP Requests, ARP probes, NS messages, as well as DAD NS messages.
An ARP probe is an ARP Request constructed with an all-zero sender IP
address that may be used by hosts for IPv4 Address Conflict Detection as
specified in [RFC5227]. A PE SHOULD NOT reply to unicast address resolution
requests (for instance, NUD NS messages)."

Forward such requests and prevent the bridge from replying to them.

Reported-by: Denis Yulevych <denisyu@nvidia.com>
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/6bf745a149ddfe5e6be8da684a63aa574a326f8d.1744123493.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Amit Cohen

net/bridge/br_arp_nd_proxy.c

@@ -160,6 +160,9 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,
 	if (br_opt_get(br, BROPT_NEIGH_SUPPRESS_ENABLED)) {
 		if (br_is_neigh_suppress_enabled(p, vid))
 			return;
+		if (is_unicast_ether_addr(eth_hdr(skb)->h_dest) &&
+		    parp->ar_op == htons(ARPOP_REQUEST))
+			return;
 		if (parp->ar_op != htons(ARPOP_RREQUEST) &&
 		    parp->ar_op != htons(ARPOP_RREPLY) &&
 		    (ipv4_is_zeronet(sip) || sip == tip)) {
@@ -410,6 +413,10 @@ void br_do_suppress_nd(struct sk_buff *skb, struct net_bridge *br,
 	if (br_is_neigh_suppress_enabled(p, vid))
 		return;
 
+	if (is_unicast_ether_addr(eth_hdr(skb)->h_dest) &&
+	    msg->icmph.icmp6_type == NDISC_NEIGHBOUR_SOLICITATION)
+		return;
+
 	if (msg->icmph.icmp6_type == NDISC_NEIGHBOUR_ADVERTISEMENT &&
 	    !msg->icmph.icmp6_solicited) {
 		/* prevent flooding to neigh suppress ports */