linux-kernel - [PATCH 3.16 395/410] batman-adv: fix packet loss for broadcasted DHCP packets to a server

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Date:   Thu, 07 Jun 2018 15:05:21 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Linus Lüssing" <linus.luessing@...3.blue>,
        "Sven Eckelmann" <sven@...fation.org>,
        "Simon Wunderlich" <sw@...onwunderlich.de>
Subject: [PATCH 3.16 395/410] batman-adv: fix packet loss for broadcasted
 DHCP packets to a server

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Lüssing <linus.luessing@...3.blue>

commit a752c0a4524889cdc0765925258fd1fd72344100 upstream.

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: be7af5cf9cae ("batman-adv: refactoring gateway handling code")
Signed-off-by: Linus Lüssing <linus.luessing@...3.blue>
Signed-off-by: Sven Eckelmann <sven@...fation.org>
Signed-off-by: Simon Wunderlich <sw@...onwunderlich.de>
[bwh: Backported to 3.16: Drop redundant change to initialisation]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/net/batman-adv/gateway_client.c
+++ b/net/batman-adv/gateway_client.c
@@ -804,6 +804,9 @@ bool batadv_gw_out_of_range(struct batad

 	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)