[PATCH net-next v2 3/5] bridge: Validate NS/NA messages using ndisc_check_ns_na()

[Danielle Ratson <danieller@nvidia.com>]

The bridge performs neighbor suppression by snooping NS/NA messages, but
previously only checked the ICMPv6 type and code. This leaves it open to
acting on malformed or spoofed packets that any RFC-compliant node should
reject.

Wire br_is_nd_neigh_msg() into the new ndisc_check_ns_na() helper, which
enforces the full RFC 4861 section 7.1.1/7.1.2 receive validation:
hop limit of 255, valid checksum, correct code, and type-specific rules
(NS target not multicast; NA solicited flag clear for multicast
destinations).

MLD messages are already validated by ipv6_mc_check_mld() before the
bridge acts on them; this brings NS/NA to the same standard.

As a side effect, the skb parameter of br_is_nd_neigh_msg() changes from
const to non-const, since ndisc_check_ns_na() may reallocate the skb head
via pskb_may_pull() and sets the transport header. The returned pointer is
now derived from skb_transport_header() rather than a direct cast.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
---
 net/bridge/br_arp_nd_proxy.c | 11 ++++-------
 net/bridge/br_private.h      |  2 +-
 2 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/net/bridge/br_arp_nd_proxy.c b/net/bridge/br_arp_nd_proxy.c
index db08c3272001..445c930ed59b 100644
--- a/net/bridge/br_arp_nd_proxy.c
+++ b/net/bridge/br_arp_nd_proxy.c
@@ -19,6 +19,7 @@
 #include <net/addrconf.h>
 #if IS_ENABLED(CONFIG_IPV6)
 #include <net/ip6_checksum.h>
+#include <net/ndisc.h>
 #endif
 
 #include "br_private.h"
@@ -234,16 +235,12 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,
 #endif
 
 #if IS_ENABLED(CONFIG_IPV6)
-struct nd_msg *br_is_nd_neigh_msg(const struct sk_buff *skb)
+struct nd_msg *br_is_nd_neigh_msg(struct sk_buff *skb)
 {
-	struct nd_msg *m = (struct nd_msg *)(ipv6_hdr(skb) + 1);
-
-	if (m->icmph.icmp6_code != 0 ||
-	    (m->icmph.icmp6_type != NDISC_NEIGHBOUR_SOLICITATION &&
-	     m->icmph.icmp6_type != NDISC_NEIGHBOUR_ADVERTISEMENT))
+	if (ndisc_check_ns_na(skb))
 		return NULL;
 
-	return m;
+	return (struct nd_msg *)skb_transport_header(skb);
 }
 
 static void br_nd_send(struct net_bridge *br, struct net_bridge_port *p,
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index e31439cb4420..d337b1cfb980 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -2367,7 +2367,7 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,
 			      u16 vid, struct net_bridge_port *p);
 void br_do_suppress_nd(struct sk_buff *skb, struct net_bridge *br,
 		       u16 vid, struct net_bridge_port *p, struct nd_msg *msg);
-struct nd_msg *br_is_nd_neigh_msg(const struct sk_buff *skb);
+struct nd_msg *br_is_nd_neigh_msg(struct sk_buff *skb);
 bool br_is_neigh_suppress_enabled(const struct net_bridge_port *p, u16 vid);
 bool br_is_neigh_forward_grat_enabled(const struct net_bridge_port *p, u16 vid);
 #endif
-- 
2.54.0