Re: [PATCH net-next v2 4/5] bridge: Linearize skb once the ND message type is validated

br_nd_send() parses ND options from ns->opt[] and therefore needs the skb
to be linear. Commit a01aee7cafc5 ("bridge: br_nd_send: linearize skb
before parsing ND options") ensured that by linearizing inside
br_nd_send() itself.

Move the linearization up into br_is_nd_neigh_msg(), right after
ndisc_check_ns_na() has validated the message as an NS/NA. This makes a
linear buffer a property of every recognized ND message, so that this and
any future ND message handling operate on a linear skb and cannot
reintroduce that class of bug by forgetting to linearize.

Since the skb is now linear by the time br_nd_send() runs, drop the
linearization there and derive ns from the transport header set by
ndisc_check_ns_na(), instead of recomputing it from the network header.

If linearization fails under memory pressure, br_is_nd_neigh_msg() returns
NULL and the packet falls back to normal forwarding rather than being
suppressed.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Danielle Ratson <redacted>
---

Notes:
    v2:
    	* Add a comment noting that br_is_nd_neigh_msg() also linearizes the
    	  skb.

 net/bridge/br_arp_nd_proxy.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/bridge/br_arp_nd_proxy.c b/net/bridge/br_arp_nd_proxy.c
index 445c930ed59b..6b6de0eff38c 100644
--- a/net/bridge/br_arp_nd_proxy.c
+++ b/net/bridge/br_arp_nd_proxy.c

@@ -235,11 +235,17 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,
 #endif
 
 #if IS_ENABLED(CONFIG_IPV6)
+/* Validate skb as an NS/NA and linearize it for br_nd_send()'s ND
+ * option parsing; returns the nd_msg, or NULL on failure.
+ */
 struct nd_msg *br_is_nd_neigh_msg(struct sk_buff *skb)
 {
 	if (ndisc_check_ns_na(skb))
 		return NULL;
 
+	if (skb_linearize(skb))
+		return NULL;
+
 	return (struct nd_msg *)skb_transport_header(skb);
 }
 

@@ -259,7 +265,7 @@ static void br_nd_send(struct net_bridge *br, struct net_bridge_port *p,
 	bool dad;
 	u16 pvid;
 
-	if (!dev || skb_linearize(request))
+	if (!dev)
 		return;
 
 	len = LL_RESERVED_SPACE(dev) + sizeof(struct ipv6hdr) +

@@ -276,8 +282,7 @@ static void br_nd_send(struct net_bridge *br, struct net_bridge_port *p,
 	skb_set_mac_header(reply, 0);
 
 	daddr = eth_hdr(request)->h_source;
-	ns = (struct nd_msg *)(skb_network_header(request) +
-			       sizeof(struct ipv6hdr));
+	ns = (struct nd_msg *)skb_transport_header(request);
 
 	/* Do we need option processing ? */
 	ns_olen = request->len - (skb_network_offset(request) +

-- 
2.54.0