Re: [PATCH net-next 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>
---
  net/bridge/br_arp_nd_proxy.c | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/bridge/br_arp_nd_proxy.c b/net/bridge/br_arp_nd_proxy.c
index 445c930ed59b..46779d9fad61 100644
--- a/net/bridge/br_arp_nd_proxy.c
+++ b/net/bridge/br_arp_nd_proxy.c

@@ -240,6 +240,9 @@ 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 +262,7 @@ 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 +279,7 @@ 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) +