tunnel: drop packet if ECN present with not-ECT

Linux tunnels were written before RFC6040 and therefore never
implemented the corner case of ECN getting set in the outer header
and the inner header not being ready for it.

Section 4.2.  Default Tunnel Egress Behaviour.
 o If the inner ECN field is Not-ECT, the decapsulator MUST NOT
      propagate any other ECN codepoint onwards.  This is because the
      inner Not-ECT marking is set by transports that rely on dropped
      packets as an indication of congestion and would not understand or
      respond to any other ECN codepoint [RFC4774].  Specifically:

      *  If the inner ECN field is Not-ECT and the outer ECN field is
         CE, the decapsulator MUST drop the packet.

      *  If the inner ECN field is Not-ECT and the outer ECN field is
         Not-ECT, ECT(0), or ECT(1), the decapsulator MUST forward the
         outgoing packet with the ECN field cleared to Not-ECT.

This patch moves the ECN decap logic out of the individual tunnels
into a common place.

It also adds logging to allow detecting broken systems that
set ECN bits incorrectly when tunneling (or an intermediate
router might be changing the header).

Overloads rx_frame_error to keep track of ECN related error.

Thanks to Chris Wright who caught this while reviewing the new VXLAN
tunnel.

This code was tested by injecting faulty logic in other end GRE
to send incorrectly encapsulated packets.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 76 insertions, 0 deletions

diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
index 2fa1469..aab7375 100644
--- a/include/net/inet_ecn.h
+++ b/include/net/inet_ecn.h
@@ -15,6 +15,8 @@ enum {
 	INET_ECN_MASK = 3,
 };
 
+extern int sysctl_tunnel_ecn_log;
+
 static inline int INET_ECN_is_ce(__u8 dsfield)
 {
 	return (dsfield & INET_ECN_MASK) == INET_ECN_CE;
@@ -145,4 +147,78 @@ static inline int INET_ECN_set_ce(struct sk_buff *skb)
 	return 0;
 }
 
+/*
+ * RFC 6080 4.2
+ *  To decapsulate the inner header at the tunnel egress, a compliant
+ *  tunnel egress MUST set the outgoing ECN field to the codepoint at the
+ *  intersection of the appropriate arriving inner header (row) and outer
+ *  header (column) in Figure 4
+ *
+ *      +---------+------------------------------------------------+
+ *      |Arriving |            Arriving Outer Header               |
+ *      |   Inner +---------+------------+------------+------------+
+ *      |  Header | Not-ECT | ECT(0)     | ECT(1)     |     CE     |
+ *      +---------+---------+------------+------------+------------+
+ *      | Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| <drop>(!!!)|
+ *      |  ECT(0) |  ECT(0) | ECT(0)     | ECT(1)     |     CE     |
+ *      |  ECT(1) |  ECT(1) | ECT(1) (!) | ECT(1)     |     CE     |
+ *      |    CE   |      CE |     CE     |     CE(!!!)|     CE     |
+ *      +---------+---------+------------+------------+------------+
+ *
+ *             Figure 4: New IP in IP Decapsulation Behaviour
+ *
+ *  returns 0 on success
+ *          1 if something is broken and should be logged (!!! above)
+ *          2 if packet should be dropped
+ */
+static inline int INET_ECN_decapsulate(struct sk_buff *skb,
+				       __u8 outer, __u8 inner)
+{
+	if (INET_ECN_is_not_ect(inner)) {
+		switch (outer & INET_ECN_MASK) {
+		case INET_ECN_NOT_ECT:
+			return 0;
+		case INET_ECN_ECT_0:
+		case INET_ECN_ECT_1:
+			return 1;
+		case INET_ECN_CE:
+			return 2;
+		}
+	}
+
+	if (INET_ECN_is_ce(outer))
+		INET_ECN_set_ce(skb);
+
+	return 0;
+}
+
+static inline int IP_ECN_decapsulate(const struct iphdr *oiph,
+				     struct sk_buff *skb)
+{
+	__u8 inner;
+
+	if (skb->protocol == htons(ETH_P_IP))
+		inner = ip_hdr(skb)->tos;
+	else if (skb->protocol == htons(ETH_P_IPV6))
+		inner = ipv6_get_dsfield(ipv6_hdr(skb));
+	else
+		return 0;
+
+	return INET_ECN_decapsulate(skb, oiph->tos, inner);
+}
+
+static inline int IP6_ECN_decapsulate(const struct ipv6hdr *oipv6h,
+				      struct sk_buff *skb)
+{
+	__u8 inner;
+
+	if (skb->protocol == htons(ETH_P_IP))
+		inner = ip_hdr(skb)->tos;
+	else if (skb->protocol == htons(ETH_P_IPV6))
+		inner = ipv6_get_dsfield(ipv6_hdr(skb));
+	else
+		return 0;
+
+	return INET_ECN_decapsulate(skb, ipv6_get_dsfield(oipv6h), inner);
+}
 #endif