ipv6: implement dataplane support for rthdr type 4 (Segment Routing Header)

Implement minimal support for processing of SR-enabled packets
as described in
https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header-02.

This patch implements the following operations:
- Intermediate segment endpoint: incrementation of active segment and rerouting.
- Egress for SR-encapsulated packets: decapsulation of outer IPv6 header + SRH
  and routing of inner packet.
- Cleanup flag support for SR-inlined packets: removal of SRH if we are the
  penultimate segment endpoint.

A per-interface sysctl seg6_enabled is provided, to accept/deny SR-enabled
packets. Default is deny.

This patch does not provide support for HMAC-signed packets.

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 175 insertions, 0 deletions

diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index 139ceb6..b8ba396 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -47,6 +47,8 @@
 #if IS_ENABLED(CONFIG_IPV6_MIP6)
 #include <net/xfrm.h>
 #endif
+#include <linux/seg6.h>
+#include <net/seg6.h>
 
 #include <linux/uaccess.h>
 
@@ -286,6 +288,175 @@ static int ipv6_destopt_rcv(struct sk_buff *skb)
 	return -1;
 }
 
+static void seg6_update_csum(struct sk_buff *skb)
+{
+	struct ipv6_sr_hdr *hdr;
+	struct in6_addr *addr;
+	__be32 from, to;
+
+	/* srh is at transport offset and seg_left is already decremented
+	 * but daddr is not yet updated with next segment
+	 */
+
+	hdr = (struct ipv6_sr_hdr *)skb_transport_header(skb);
+	addr = hdr->segments + hdr->segments_left;
+
+	hdr->segments_left++;
+	from = *(__be32 *)hdr;
+
+	hdr->segments_left--;
+	to = *(__be32 *)hdr;
+
+	/* update skb csum with diff resulting from seg_left decrement */
+
+	update_csum_diff4(skb, from, to);
+
+	/* compute csum diff between current and next segment and update */
+
+	update_csum_diff16(skb, (__be32 *)(&ipv6_hdr(skb)->daddr),
+			   (__be32 *)addr);
+}
+
+static int ipv6_srh_rcv(struct sk_buff *skb)
+{
+	struct inet6_skb_parm *opt = IP6CB(skb);
+	struct net *net = dev_net(skb->dev);
+	struct ipv6_sr_hdr *hdr;
+	struct inet6_dev *idev;
+	struct in6_addr *addr;
+	bool cleanup = false;
+	int accept_seg6;
+
+	hdr = (struct ipv6_sr_hdr *)skb_transport_header(skb);
+
+	idev = __in6_dev_get(skb->dev);
+
+	accept_seg6 = net->ipv6.devconf_all->seg6_enabled;
+	if (accept_seg6 > idev->cnf.seg6_enabled)
+		accept_seg6 = idev->cnf.seg6_enabled;
+
+	if (!accept_seg6) {
+		kfree_skb(skb);
+		return -1;
+	}
+
+looped_back:
+	if (hdr->segments_left > 0) {
+		if (hdr->nexthdr != NEXTHDR_IPV6 && hdr->segments_left == 1 &&
+		    sr_has_cleanup(hdr))
+			cleanup = true;
+	} else {
+		if (hdr->nexthdr == NEXTHDR_IPV6) {
+			int offset = (hdr->hdrlen + 1) << 3;
+
+			skb_postpull_rcsum(skb, skb_network_header(skb),
+					   skb_network_header_len(skb));
+
+			if (!pskb_pull(skb, offset)) {
+				kfree_skb(skb);
+				return -1;
+			}
+			skb_postpull_rcsum(skb, skb_transport_header(skb),
+					   offset);
+
+			skb_reset_network_header(skb);
+			skb_reset_transport_header(skb);
+			skb->encapsulation = 0;
+
+			__skb_tunnel_rx(skb, skb->dev, net);
+
+			netif_rx(skb);
+			return -1;
+		}
+
+		opt->srcrt = skb_network_header_len(skb);
+		opt->lastopt = opt->srcrt;
+		skb->transport_header += (hdr->hdrlen + 1) << 3;
+		opt->nhoff = (&hdr->nexthdr) - skb_network_header(skb);
+
+		return 1;
+	}
+
+	if (hdr->segments_left >= (hdr->hdrlen >> 1)) {
+		__IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
+				IPSTATS_MIB_INHDRERRORS);
+		icmpv6_param_prob(skb, ICMPV6_HDR_FIELD,
+				  ((&hdr->segments_left) -
+				   skb_network_header(skb)));
+		kfree_skb(skb);
+		return -1;
+	}
+
+	if (skb_cloned(skb)) {
+		if (pskb_expand_head(skb, 0, 0, GFP_ATOMIC)) {
+			__IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
+					IPSTATS_MIB_OUTDISCARDS);
+			kfree_skb(skb);
+			return -1;
+		}
+	}
+
+	hdr = (struct ipv6_sr_hdr *)skb_transport_header(skb);
+
+	hdr->segments_left--;
+	addr = hdr->segments + hdr->segments_left;
+
+	skb_push(skb, sizeof(struct ipv6hdr));
+
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		seg6_update_csum(skb);
+
+	ipv6_hdr(skb)->daddr = *addr;
+
+	if (cleanup) {
+		int srhlen = (hdr->hdrlen + 1) << 3;
+		int nh = hdr->nexthdr;
+
+		skb_pull_rcsum(skb, sizeof(struct ipv6hdr) + srhlen);
+		memmove(skb_network_header(skb) + srhlen,
+			skb_network_header(skb),
+			(unsigned char *)hdr - skb_network_header(skb));
+		skb->network_header += srhlen;
+		ipv6_hdr(skb)->nexthdr = nh;
+		ipv6_hdr(skb)->payload_len = htons(skb->len -
+						   sizeof(struct ipv6hdr));
+		skb_push_rcsum(skb, sizeof(struct ipv6hdr));
+	}
+
+	skb_dst_drop(skb);
+
+	ip6_route_input(skb);
+
+	if (skb_dst(skb)->error) {
+		dst_input(skb);
+		return -1;
+	}
+
+	if (skb_dst(skb)->dev->flags & IFF_LOOPBACK) {
+		if (ipv6_hdr(skb)->hop_limit <= 1) {
+			__IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
+					IPSTATS_MIB_INHDRERRORS);
+			icmpv6_send(skb, ICMPV6_TIME_EXCEED,
+				    ICMPV6_EXC_HOPLIMIT, 0);
+			kfree_skb(skb);
+			return -1;
+		}
+		ipv6_hdr(skb)->hop_limit--;
+
+		/* be sure that srh is still present before reinjecting */
+		if (!cleanup) {
+			skb_pull(skb, sizeof(struct ipv6hdr));
+			goto looped_back;
+		}
+		skb_set_transport_header(skb, sizeof(struct ipv6hdr));
+		IP6CB(skb)->nhoff = offsetof(struct ipv6hdr, nexthdr);
+	}
+
+	dst_input(skb);
+
+	return -1;
+}
+
 /********************************
   Routing header.
  ********************************/
@@ -326,6 +497,10 @@ static int ipv6_rthdr_rcv(struct sk_buff *skb)
 		return -1;
 	}
 
+	/* segment routing */
+	if (hdr->type == IPV6_SRCRT_TYPE_4)
+		return ipv6_srh_rcv(skb);
+
 looped_back:
 	if (hdr->segments_left == 0) {
 		switch (hdr->type) {