icmp: ICMPV6: Examine invoking packet for Segment Route Headers.

RFC8754 says:

ICMP error packets generated within the SR domain are sent to source
nodes within the SR domain.  The invoking packet in the ICMP error
message may contain an SRH.  Since the destination address of a packet
with an SRH changes as each segment is processed, it may not be the
destination used by the socket or application that generated the
invoking packet.

For the source of an invoking packet to process the ICMP error
message, the ultimate destination address of the IPv6 header may be
required.  The following logic is used to determine the destination
address for use by protocol-error handlers.

*  Walk all extension headers of the invoking IPv6 packet to the
   routing extension header preceding the upper-layer header.

   -  If routing header is type 4 Segment Routing Header (SRH)

      o  The SID at Segment List[0] may be used as the destination
         address of the invoking packet.

Mangle the skb so the network header points to the invoking packet
inside the ICMP packet. The seg6 helpers can then be used on the skb
to find any segment routing headers. If found, mark this fact in the
IPv6 control block of the skb, and store the offset into the packet of
the SRH. Then restore the skb back to its old state.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 30 insertions, 0 deletions

diff --git a/net/ipv6/seg6.c b/net/ipv6/seg6.c
index 5bc9bf892199..73aaabf0e966 100644
--- a/net/ipv6/seg6.c
+++ b/net/ipv6/seg6.c
@@ -104,6 +104,36 @@ struct ipv6_sr_hdr *seg6_get_srh(struct sk_buff *skb, int flags)
 	return srh;
 }
 
+/* Determine if an ICMP invoking packet contains a segment routing
+ * header.  If it does, extract the offset to the true destination
+ * address, which is in the first segment address.
+ */
+void seg6_icmp_srh(struct sk_buff *skb, struct inet6_skb_parm *opt)
+{
+	__u16 network_header = skb->network_header;
+	struct ipv6_sr_hdr *srh;
+
+	/* Update network header to point to the invoking packet
+	 * inside the ICMP packet, so we can use the seg6_get_srh()
+	 * helper.
+	 */
+	skb_reset_network_header(skb);
+
+	srh = seg6_get_srh(skb, 0);
+	if (!srh)
+		goto out;
+
+	if (srh->type != IPV6_SRCRT_TYPE_4)
+		goto out;
+
+	opt->flags |= IP6SKB_SEG6;
+	opt->srhoff = (unsigned char *)srh - skb->data;
+
+out:
+	/* Restore the network header back to the ICMP packet */
+	skb->network_header = network_header;
+}
+
 static struct genl_family seg6_genl_family;
 
 static const struct nla_policy seg6_genl_policy[SEG6_ATTR_MAX + 1] = {