Fix write to cloned skb in ipv6_hop_ioam()

ioam6_fill_trace_data() writes inside the skb payload without ensuring
it's writeable (e.g., not cloned). This function is called both from the
input and output path. The output path (ioam6_iptunnel) already does the
check. This commit provides a fix for the input path, inside
ipv6_hop_ioam(). It also updates ip6_parse_tlv() to refresh the network
header pointer ("nh") when returning from ipv6_hop_ioam().

Fixes: 9ee11f0f ("ipv6: ioam: Data plane support for Pre-allocated Trace")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv6/exthdrs.c

@@ -177,6 +177,8 @@ static bool ip6_parse_tlv(bool hopbyhop,
 				case IPV6_TLV_IOAM:
 					if (!ipv6_hop_ioam(skb, off))
 						return false;
+
+					nh = skb_network_header(skb);
 					break;
 				case IPV6_TLV_JUMBO:
 					if (!ipv6_hop_jumbo(skb, off))
@@ -943,6 +945,14 @@ static bool ipv6_hop_ioam(struct sk_buff *skb, int optoff)
 		if (!skb_valid_dst(skb))
 			ip6_route_input(skb);
 
+		/* About to mangle packet header */
+		if (skb_ensure_writable(skb, optoff + 2 + hdr->opt_len))
+			goto drop;
+
+		/* Trace pointer may have changed */
+		trace = (struct ioam6_trace_hdr *)(skb_network_header(skb)
+						   + optoff + sizeof(*hdr));
+
 		ioam6_fill_trace_data(skb, ns, trace, true);
 		break;
 	default: