net/ipv6: Fix linklocal to global address with VRF

Example setup:
host: ip -6 addr add dev eth1 2001:db8:104::4
where eth1 is enslaved to a VRF

switch: ip -6 ro add 2001:db8:104::4/128 dev br1
where br1 only has an LLA

ping6 2001:db8:104::4
ssh 2001:db8:104::4

(NOTE: UDP works fine if the PKTINFO has the address set to the global
address and ifindex is set to the index of eth1 with a destination an
LLA).

For ICMP, icmp6_iif needs to be updated to check if skb->dev is an
L3 master. If it is then return the ifindex from rt6i_idev similar
to what is done for loopback.

For TCP, restore the original tcp_v6_iif definition which is needed in
most places and add a new tcp_v6_iif_l3_slave that considers the
l3_slave variability. This latter check is only needed for socket
lookups.

Fixes: 9ff74384600a ("net: vrf: Handle ipv6 multicast and link-local addresses")
Signed-off-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/tcp.h

···
  */
 static inline int tcp_v6_iif(const struct sk_buff *skb)
 {
+	return TCP_SKB_CB(skb)->header.h6.iif;
+}
+
+static inline int tcp_v6_iif_l3_slave(const struct sk_buff *skb)
+{
 	bool l3_slave = ipv6_l3mdev_skb(TCP_SKB_CB(skb)->header.h6.flags);
 
 	return l3_slave ? skb->skb_iif : TCP_SKB_CB(skb)->header.h6.iif;

net/ipv6/icmp.c

···
 
 	/* for local traffic to local address, skb dev is the loopback
 	 * device. Check if there is a dst attached to the skb and if so
-	 * get the real device index.
+	 * get the real device index. Same is needed for replies to a link
+	 * local address on a device enslaved to an L3 master device
 	 */
-	if (unlikely(iif == LOOPBACK_IFINDEX)) {
+	if (unlikely(iif == LOOPBACK_IFINDEX || netif_is_l3_master(skb->dev))) {
 		const struct rt6_info *rt6 = skb_rt6_info(skb);
 
 		if (rt6)

net/ipv6/tcp_ipv6.c

···
 					   &tcp_hashinfo, NULL, 0,
 					   &ipv6h->saddr,
 					   th->source, &ipv6h->daddr,
-					   ntohs(th->source), tcp_v6_iif(skb),
+					   ntohs(th->source),
+					   tcp_v6_iif_l3_slave(skb),
 					   tcp_v6_sdif(skb));
 		if (!sk1)
 			goto out;
···
 					    skb, __tcp_hdrlen(th),
 					    &ipv6_hdr(skb)->saddr, th->source,
 					    &ipv6_hdr(skb)->daddr,
-					    ntohs(th->dest), tcp_v6_iif(skb),
+					    ntohs(th->dest),
+					    tcp_v6_iif_l3_slave(skb),
 					    sdif);
 		if (sk2) {
 			struct inet_timewait_sock *tw = inet_twsk(sk);