ipv6: fix reachability confirmation with proxy_ndp

When proxying IPv6 NDP requests, the adverts to the initial multicast
solicits are correct and working.  On the other hand, when later a
reachability confirmation is requested (on unicast), no reply is sent.

This causes the neighbor entry expiring on the sending node, which is
mostly a non-issue, as a new multicast request is sent.  There are
routers, where the multicast requests are intentionally delayed, and in
these environments the current implementation causes periodic packet
loss for the proxied endpoints.

The root cause is the erroneous decrease of the hop limit, as this
is checked in ndisc.c and no answer is generated when it's 254 instead
of the correct 255.

Cc: stable@vger.kernel.org
Fixes: 46c7655f ("ipv6: decrease hop limit counter in ip6_forward()")
Signed-off-by: Gergely Risko <gergely.risko@gmail.com>
Tested-by: Gergely Risko <gergely.risko@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -547,7 +547,20 @@ int ip6_forward(struct sk_buff *skb)
 	    pneigh_lookup(&nd_tbl, net, &hdr->daddr, skb->dev, 0)) {
 		int proxied = ip6_forward_proxy_check(skb);
 		if (proxied > 0) {
-			hdr->hop_limit--;
+			/* It's tempting to decrease the hop limit
+			 * here by 1, as we do at the end of the
+			 * function too.
+			 *
+			 * But that would be incorrect, as proxying is
+			 * not forwarding.  The ip6_input function
+			 * will handle this packet locally, and it
+			 * depends on the hop limit being unchanged.
+			 *
+			 * One example is the NDP hop limit, that
+			 * always has to stay 255, but other would be
+			 * similar checks around RA packets, where the
+			 * user can even change the desired limit.
+			 */
 			return ip6_input(skb);
 		} else if (proxied < 0) {
 			__IP6_INC_STATS(net, idev, IPSTATS_MIB_INDISCARDS);