ipv6: Fixed source specific default route handling.

If there are only IPv6 source specific default routes present, the
host gets -ENETUNREACH on e.g. connect() because ip6_dst_lookup_tail
calls ip6_route_output first, and given source address any, it fails,
and ip6_route_get_saddr is never called.

The change is to use the ip6_route_get_saddr, even if the initial
ip6_route_output fails, and then doing ip6_route_output _again_ after
we have appropriate source address available.

Note that this is '99% fix' to the problem; a correct fix would be to
do route lookups only within addrconf.c when picking a source address,
and never call ip6_route_output before source address has been
populated.
Signed-off-by: Markus Stenberg <markus.stenberg@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -886,21 +886,44 @@ static int ip6_dst_lookup_tail(struct sock *sk,
 #endif
 	int err;
 
-	if (!*dst)
-		*dst = ip6_route_output(net, sk, fl6);
-
-	err = (*dst)->error;
-	if (err)
-		goto out_err_release;
+	/* The correct way to handle this would be to do
+	 * ip6_route_get_saddr, and then ip6_route_output; however,
+	 * the route-specific preferred source forces the
+	 * ip6_route_output call _before_ ip6_route_get_saddr.
+	 *
+	 * In source specific routing (no src=any default route),
+	 * ip6_route_output will fail given src=any saddr, though, so
+	 * that's why we try it again later.
+	 */
+	if (ipv6_addr_any(&fl6->saddr) && (!*dst || !(*dst)->error)) {
+		struct rt6_info *rt;
+		bool had_dst = *dst != NULL;
 
-	if (ipv6_addr_any(&fl6->saddr)) {
-		struct rt6_info *rt = (struct rt6_info *) *dst;
+		if (!had_dst)
+			*dst = ip6_route_output(net, sk, fl6);
+		rt = (*dst)->error ? NULL : (struct rt6_info *)*dst;
 		err = ip6_route_get_saddr(net, rt, &fl6->daddr,
 					  sk ? inet6_sk(sk)->srcprefs : 0,
 					  &fl6->saddr);
 		if (err)
 			goto out_err_release;
+
+		/* If we had an erroneous initial result, pretend it
+		 * never existed and let the SA-enabled version take
+		 * over.
+		 */
+		if (!had_dst && (*dst)->error) {
+			dst_release(*dst);
+			*dst = NULL;
 		}
+	}
+
+	if (!*dst)
+		*dst = ip6_route_output(net, sk, fl6);
+
+	err = (*dst)->error;
+	if (err)
+		goto out_err_release;
 
 #ifdef CONFIG_IPV6_OPTIMISTIC_DAD
 	/*

net/ipv6/route.c

@@ -2245,9 +2245,10 @@ int ip6_route_get_saddr(struct net *net,
 			unsigned int prefs,
 			struct in6_addr *saddr)
 {
-	struct inet6_dev *idev = ip6_dst_idev((struct dst_entry *)rt);
+	struct inet6_dev *idev =
+		rt ? ip6_dst_idev((struct dst_entry *)rt) : NULL;
 	int err = 0;
-	if (rt->rt6i_prefsrc.plen)
+	if (rt && rt->rt6i_prefsrc.plen)
 		*saddr = rt->rt6i_prefsrc.addr;
 	else
 		err = ipv6_dev_get_saddr(net, idev ? idev->dev : NULL,