ipv6: don't reject link-local nexthop on other interface

48ed7b26faa7 ("ipv6: reject locally assigned nexthop addresses") is too
strict; it rejects following corner-case:

ip -6 route add default via fe80::1:2:3 dev eth1

[ where fe80::1:2:3 is assigned to a local interface, but not eth1 ]

Fix this by restricting search to given device if nh is linklocal.

Joint work with Hannes Frederic Sowa.

Fixes: 48ed7b26faa7 ("ipv6: reject locally assigned nexthop addresses")
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1831,6 +1831,7 @@ int ip6_route_add(struct fib6_config *cfg)
 		int gwa_type;
 
 		gw_addr = &cfg->fc_gateway;
+		gwa_type = ipv6_addr_type(gw_addr);
 
 		/* if gw_addr is local we will fail to detect this in case
 		 * address is still TENTATIVE (DAD in progress). rt6_lookup()
@@ -1838,11 +1839,12 @@ int ip6_route_add(struct fib6_config *cfg)
 		 * prefix route was assigned to, which might be non-loopback.
 		 */
 		err = -EINVAL;
-		if (ipv6_chk_addr_and_flags(net, gw_addr, NULL, 0, 0))
+		if (ipv6_chk_addr_and_flags(net, gw_addr,
+					    gwa_type & IPV6_ADDR_LINKLOCAL ?
+					    dev : NULL, 0, 0))
 			goto out;
 
 		rt->rt6i_gateway = *gw_addr;
-		gwa_type = ipv6_addr_type(gw_addr);
 
 		if (gwa_type != (IPV6_ADDR_LINKLOCAL|IPV6_ADDR_UNICAST)) {
 			struct rt6_info *grt;