Merge branch 'do-not-allow-adding-routes-if-disable_ipv6-is-enabled'

Lorenzo Bianconi says:

====================
do not allow adding routes if disable_ipv6 is enabled

Do not allow userspace to add static ipv6 routes if disable_ipv6 is enabled.
Update disable_ipv6 documentation according to that change

Changes since v1:
- added an extack message telling the user that IPv6 is disabled on the nexthop
  device
- rebased on-top of net-next
====================
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1703,7 +1703,9 @@ disable_ipv6 - BOOLEAN
 	interface and start Duplicate Address Detection, if necessary.
 
 	When this value is changed from 0 to 1 (IPv6 is being disabled),
-	it will dynamically delete all address on the given interface.
+	it will dynamically delete all addresses and routes on the given
+	interface. From now on it will not possible to add addresses/routes
+	to the selected interface.
 
 accept_dad - INTEGER
 	Whether to accept DAD (Duplicate Address Detection).

net/ipv6/route.c

@@ -2917,6 +2917,12 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg,
 	if (!dev)
 		goto out;
 
+	if (idev->cnf.disable_ipv6) {
+		NL_SET_ERR_MSG(extack, "IPv6 is disabled on nexthop device");
+		err = -EACCES;
+		goto out;
+	}
+
 	if (!(dev->flags & IFF_UP)) {
 		NL_SET_ERR_MSG(extack, "Nexthop device is not up");
 		err = -ENETDOWN;