rtnetlink: add restricted rtnl groups for ipv4 and ipv6 mroute

Add RTNLGRP_{IPV4,IPV6}_MROUTE_R as two new restricted groups for the
NETLINK_ROUTE family.
Binding to these groups specifically requires CAP_NET_ADMIN to allow
multicast of sensitive messages (e.g. mroute cache reports).

Suggested-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: Julien Gomes <[e-mail]>
Signed-off-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/rtnetlink.h

···
 #define RTNLGRP_NSID		RTNLGRP_NSID
 	RTNLGRP_MPLS_NETCONF,
 #define RTNLGRP_MPLS_NETCONF	RTNLGRP_MPLS_NETCONF
+	RTNLGRP_IPV4_MROUTE_R,
+#define RTNLGRP_IPV4_MROUTE_R	RTNLGRP_IPV4_MROUTE_R
+	RTNLGRP_IPV6_MROUTE_R,
+#define RTNLGRP_IPV6_MROUTE_R	RTNLGRP_IPV6_MROUTE_R
 	__RTNLGRP_MAX
 };
 #define RTNLGRP_MAX	(__RTNLGRP_MAX - 1)

net/core/rtnetlink.c

···
 	rtnl_unlock();
 }
 
+static int rtnetlink_bind(struct net *net, int group)
+{
+	switch (group) {
+	case RTNLGRP_IPV4_MROUTE_R:
+	case RTNLGRP_IPV6_MROUTE_R:
+		if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+			return -EPERM;
+		break;
+	}
+	return 0;
+}
+
 static int rtnetlink_event(struct notifier_block *this, unsigned long event, void *ptr)
 {
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
···
 		.input		= rtnetlink_rcv,
 		.cb_mutex	= &rtnl_mutex,
 		.flags		= NL_CFG_F_NONROOT_RECV,
+		.bind		= rtnetlink_bind,
 	};
 
 	sk = netlink_kernel_create(net, NETLINK_ROUTE, &cfg);