rtnetlink: require unique netns identifier

Since we've added support for IFLA_IF_NETNSID for RTM_{DEL,GET,SET,NEW}LINK
it is possible for userspace to send us requests with three different
properties to identify a target network namespace. This affects at least
RTM_{NEW,SET}LINK. Each of them could potentially refer to a different
network namespace which is confusing. For legacy reasons the kernel will
pick the IFLA_NET_NS_PID property first and then look for the
IFLA_NET_NS_FD property but there is no reason to extend this type of
behavior to network namespace ids. The regression potential is quite
minimal since the rtnetlink requests in question either won't allow
IFLA_IF_NETNSID requests before 4.16 is out (RTM_{NEW,SET}LINK) or don't
support IFLA_NET_NS_{PID,FD} (RTM_{DEL,GET}LINK) in the first place.

Signed-off-by: Christian Brauner <[e-mail]>
Acked-by: Jiri Benc <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/rtnetlink.c

···
 	return net;
 }
 
+/* Verify that rtnetlink requests do not pass additional properties
+ * potentially referring to different network namespaces.
+ */
+static int rtnl_ensure_unique_netns(struct nlattr *tb[],
+				    struct netlink_ext_ack *extack,
+				    bool netns_id_only)
+{
+
+	if (netns_id_only) {
+		if (!tb[IFLA_NET_NS_PID] && !tb[IFLA_NET_NS_FD])
+			return 0;
+
+		NL_SET_ERR_MSG(extack, "specified netns attribute not supported");
+		return -EOPNOTSUPP;
+	}
+
+	if (tb[IFLA_IF_NETNSID] && (tb[IFLA_NET_NS_PID] || tb[IFLA_NET_NS_FD]))
+		goto invalid_attr;
+
+	if (tb[IFLA_NET_NS_PID] && (tb[IFLA_IF_NETNSID] || tb[IFLA_NET_NS_FD]))
+		goto invalid_attr;
+
+	if (tb[IFLA_NET_NS_FD] && (tb[IFLA_IF_NETNSID] || tb[IFLA_NET_NS_PID]))
+		goto invalid_attr;
+
+	return 0;
+
+invalid_attr:
+	NL_SET_ERR_MSG(extack, "multiple netns identifying attributes specified");
+	return -EINVAL;
+}
+
 static int validate_linkmsg(struct net_device *dev, struct nlattr *tb[])
 {
 	if (dev) {
···
 	if (err < 0)
 		goto errout;
 
+	err = rtnl_ensure_unique_netns(tb, extack, false);
+	if (err < 0)
+		goto errout;
+
 	if (tb[IFLA_IFNAME])
 		nla_strlcpy(ifname, tb[IFLA_IFNAME], IFNAMSIZ);
 	else
···
 	int netnsid = -1;
 
 	err = nlmsg_parse(nlh, sizeof(*ifm), tb, IFLA_MAX, ifla_policy, extack);
+	if (err < 0)
+		return err;
+
+	err = rtnl_ensure_unique_netns(tb, extack, true);
 	if (err < 0)
 		return err;
 
···
 replay:
 #endif
 	err = nlmsg_parse(nlh, sizeof(*ifm), tb, IFLA_MAX, ifla_policy, extack);
+	if (err < 0)
+		return err;
+
+	err = rtnl_ensure_unique_netns(tb, extack, false);
 	if (err < 0)
 		return err;
 
···
 	u32 ext_filter_mask = 0;
 
 	err = nlmsg_parse(nlh, sizeof(*ifm), tb, IFLA_MAX, ifla_policy, extack);
+	if (err < 0)
+		return err;
+
+	err = rtnl_ensure_unique_netns(tb, extack, true);
 	if (err < 0)
 		return err;