rtnetlink: add missing netlink_ns_capable() check for peer netns

rtnl_newlink() lacks a CAP_NET_ADMIN capability check on the peer
network namespace when creating paired devices (veth, vxcan,
netkit). This allows an unprivileged user with a user namespace
to create interfaces in arbitrary network namespaces, including
init_net.

Add a netlink_ns_capable() check for CAP_NET_ADMIN in the peer
namespace before allowing device creation to proceed.

Fixes: 81adee4 ("net: Support specifying the network namespace upon device creation.")
Signed-off-by: Nikolaos Gkarlis <nickgarlis@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260402181432.4126920-1-nickgarlis@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: nickgarlis

net/core/rtnetlink.c

@@ -3894,28 +3894,42 @@ static int rtnl_newlink_create(struct sk_buff *skb, struct ifinfomsg *ifm,
 	goto out;
 }
 
-static struct net *rtnl_get_peer_net(const struct rtnl_link_ops *ops,
+static struct net *rtnl_get_peer_net(struct sk_buff *skb,
+				     const struct rtnl_link_ops *ops,
 				     struct nlattr *tbp[],
 				     struct nlattr *data[],
 				     struct netlink_ext_ack *extack)
 {
-	struct nlattr *tb[IFLA_MAX + 1];
+	struct nlattr *tb[IFLA_MAX + 1], **attrs;
+	struct net *net;
 	int err;
 
-	if (!data || !data[ops->peer_type])
-		return rtnl_link_get_net_ifla(tbp);
-
-	err = rtnl_nla_parse_ifinfomsg(tb, data[ops->peer_type], extack);
-	if (err < 0)
-		return ERR_PTR(err);
-
-	if (ops->validate) {
-		err = ops->validate(tb, NULL, extack);
+	if (!data || !data[ops->peer_type]) {
+		attrs = tbp;
+	} else {
+		err = rtnl_nla_parse_ifinfomsg(tb, data[ops->peer_type], extack);
 		if (err < 0)
 			return ERR_PTR(err);
+
+		if (ops->validate) {
+			err = ops->validate(tb, NULL, extack);
+			if (err < 0)
+				return ERR_PTR(err);
+		}
+
+		attrs = tb;
 	}
 
-	return rtnl_link_get_net_ifla(tb);
+	net = rtnl_link_get_net_ifla(attrs);
+	if (IS_ERR_OR_NULL(net))
+		return net;
+
+	if (!netlink_ns_capable(skb, net->user_ns, CAP_NET_ADMIN)) {
+		put_net(net);
+		return ERR_PTR(-EPERM);
+	}
+
+	return net;
 }
 
 static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
@@ -4054,7 +4068,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		}
 
 		if (ops->peer_type) {
-			peer_net = rtnl_get_peer_net(ops, tb, data, extack);
+			peer_net = rtnl_get_peer_net(skb, ops, tb, data, extack);
 			if (IS_ERR(peer_net)) {
 				ret = PTR_ERR(peer_net);
 				goto put_ops;