net: hsr: prevent creation of HSR device with slaves from another netns

HSR/PRP driver does not handle correctly having slaves/interlink devices
in a different net namespace. Currently, it is possible to create a HSR
link in a different net namespace than the slaves/interlink with the
following command:

 ip link add hsr0 netns hsr-ns type hsr slave1 eth1 slave2 eth2

As there is no use-case on supporting this scenario, enforce that HSR
device link matches netns defined by IFLA_LINK_NETNSID.

The iproute2 command mentioned above will throw the following error:

 Error: hsr: HSR slaves/interlink must be on the same net namespace than HSR link.

Fixes: f421436 ("net/hsr: Add support for the High-availability Seamless Redundancy protocol (HSRv0)")
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20251020135533.9373-1-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: ffmancera

net/hsr/hsr_netlink.c

@@ -34,12 +34,18 @@ static int hsr_newlink(struct net_device *dev,
 		       struct netlink_ext_ack *extack)
 {
 	struct net *link_net = rtnl_newlink_link_net(params);
+	struct net_device *link[2], *interlink = NULL;
 	struct nlattr **data = params->data;
 	enum hsr_version proto_version;
 	unsigned char multicast_spec;
 	u8 proto = HSR_PROTOCOL_HSR;
 
-	struct net_device *link[2], *interlink = NULL;
+	if (!net_eq(link_net, dev_net(dev))) {
+		NL_SET_ERR_MSG_MOD(extack,
+				   "HSR slaves/interlink must be on the same net namespace than HSR link");
+		return -EINVAL;
+	}
+
 	if (!data) {
 		NL_SET_ERR_MSG_MOD(extack, "No slave devices specified");
 		return -EINVAL;