gtp: Require 'CAP_NET_ADMIN' when joining the gtp multicast group

The gtp generic netlink family sends notifications on its "gtp"
multicast group when a PDP context is added or deleted, and for GTP-U
echo responses. They carry the same attributes as a GETPDP reply: the
MS and peer addresses, and the TEIDs, or the TID and the flow label.

GETPDP requires CAP_NET_ADMIN, but generic netlink lets any user join a
multicast group by default, so an unprivileged user in the network
namespace of the gtp device can read all of this. Found by code
inspection.

Fix by marking the group with GENL_MCAST_CAP_NET_ADMIN, like the psample
"packets" group, and with uns-admin-perm in the gtp netlink spec. Only
users with CAP_NET_ADMIN in the user namespace owning the network
namespace can then join it.

This is still weaker than the check on GETPDP, which needs
CAP_NET_ADMIN in the initial user namespace: in a network namespace
owned by another user namespace, its root can join the group, but not
use GETPDP. Genetlink has no multicast group flag for the initial user
namespace, and ioam6 pairs its ops and its group the same way. This fix
blocks the unprivileged users.

Tested by joining the group as root, as root without CAP_NET_ADMIN, and
as an unprivileged user:

Before:
  root: joined
  root without CAP_NET_ADMIN: joined
  nobody: joined

After:
  root: joined
  root without CAP_NET_ADMIN: EPERM
  nobody: EPERM

Fixes: 50aba46c23 ("gtp: add notification mechanism")
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20261027130623.3136658-1-anilkaushikwireless@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/netlink/specs/gtp.yaml

@@ -194,3 +194,4 @@ mcast-groups:
   list:
     -
       name: gtp
+      flags: [uns-admin-perm]

drivers/net/gtp.c

@@ -118,7 +118,8 @@ enum gtp_multicast_groups {
 };
 
 static const struct genl_multicast_group gtp_genl_mcgrps[] = {
-	[GTP_GENL_MCGRP] = { .name = GTP_GENL_MCGRP_NAME },
+	[GTP_GENL_MCGRP] = { .name = GTP_GENL_MCGRP_NAME,
+			     .flags = GENL_MCAST_CAP_NET_ADMIN, },
 };
 
 static void pdp_context_delete(struct pdp_ctx *pctx);