[PATCH net v2] gtp: Require 'CAP_NET_ADMIN' when joining the gtp multicast group

[Anil Kaushik <anilkaushikwireless@gmail.com>]

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. 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: 50aba46c234e ("gtp: add notification mechanism")
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---

Notes:
    v2:
     - say that the group check is still weaker than the one on GETPDP
       (Sashiko)
     - say how it was found
    
    v1: https://lore.kernel.org/netdev/20261021065750.2069795-1-anilkaushikwireless@gmail.com/

 drivers/net/gtp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index 15b5d845fc3b..8b29c35d7cdb 100644
--- a/drivers/net/gtp.c
+++ b/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);
-- 
2.25.1