A tunnel changelink() operates on at most two netns, dev_net(dev) and
the tunnel link netns t->net. They differ once the device is created in
or moved to a netns other than the one the request runs in. The rtnl
changelink path checks CAP_NET_ADMIN only against dev_net(dev), so a
caller privileged there but not in the link netns can rewrite a tunnel
that lives in the link netns. Commit 8b484efd5cb4 ("ip6: vti: Use
ip6_tnl.net in vti6_siocdevprivate().") added the same check on the
ioctl path. This series adds it on the RTM_NEWLINK path.

Each changelink is gated at the top of the op, before any attribute is
parsed, because the per-type parsers can update live tunnel fields first
(for example ipgre_netlink_parms() sets t->collect_md). The check is
skipped when the link netns is dev_net(dev), where the rtnl path already
checked the cap.

The check goes through a new helper, rtnl_dev_link_net_capable(), added
in patch 1 next to rtnl_get_net_ns_capable() in net/core/rtnetlink.c.

Tested on net/main. For every tunnel type in the series a migrated
fake-root changelink is rejected with EPERM. For vti6 SIOCGETTUNNEL
confirms the link netns hash is left unchanged. Legit non-migrated
changelinks still succeed.

v5 -> v6, addressing Kuniyuki Iwashima's review:
 - Correct the Fixes: tags to the commits that first added x-netns
   support / namespace changing for each tunnel type, where the cap gap
   has existed, instead of the later "fix namespaces move" commits.
 - Add Kuniyuki Iwashima's Reviewed-by to all patches.

v5: https://lore.kernel.org/netdev/20260611062814.2528793-1-maoyixie.tju@gmail.com/ (local)
v4: https://lore.kernel.org/netdev/20260609163110.1717419-1-maoyixie.tju@gmail.com/ (local)
v3: https://lore.kernel.org/netdev/20260604125055.3254652-1-maoyixie.tju@gmail.com/ (local)
v2: https://lore.kernel.org/netdev/20260601034148.1272080-1-maoyixie.tju@gmail.com/ (local)
v1: https://lore.kernel.org/netdev/20260527070824.2677331-1-maoyixie.tju@gmail.com/ (local)

Maoyi Xie (7):
  net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink
  net: ipip: require CAP_NET_ADMIN in the device netns for changelink
  net: ip_vti: require CAP_NET_ADMIN in the device netns for changelink
  net: ip6_tunnel: require CAP_NET_ADMIN in the device netns for
    changelink
  net: ip6_gre: require CAP_NET_ADMIN in the device netns for changelink
  net: ip6_vti: require CAP_NET_ADMIN in the device netns for changelink
  xfrm: xfrm_interface: require CAP_NET_ADMIN in the device netns for
    changelink

 include/net/rtnetlink.h        | 2 ++
 net/core/rtnetlink.c           | 8 ++++++++
 net/ipv4/ip_gre.c              | 6 ++++++
 net/ipv4/ip_vti.c              | 3 +++
 net/ipv4/ipip.c                | 3 +++
 net/ipv6/ip6_gre.c             | 6 ++++++
 net/ipv6/ip6_tunnel.c          | 3 +++
 net/ipv6/ip6_vti.c             | 3 +++
 net/xfrm/xfrm_interface_core.c | 3 +++
 9 files changed, 37 insertions(+)


base-commit: 512db8267b73a220a64180d95ab5eebe7c4964a8
-- 
2.34.1