net: ip6_gre: require CAP_NET_ADMIN in the device netns for changelink

net: ip6_gre: require CAP_NET_ADMIN in the device netns for changelink

ip6gre_changelink() and ip6erspan_changelink() operate 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 t->net can
rewrite a tunnel that lives in t->net.

Gate both ops on rtnl_dev_link_net_capable() at their top, before any
attribute is parsed.

Reported-by: Xiao Liang <shaw.leon@gmail.com>
Closes: https://lore.kernel.org/netdev/CABAhCOSzP1vaThGV35_VnsRCb=87_CPjPVsTHbq905k8A+BuUg@mail.gmail.com/
Fixes: 690afc165bb3 ("net: ip6_gre: fix moving ip6gre between namespaces")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260612085941.3158249-6-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>