net: ip6_tunnel: require CAP_NET_ADMIN in the device netns for changelink

ip6_tnl_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 t->net can rewrite a tunnel that
lives in t->net.

Gate ip6_tnl_changelink() on rtnl_dev_link_net_capable() at its 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: 0bd8762824e7 ("ip6tnl: add x-netns support")
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/20260612085941.3158249-5-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 73fc5a0b8203a0109bce022b46c92f31da674ae0..d7c90a8533ec41e710538a7aa5fa301d72280a4f 100644 (file)
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -2103,6 +2103,9 @@ static int ip6_tnl_changelink(struct net_device *dev, struct nlattr *tb[],
        struct ip6_tnl_net *ip6n = net_generic(net, ip6_tnl_net_id);
        struct ip_tunnel_encap ipencap;
 
+       if (!rtnl_dev_link_net_capable(dev, net))
+               return -EPERM;
+
        if (dev == ip6n->fb_tnl_dev) {
                if (ip_tunnel_netlink_encap_parms(data, &ipencap)) {
                        /* iproute2 always sets TUNNEL_ENCAP_FLAG_CSUM6, so