[PATCH net v2] net: require CAP_NET_ADMIN in the device netns for tunnel changelink

[Maoyi Xie <maoyixie.tju@gmail.com>]

A tunnel changelink mutates the tunnel hash of the device's creation
netns. ip_tunnel_changelink(), ip6_tnl_changelink(), vti6_changelink(),
ip6gre_changelink(), ip6erspan_changelink() and xfrmi_changelink() all
look up and update through t->net.

The rtnl path into changelink only checks CAP_NET_ADMIN against
tgt_net. After IFLA_NET_NS_FD migration the creation netns differs from
the caller's netns. A caller with caps only in its current netns can
then rewrite an entry in the creation netns hash. They pick the
endpoint addresses. Commit 8b484efd5cb4 ("ip6: vti: Use ip6_tnl.net in
vti6_siocdevprivate().") added the same check on the ioctl path. This
adds it on the RTM_NEWLINK path.

Check ns_capable(t->net->user_ns, CAP_NET_ADMIN) in each changelink
before the lookup and update. The newlink path has long checked the
capability in the link netns. The changelink path never did.

Reported-by: Xiao Liang <shaw.leon@gmail.com>
Closes: https://lore.kernel.org/netdev/CABAhCOSzP1vaThGV35_VnsRCb=87_CPjPVsTHbq905k8A+BuUg@mail.gmail.com/
Fixes: d0f418516022 ("net, ip_tunnel: fix namespaces move")
Fixes: 5311a69aaca3 ("net, ip6_tunnel: fix namespaces move")
Fixes: 690afc165bb3 ("net: ip6_gre: fix moving ip6gre between namespaces")
Fixes: f203b76d7809 ("xfrm: Add virtual xfrm interfaces")
Fixes: 11b326fb0a37 ("ip6: vti: Use ip6_tnl.net in vti6_changelink().")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
---
v2: Reworked per Kuniyuki Iwashima's review. v1 gated the check on
    dev->rtnl_link_ops->get_link_net in __rtnl_newlink(). That gate is
    too broad. For peer types like netkit and veth get_link_net returns
    the peer netns, which changelink does not mutate, so the core check
    would wrongly require CAP_NET_ADMIN there. Move the check into the
    changelink path of the tunnel types that mutate t->net, against
    t->net->user_ns. This mirrors the ioctl side in 8b484efd5cb4.

v1: https://lore.kernel.org/netdev/20260527070824.2677331-1-maoyixie.tju@gmail.com/

 net/ipv4/ip_tunnel.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 +++
 5 files changed, 18 insertions(+)

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 50d0f5fe4e4c..51d8787318f3 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -1251,6 +1251,9 @@ int ip_tunnel_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct net *net = tunnel->net;
 	struct ip_tunnel_net *itn = net_generic(net, tunnel->ip_tnl_net_id);
 
+	if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
 	if (dev == itn->fb_tunnel_dev)
 		return -EINVAL;
 
diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 365b4059eb20..0de4994bc92f 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -2047,6 +2047,9 @@ static int ip6gre_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct ip6gre_net *ign = net_generic(t->net, ip6gre_net_id);
 	struct __ip6_tnl_parm p;
 
+	if (!ns_capable(t->net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
 	t = ip6gre_changelink_common(dev, tb, data, &p, extack);
 	if (IS_ERR(t))
 		return PTR_ERR(t);
@@ -2266,6 +2269,9 @@ static int ip6erspan_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct __ip6_tnl_parm p;
 	struct ip6gre_net *ign;
 
+	if (!ns_capable(t->net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
 	ign = net_generic(t->net, ip6gre_net_id);
 	t = ip6gre_changelink_common(dev, tb, data, &p, extack);
 	if (IS_ERR(t))
diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 9d1037ac082f..2834004c7011 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -2102,6 +2102,9 @@ 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 (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
 	if (dev == ip6n->fb_tnl_dev) {
 		if (ip_tunnel_netlink_encap_parms(data, &ipencap)) {
 			/* iproute2 always sets TUNNEL_ENCAP_FLAG_CSUM6, so
diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
index df793c8bfffb..7b05e0c491db 100644
--- a/net/ipv6/ip6_vti.c
+++ b/net/ipv6/ip6_vti.c
@@ -1044,6 +1044,9 @@ static int vti6_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct __ip6_tnl_parm p;
 	struct vti6_net *ip6n;
 
+	if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
 	ip6n = net_generic(net, vti6_net_id);
 	if (dev == ip6n->fb_tnl_dev)
 		return -EINVAL;
diff --git a/net/xfrm/xfrm_interface_core.c b/net/xfrm/xfrm_interface_core.c
index 330a05286a56..a1029a829406 100644
--- a/net/xfrm/xfrm_interface_core.c
+++ b/net/xfrm/xfrm_interface_core.c
@@ -869,6 +869,9 @@ static int xfrmi_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct net *net = xi->net;
 	struct xfrm_if_parms p = {};
 
+	if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
 	xfrmi_netlink_parms(data, &p);
 	if (!p.if_id) {
 		NL_SET_ERR_MSG(extack, "if_id must be non zero");
-- 
2.34.1