[PATCH 6.1.y] net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink

[Sasha Levin <sashal@kernel.org>]

From: Maoyi Xie <maoyixie.tju@gmail.com>

[ Upstream commit 8165f7ff57d9667d2bb477ef6af83ede7fed4ad7 ]

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

Add rtnl_dev_link_net_capable() next to rtnl_get_net_ns_capable() in
net/core/rtnetlink.c. It requires CAP_NET_ADMIN in the link netns and is
skipped when the link netns is dev_net(dev), where the rtnl path already
checked it. The other patches in this series use the same helper.

Gate ipgre_changelink() and erspan_changelink() with it, at the top of
the op before any attribute is parsed, because the parsers update live
tunnel fields first. ipgre_netlink_parms() sets t->collect_md before
ip_tunnel_changelink() runs.

Commit 8b484efd5cb4 ("ip6: vti: Use ip6_tnl.net in
vti6_siocdevprivate().") added the same check on the ioctl path. This
adds it on RTM_NEWLINK.

Reported-by: Xiao Liang <shaw.leon@gmail.com>
Closes: https://lore.kernel.org/netdev/CABAhCOSzP1vaThGV35_VnsRCb=87_CPjPVsTHbq905k8A+BuUg@mail.gmail.com/
Fixes: b57708add314 ("gre: 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/20260612085941.3158249-2-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/net/rtnetlink.h | 2 ++
 net/core/rtnetlink.c    | 8 ++++++++
 net/ipv4/ip_gre.c       | 6 ++++++
 3 files changed, 16 insertions(+)

diff --git a/include/net/rtnetlink.h b/include/net/rtnetlink.h
index 0bd400be3f8d9f..983f01bf1bd6ba 100644
--- a/include/net/rtnetlink.h
+++ b/include/net/rtnetlink.h
@@ -210,6 +210,8 @@ int rtnl_configure_link(struct net_device *dev, const struct ifinfomsg *ifm,
 int rtnl_nla_parse_ifinfomsg(struct nlattr **tb, const struct nlattr *nla_peer,
 			     struct netlink_ext_ack *exterr);
 struct net *rtnl_get_net_ns_capable(struct sock *sk, int netnsid);
+bool rtnl_dev_link_net_capable(const struct net_device *dev,
+			       const struct net *link_net);
 
 #define MODULE_ALIAS_RTNL_LINK(kind) MODULE_ALIAS("rtnl-link-" kind)
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index a4e017c2cbf213..b97d348a5b2a93 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -2110,6 +2110,14 @@ struct net *rtnl_get_net_ns_capable(struct sock *sk, int netnsid)
 }
 EXPORT_SYMBOL_GPL(rtnl_get_net_ns_capable);
 
+bool rtnl_dev_link_net_capable(const struct net_device *dev,
+			       const struct net *link_net)
+{
+	return net_eq(link_net, dev_net(dev)) ||
+	       ns_capable(link_net->user_ns, CAP_NET_ADMIN);
+}
+EXPORT_SYMBOL_GPL(rtnl_dev_link_net_capable);
+
 static int rtnl_valid_dump_ifinfo_req(const struct nlmsghdr *nlh,
 				      bool strict_check, struct nlattr **tb,
 				      struct netlink_ext_ack *extack)
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 2c1228ddaa22f7..b635d2fedd2dbd 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -1406,6 +1406,9 @@ static int ipgre_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct ip_tunnel_parm p;
 	int err;
 
+	if (!rtnl_dev_link_net_capable(dev, t->net))
+		return -EPERM;
+
 	err = ipgre_newlink_encap_setup(dev, data);
 	if (err)
 		return err;
@@ -1435,6 +1438,9 @@ static int erspan_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct ip_tunnel_parm p;
 	int err;
 
+	if (!rtnl_dev_link_net_capable(dev, t->net))
+		return -EPERM;
+
 	err = ipgre_newlink_encap_setup(dev, data);
 	if (err)
 		return err;
-- 
2.53.0