xfrmi_changelink() rewrites the interface in its creation netns. After an
IFLA_NET_NS_FD migration that netns is not the caller's, but the rtnl
changelink path only checks CAP_NET_ADMIN against the caller's netns. A
caller with caps only in its current netns can then rewrite an interface
in another netns.

Gate the op on net_admin_capable() at its top, before any attribute is
parsed. The check is skipped when the interface netns is the device's
current netns, where the rtnl path already checked the cap.

Reported-by: Xiao Liang <redacted>
Closes: https://lore.kernel.org/netdev/CABAhCOSzP1vaThGV35_VnsRCb=87_CPjPVsTHbq905k8A+BuUg@mail.gmail.com/ (local)
Fixes: f203b76d7809 ("xfrm: Add virtual xfrm interfaces")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <redacted>
---
 net/xfrm/xfrm_interface_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/xfrm/xfrm_interface_core.c b/net/xfrm/xfrm_interface_core.c
index 330a05286a56..8fd3842d20c2 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 (!net_admin_capable(net, dev_net(dev)))
+		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