ipip6_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 ipip6_changelink() on rtnl_dev_link_net_capable() at its top,
before any attribute is parsed. sit was the one tunnel type not covered
by the recent series that added this check to the other changelink()
handlers.

Fixes: 5e6700b3bf98 ("sit: add support of x-netns")
Link: https://lore.kernel.org/netdev/20260612085941.3158249-1-maoyixie.tju@gmail.com/ (local)
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <redacted>
---
 net/ipv6/sit.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 64f0d1b..a38b24f 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c

@@ -1613,6 +1613,9 @@ static int ipip6_changelink(struct net_device *dev, struct nlattr *tb[],
 	__u32 fwmark = t->fwmark;
 	int err;
 
+	if (!rtnl_dev_link_net_capable(dev, net))
+		return -EPERM;
+
 	if (dev == sitn->fb_tunnel_dev)
 		return -EINVAL;
 
--

2.43.0