ip6_gre: Use cached t->net in ip6erspan_changelink().

After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
ip6gre hash via link_net. ip6erspan_changelink() was not converted in
that series and still uses dev_net(dev), which diverges from the
device's creation netns after IFLA_NET_NS_FD migration.

This re-inserts the tunnel into the wrong per-netns hash. The
original netns keeps a stale entry. When that netns is later
destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
slab-use-after-free reported by KASAN, followed by a kernel BUG at
net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().

Reachable from an unprivileged user namespace (unshare --user
--map-root-user --net).

ip6gre_changelink() earlier in the same file already uses the cached
t->net; only ip6erspan_changelink() has the wrong shape.

Fixes: 2d665034f239 ("net: ip6_gre: Fix ip6erspan hlen calculation")
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260430103318.3206018-1-maoyi.xie@ntu.edu.sg
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 63fc8556b475e5166f150ed1aff39319f096f758..365b4059eb20354c256c491a16db0e606e0a9790 100644 (file)
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -2262,10 +2262,11 @@ static int ip6erspan_changelink(struct net_device *dev, struct nlattr *tb[],
                                struct nlattr *data[],
                                struct netlink_ext_ack *extack)
 {
-       struct ip6gre_net *ign = net_generic(dev_net(dev), ip6gre_net_id);
+       struct ip6_tnl *t = netdev_priv(dev);
        struct __ip6_tnl_parm p;
-       struct ip6_tnl *t;
+       struct ip6gre_net *ign;
 
+       ign = net_generic(t->net, ip6gre_net_id);
        t = ip6gre_changelink_common(dev, tb, data, &p, extack);
        if (IS_ERR(t))
                return PTR_ERR(t);