[PATCH net 2/2] ip6_gre: Use cached t->net in ip6erspan_changelink().

[Maoyi Xie]

From: Maoyi Xie <maoyi.xie@xxxxxxxxxx>

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, leaving a
stale entry in the original creation netns. 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"); cross-tenant scope on container hosts.

Note: ip6gre_changelink() (the non-erspan sibling earlier in the same
file) already uses the cached t->net correctly. The bug is specific
to ip6erspan_changelink() copying the wrong shape.

Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
Reported-by: Maoyi Xie <maoyi.xie@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v5.15+
Signed-off-by: Maoyi Xie <maoyi.xie@xxxxxxxxxx>
---
 net/ipv6/ip6_gre.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index dafcc0dcd..38ac14cc0 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -2261,7 +2261,8 @@ 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 *nt = netdev_priv(dev);
+	struct ip6gre_net *ign = net_generic(nt->net, ip6gre_net_id);
 	struct __ip6_tnl_parm p;
 	struct ip6_tnl *t;
 
-- 
2.34.1