[PATCH net 1/2] ip6: vti: Use ip6_tnl.net in vti6_changelink().

[Maoyi Xie]

From: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>

ip netns add ns1
ip netns add ns2
ip -n ns1 link add vti6_test type vti6 remote ::1 local ::2 key 7
ip -n ns1 link set vti6_test netns ns2
ip -n ns2 link set vti6_test type vti6 remote ::3 local ::4 key 9
ip netns del ns2
ip netns del ns1
[  132.495484] ------------[ cut here ]------------
[  132.497609] kernel BUG at net/core/dev.c:12376!

After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
rtnl_link_ops"), vti6_newlink() correctly resolves the per-netns vti6
hash via link_net. vti6_changelink() and vti6_update() were not
converted in that series and still read dev_net(dev) /
dev_net(t->dev), which diverge from the device's creation netns
after IFLA_NET_NS_FD migration. The result is a stale per-netns hash
entry; cleanup_net() of the original netns then walks freed memory.

Reachable from an unprivileged user namespace ("unshare --user
--map-root-user --net"); cross-tenant scope on container hosts.

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: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
---
 net/ipv6/ip6_vti.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
index ad5290be4..dcb257411 100644
--- a/net/ipv6/ip6_vti.c
+++ b/net/ipv6/ip6_vti.c
@@ -722,10 +722,11 @@ vti6_tnl_change(struct ip6_tnl *t, const struct __ip6_tnl_parm *p,
 static int vti6_update(struct ip6_tnl *t, struct __ip6_tnl_parm *p,
 		       bool keep_mtu)
 {
-	struct net *net = dev_net(t->dev);
-	struct vti6_net *ip6n = net_generic(net, vti6_net_id);
+	struct net *net = t->net;
+	struct vti6_net *ip6n;
 	int err;
 
+	ip6n = net_generic(net, vti6_net_id);
 	vti6_tnl_unlink(ip6n, t);
 	synchronize_net();
 	err = vti6_tnl_change(t, p, keep_mtu);
@@ -1031,11 +1032,12 @@ static int vti6_changelink(struct net_device *dev, struct nlattr *tb[],
 			   struct nlattr *data[],
 			   struct netlink_ext_ack *extack)
 {
-	struct ip6_tnl *t;
+	struct ip6_tnl *t = netdev_priv(dev);
+	struct net *net = t->net;
 	struct __ip6_tnl_parm p;
-	struct net *net = dev_net(dev);
-	struct vti6_net *ip6n = net_generic(net, vti6_net_id);
+	struct vti6_net *ip6n;
 
+	ip6n = net_generic(net, vti6_net_id);
 	if (dev == ip6n->fb_tnl_dev)
 		return -EINVAL;
 
-- 
2.34.1