In the Linux kernel, the following vulnerability has been resolved:

ip6: vti: Use ip6_tnl.net in vti6_siocdevprivate().

After patch 1/2 in this series, vti6_update() unlinks and relinks
the tunnel through t->net. vti6_siocdevprivate() still uses
dev_net(dev) for the collision lookup. For a tunnel moved through
IFLA_NET_NS_FD, dev_net(dev) is the new netns, not t->net.

SIOCCHGTUNNEL on a migrated tunnel then runs:

  net = dev_net(dev)                    /* migrated netns */
  t   = vti6_locate(net, &p1, false)    /* misses target in t->net */
  ...
  t   = netdev_priv(dev)
  vti6_update(t, &p1, false)            /* mutates t->net's hash */

A caller in the migrated netns picks params that match a tunnel
in the creation netns. The lookup in dev_net(dev) finds nothing.
vti6_update() prepends the migrated tunnel at the head of the
creation netns hash bucket for those params. Later lookups in
the creation netns resolve to the migrated device. xfrm receive
delivers the matched packets through a device the caller controls.

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

Switch the SIOCCHGTUNNEL path on a non fallback device to use
t->net for the lookup. The lookup now matches the netns
vti6_update() operates on.

Also add ns_capable(self->net->user_ns, CAP_NET_ADMIN) before
the lookup. The check at the top of the case is against
dev_net(dev)->user_ns, which after migration is the attacker's
netns. A caller there can pick params absent from self->net,
the lookup returns NULL, t becomes self, and vti6_update()
inserts the device into the creation netns hash. The new check
requires CAP_NET_ADMIN in the creation netns user_ns too.

SIOCADDTUNNEL and SIOCCHGTUNNEL on the fallback device keep
dev_net(dev), which equals init_net there.

https://git.kernel.org/stable/c/1acfb7d9c6fc7e209ed7789392697e97e03edd33

https://git.kernel.org/stable/c/44d2ff7d2178503b93151140a45dfa2ad49c9906

https://git.kernel.org/stable/c/596f6354c96a891e58c04a09cbfb7b0d1ec00dab

https://git.kernel.org/stable/c/853f6ea482dfcd3404bbef458ab4d68364eed838

https://git.kernel.org/stable/c/8b484efd5cb4eeef9021a661e198edc5349dacf6

https://git.kernel.org/stable/c/94ff740a7f9ef5c010784a325dca00cbf228f941

https://git.kernel.org/stable/c/d2236348414bdd6558385f35aa7fdc9bf5634011

https://git.kernel.org/stable/c/df42ac708acc3399bbb6dc5ca16e0540adda7bbf

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