Greetings fellow Docker enthusiasts! Have you ever come across a pesky issue with docker improper write permissions for volumes & host filesystems when deploying your containers? If so, you’re not alone. This is a common problem that many Docker users face and it can be frustrating trying to figure out how to resolve it. But don’t despair, for in this post, we’ll explore the world of volumes and host filesystems why proper write permissions are essential and how to ensure your containers have the proper permissions to write to these resources. So put on your thinking cap, pour yourself a cup of tea and let’s dive into the world of Docker volumes and host filesystems.
The Issue of Improper Write Permissions
When deploying Docker containers, it’s common to encounter a problem with improper write permissions for volumes and host filesystems. This can cause a variety of issues like containers being unable to write to the volumes or host filesystems they need to access which can lead to data loss/corruption.
Understanding Volumes and Host Filesystems
Definition of Volumes and Host Filesystems
In Docker, volumes and host filesystems refer to the storage locations used by containers to persist data.
The difference between Volumes and Host Filesystems
Within the Docker system, volumes are private storage spaces that Docker itself controls. Despite being kept apart from the host filesystem they are kept in the Docker host system.
Referring to the actual file system of the host operating system that Docker is executing on, we use the term “host filesystems.”
The Importance of Proper Write Permissions
Proper write permissions are crucial for ensuring that containers can access & write to the volumes and host filesystems they need to in order to persist data and run properly. Without proper write permissions, containers may experience data loss or corruption or be unable to access essential resources.
Here is an example of how to create a Docker volume and mount it to a container with the correct write permissions:
Create a volume
docker volume create my_volume
Start a container and mount the volume with proper write permissions
docker run -it -v my_volume:/app/data --mount type=volume,src=my_volume,dst=/app/data,volume-driver=local,volume-opt=o=uid=1000,volume-opt=gid=1000 my_image ...
In this example, we are creating a volume named “my_volume” and then mounting it to the container with the correct write permissions. The volume-opt option is used to set the correct user ID (uid) & group ID (gid) for the volume ensuring that the container has proper write permissions.
The Causes of Improper Write Permissions
How Docker Handles Write Permissions
Docker uses a combination of user IDs (UIDs) & group IDs (GIDs) to manage write permissions for volumes and host filesystems. When a container is started, Docker sets the UID and GID for the container’s processes to match those of the host user that started the container.
Common Causes of Improper Write Permissions
There are several common causes of improper write permissions in Docker, including;
- Running containers as root: By default, Docker containers run as the root user, which can lead to permissions issues if the host user and root user have different UIDs and GIDs.
- Inconsistent UIDs & GIDs: If the UIDs and GIDs of the host and container users are inconsistent, containers may be unable to write to the volumes or host filesystems they need to access.
- Incorrect volume mounting: If a volume is not mounted with the correct write permissions, containers may be unable to write to the volume.
Why Improper Write Permissions can be a Challenge
- Improper write permissions can be a challenge for Docker users because it can cause data loss or corruption or prevent containers from accessing essential resources. Additionally, it can be difficult to diagnose & resolve permissions issues, as they can be caused by a variety of factors and may not be immediately apparent.
- Here is an example of how to start a container with the correct user and group IDs to avoid improper write permissions:
Start a container as a specific host user
docker run -it --user $(id -u):$(id -g) my_image ...
In this example; the ‘–user‘ option is used to start the container as the current host user ensuring that the container has the correct UID and GID for write permissions. By using this option, you can avoid common causes of improper write permissions and ensure that your containers have the proper write permissions they need.
Real–World Scenario
Imagine you have a Docker container running a web application & you want to store the application’s data in a persistent volume. To do this, you create a volume & mount it to a directory on your host filesystem. However, when you try to write data to the volume you receive an error saying that the write operation is not permitted. This is because the write permissions on the volume are not set correctly.
Here’s an example, of how you might resolve this issue;
Create the volume
docker volume create my-web-app-data
Start the Docker container and mount the volume to a directory on the host filesystem
docker run -it --name my-web-app -v my-web-app-data:/app/data my-web-app-image
In the Docker container, try to write data to the volume
echo "This is my data" > /app/data/my-data.txt
Error: permission denied
Exit the container
exit
Change the ownership of the host directory to match the user running the Docker container
sudo chown -R $(id -u):$(id -g) /var/lib/docker/volumes/my-web-app-data/_data ...
Start the Docker container again
docker start my-web-app ...
In the Docker container, try to write data to the volume again
echo "This is my data" > /app/data/my-data.txt
Data is written successfully
In this scenario, we see how changing the ownership of the host directory to match the user running the Docker container resolves the issue of improper write permissions. By following this process, you can ensure that your volumes are set up with the correct write permissions & that your data is persistent and secure.
Mount Enumeration
Mount enumeration in Docker refers to the process of listing all the mounts (volumes and bind mounts) associated with a Docker container. This information is useful for understanding the storage configuration of a container, including which directories are being shared between the host and container & which directories are stored in a persistent volume.
To enumerate the mounts in a Docker container, you can use the docker inspect command and filter the output to show only the Mounts section. Here’s an example;
Get the ID of the Docker container you want to inspect
docker ps
Inspect the container
docker inspect <container_id> | grep -A 5 Mounts
OUTPUT
Output:
"Mounts": [
{
"Type": "bind",
"Source": "/host/path/to/mount",
"Destination": "/container/path/to/mount",
"Mode": "",
"RW": true
},
{
"Type": "volume",
"Name": "my-web-app-data",
"Source": "/var/lib/docker/volumes/my-web-app-data/_data",
"Destination": "/app/data",
"Driver": "local",
"Mode": "",
"RW": true,
"Propagation": ""
}
],
In this example; we see that the container has two mounts: a bind mount and a volume. The bind mount is sharing a directory on the host with the container, while the volume is a persistent storage area managed by Docker.
By enumerating the mounts in a Docker container, you can gain a deeper understanding of the storage configuration for your containers and ensure that your data is being stored correctly.
List Mount Content
To list the content of a mount in a Docker container, you can use the docker exec command to run a shell inside the container & then use standard shell commands to list the contents of the mount.
For example; if you have a container named my-web-app and you want to list the contents of the /app/data mount, you would run the following command;
docker exec -it my-web-app sh ...
Inside the container:
ls /app/data ...
This will show you a list of the files and directories in the /app/data mount. You can use standard shell commands like ls, cd, cat and more to explore the contents of the mount further.
It’s important to note that the contents of a mount will be different depending on whether it is a bind mount or a volume. A bind mount will reflect the contents of the host directory that is being shared while a volume will contain the data that is stored in the persistent volume managed by Docker.
Propagation in Docker
Here is an example of how to mount a volume in Docker and specify the propagation mode using the docker run command:
docker run -it --mount source=myvol,target=/app,type=bind,propagation=rw ubuntu bash ...
In this example, we are mounting a volume named myvol to the /app directory in the container and setting the propagation mode to rw (read-write).
Alternatively, you can specify the propagation mode in a docker-compose.yml file as follows:
services:
web:
image: ubuntu
volumes:
- myvol:/app:rw
volumes:
myvol:
driver: local
In this example, the volume myvol is defined under the volumes key and mounted to the /app directory in the web service with the rw propagation mode.
‘passwd’ in Docker
A user’s password can be changed in a Linux system using the passwd command. You can use this command in Docker to update a user’s password within a container. As an illustration; here’s how to use the docker exec command to modify the root user’s password in a Docker container.
docker exec -it mycontainer bash
This will open a shell inside the container. Then, you can use the passwd command to change the root password:
Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully ...
Note: It is recommended to create a non-root user in a Docker container and use that user instead of the root user for security reasons.
Some Vulnerabilities of Improper Write Permissions for Volumes and Host Filesystem
Improper write permissions for volumes and host filesystems in Docker can lead to various security vulnerabilities, such as:
Data Leakage
In case of improper write permissions, containers can read or write data to the host machine that may contain sensitive information. This could result in data leakage.
Example code to demonstrate the vulnerability
docker run -v /etc/passwd:/tmp/passwd -it alpine sh cat /tmp/passwd
Container Escalation
In case of improper write permissions, a malicious container can write data to the host machine and escalate its privileges.
Example code to demonstrate the vulnerability
docker run -v /:/hostfs -it alpine sh cd /hostfs echo "root:x:0:0:root:/root:/bin/bash" > etc/passwd ...
Malicious Code Execution
In case of improper write permissions, a malicious container can write malicious code to the host machine, leading to code execution.
Example code to demonstrate the vulnerability
docker run -v /:/hostfs -it alpine sh
cd /hostfs
echo 'os.execute("rm -rf /")' > malicious.lua
...
Solutions for Improper Write Permissions
Methods for Resolving Improper Write Permissions
There are several methods for resolving improper write permissions in Docker, including;
Setting the correct user & group IDs: By starting containers as the correct host user, you can ensure that the container has the correct UID & GID for write permissions.
Mounting volumes with the correct write permissions: By using the –mount option to mount volumes with the correct write permissions, you can ensure that containers have the access they need to persist data.
Modifying the Docker image; If you are experiencing permissions issues that cannot be resolved through the above methods you may need to modify the Docker image to include the correct write permissions.
Setting Correct Write Permissions
In order to set the correct write permissions for volumes & host filesystems you will need to understand the UIDs & GIDs of the host and container users and ensure that these matches. Additionally, you will need to ensure that volumes are mounted with the correct write permissions using the –mount option.
Example Code
Here is an example of how to set the correct write permissions for a volume using the –mount option:
Create a volume
docker volume create my_volume ...
Start a container and mount the volume with proper write permissions
docker run -it -v my_volume:/app/data --mount type=volume,src=my_volume,dst=/app/data,volume-driver=local,volume-opt=o=uid=1000,volume-opt=gid=1000 my_image ...
In this example, we are creating a volume named “my_volume” and then mounting it to the container with the correct write permissions. The volume-opt option is used to set the correct user ID (uid) & group ID (gid) for the volume ensuring that the container has proper write permissions.
Additionally, here is an example of how to start a container with the correct user and group IDs to avoid improper write permissions:
Start a container as a specific host user
docker run -it --user $(id -u):$(id -g) my_image ...
In this example; the –user option is used to start the container as the current host user ensuring that the container has the correct UID and GID for write permissions. By using this option, you can avoid common causes of improper write permissions and ensure that your containers have the proper write permissions they need.
Best Practices for Docker Volumes and Host Filesystems
Key Principles for Best Practices
There are a few fundamental rules to follow when working with host filesystems & Docker volumes in order to ensure adequate write permission & prevent any issues. These consist of;
- Understanding the UIDs and GIDs of the host & container users; It is crucial to know the UIDs & GIDs of the host and containers users so that you can set the correct write permissions.
- Mounting volumes with the correct write permission: By using the –mount option when mounting volumes you can ensure that the correct write permissions are set for the container.
- Starting containers as the appropriate host user will help to guarantee that the container has the proper UID and GID for write permissions.
Common Best Practices
To ensure that Docker volumes and host filesystems are used properly, it is important to follow best practices. Some of the most common best practices include:
- Using data volumes for data that needs to persist: By using data volumes you can store data outside of the container and make it easier to manage and maintain.
- Avoid using the root user for containers; Using the root user for containers can cause security problems, thus it’s recommended to use a host user that has the right UID & GID for write permissions.
- Keeping Docker images up-to-date; Keeping Docker images up-to-date helps to avoid any issues with write permissions & ensures that containers are running smoothly.
Why Following Best Practices is Essential
Following best practices is essential for avoiding improper write permissions when working with Docker volumes & host filesystems. Proper write permissions are necessary for containers to persist data, and failure to follow best practices can lead to issues with write permissions that can be difficult to resolve. By following best practices, you can ensure that your containers are running smoothly & that your data is secure.
Mitigation – Bind Mount in Docker
Bind mounts in Docker allow you to mount a file or directory from the host file system into a Docker container. This is different from a volume, which is a separate file system that is managed by Docker. When you use a bind mount, changes made inside the container will be reflected in the host file system and vice versa.
Here is an example of how to use a bind mount in Docker;
docker run -it -v /path/on/host:/path/in/container [image name or ID] ...
The -v option is used to specify the bind mount and it takes two arguments: the first argument is the path on the host file system and the second argument is the path in the container where the bind mount will be mounted.
By using bind mounts, you can prevent the improper write permissions issue that can occur with volumes, as the files and directories are part of the host file system and not managed by Docker. This can also provide better performance and consistency compared to volumes as the files are directly accessible from both the host and the container.
However, it’s important to keep in mind that the bind mounts still inherit the permissions and ownership of the host file system, so it’s important to ensure that the host file system is properly secured and maintained.
Mitigation – Volumes in Docker
Data can be stored outside of a container’s filesystem using volumes in Docker. When you wish to keep data around even after a container has been removed, this is especially helpful. To mitigate the risks associated with improper write permissions in the host filesystem & volumes you can use Docker volumes in the following ways;
- Use the -v or –mount flag when running a Docker container to specify the volume mount point. The syntax for mounting a volume is:
docker run -it --mount type=bind,source=<host-directory>,target=<container-directory> <image-name> ...
- You can also use a docker-compose.yml file to define your volumes & the containers that use them. This is particularly useful when working with multiple containers. The syntax for defining a volume in docker-compose.yml is;
services:
web:
image: <image-name>
volumes:
- <host-directory>:<container-directory>:<mount-options>
- Before mounting it in the container, you can set the host directory’s permissions and ownership appropriately to further safeguard your volumes. The chown and chmod commands can be used to accomplish this;
sudo chown <user-name>:<group-name> <host-directory> sudo chmod 700 <host-directory>
By using these best practices, you can ensure that the data stored in your volumes is secure and protected from improper write permissions.
Conclusion
In this post, we explored the issue of improper write permissions for Docker volumes and host filesystems. We discussed the importance of proper write permissions and the difference between volumes and host filesystems. We also covered the causes of improper write permissions and the various methods for resolving them. Finally, we went over best practices for Docker volumes and host filesystems & why following them is essential for avoiding improper write permissions.
The world of Docker is constantly evolving and there is always more to learn and explore. We encourage you to continue learning and exploring the world of Docker and to stay up-to-date on the latest developments and best practices. Whether you are a seasoned Docker user or just starting out there is always something new to learn & discover in this exciting field.