Mounting a Host Directory with Write Access in an LXC Container: A Journey

Introduction

I recently set up a BitTorrent download service on my Raspberry Pi using an LXC (Linux Containers) container. While LXC containers are convenient for isolating applications and managing resources, I encountered a challenge when attempting to mount a host directory with write access inside the container. Persistent storage and data sharing between the host and container are essential for applications like BitTorrent, which require writing and reading files from a shared location.

Initial Attempts

Initially, I thought attaching a host directory to the container would be a straightforward process using the lxc config device add command. However, when trying to access the mounted directory from within the container, I encountered permission denied errors, preventing me from writing or modifying files.

Understanding the Problem

After failed attempts and research, I realized that the root cause of the issue was the user and group ID mapping between the host and container environments. For security reasons, LXC containers run with different user and group IDs than the host system. The user IDs inside the container are mapped to a specific range of user IDs on the host. For example, the root user (UID=0) in the container might be mapped to UID=100000 on the host, and UID=1 in the container could be mapped to UID=100001 on the host.

Unless a host directory is configured with permissive permissions (e.g., mode 777, allowing all users full control), a container would not be able to access or modify files within that directory. However, setting broad permissions is generally not recommended for security reasons.

Failed Solutions

Initially, I tried various solutions suggested by ChatGPT, Claude, and other online resources. These solutions often involved using the idmap technique to force a mapping between a specific container user and a designated host user within the container configuration. For example:

lxc config set <container-name> raw.idmap "both 1001 1001"

However, for unknown reasons (possibly related to the Raspbian kernel configuration), my container failed to boot after setting the idmap configuration. Additionally, the values in /etc/subuid and /etc/subgid (which should control the user and group ID mapping ranges) did not seem to be respected by the system.

My Solution

After investigating further, I discovered that the root user (UID=0) inside my containers was being mapped to UID=1000000 on the host system. Instead of relying on the idmap configuration, I decided to create a new user (bt) inside the container and set the permissions on the host directory explicitly for the mapped UID and GID of the bt user.

Here are the steps I followed:

Create the bt user in the container:

groupadd -g 1001 bt
adduser bt -u 1001 -g 1001

Set permissions on the host directory for the bt user:

sudo chown 1001001:1001001 /path/to/host/directory
sudo chmod 770 /path/to/host/directory

Map the new user/group IDs to the corresponding IDs within the container:
Since the root user (UID=0) inside the container was mapped to UID=1000000 on the host, and the new user bt (UID=1001) inside the container would be mapped to UID=1001001 on the host, no further mapping configuration was needed.

Run the BitTorrent service as the new user inside the container:
Within the container, I ran the BitTorrent service as the new user bt, and now it had write access to the mounted host directory.

Security Considerations

By running the BitTorrent service as a non-root user (bt) inside the container, I limited the potential damage in case of a breach or compromise. If the bt user account was compromised, the attacker would not gain root privileges within the container or access to other containers or resources on the host system.

Additionally, since all containers' root users were mapped to the same host user ID (UID=1000000), any resources shared among containers would be accessible to the root user of any compromised container. By explicitly granting permissions only to the bt user, I ensured that a breach of the BitTorrent container would not grant access to resources shared by other containers.

Learnings and Conclusion

This experience taught me several valuable lessons:

Understand user and permission mappings: When working with containers, it's crucial to understand how user and group IDs are mapped between the host and container environments, as well as the security implications of these mappings.

Limitations of generic solutions: Generic solutions found online may not always work as expected, especially when dealing with specific system configurations or kernel versions. Tailored configurations based on thorough understanding and investigation are often necessary.

Security-conscious configurations: Granting broad permissions (e.g., mode 777) should be avoided whenever possible. Instead, configure permissions specifically for the users and groups required by the application, following the principle of least privilege.

Containerization best practices: Running services as non-root users within containers and limiting resource sharing between containers can help mitigate the impact of potential breaches or compromises.

While initially seeming like a trivial task, mounting a host directory with write access inside an LXC container proved to be a complex challenge. However, through research, experimentation, and a deeper understanding of the underlying concepts, I was able to implement a secure and tailored solution for my BitTorrent download service.

Appendix: Root Access to Mounted Host Directories

After successfully setting up the bt user and granting it the necessary permissions to access the mounted host directory, I noticed a peculiar behavior: the root user inside the container could also access and modify files within the mounted host directory, despite not explicitly granting it those permissions.

Initially, I found this puzzling since I had carefully configured the permissions to allow only the bt user (UID=1001, mapped to UID=1001001 on the host) access to the host directory. However, further investigation revealed that this behavior is an intentional design choice in LXC containers.

According to the LXC documentation and community discussions, once a resource (such as a mounted host directory) is granted to any user inside a container, the root user of that container automatically gains the same access to that resource. This is because the root user has unrestricted privileges within the container's namespace, regardless of the permissions set for other users.

The rationale behind this design decision is to maintain the expected behavior and functionality of the root user within the container environment. In traditional Linux systems, the root user has complete control over the system, including the ability to override permissions and access any resource. LXC containers aim to emulate this behavior as closely as possible while still providing isolation and security boundaries.

While this behavior may seem counterintuitive from a strict permission standpoint, it aligns with the principles of containerization and the separation of concerns between the host and container environments. The host system's permissions are respected for controlling access to resources from the host, while the container's root user retains its privileged status within the container's namespace.

It's important to note that this behavior does not compromise the overall security of the system, as the root user within the container is still confined to the container's isolated environment and cannot directly access or modify resources on the host system without proper permissions.

In my case, although the root user inside the BitTorrent container could access the mounted host directory, it remained confined within the container's boundaries. By running the BitTorrent service as the non-root bt user, I limited the potential damage in case of a breach or compromise, as an attacker would not gain root privileges within the container or access to other containers or resources on the host system.

While this "weird thing" may seem like a quirk or potential security concern at first glance, it is an intentional design choice in LXC containers to maintain the expected behavior and functionality of the root user within the container's namespace, while still providing isolation and security boundaries between the host and container environments.

Appendix: Using idmap to Mitigate Permission Overlaps

While the default user and group ID mapping between the host and containers can be a convenient and straightforward approach, it can also lead to potential permission overlaps between containers, which may introduce security risks.

To mitigate this risk and ensure better isolation of permissions between containers, the idmap technique can be employed. Instead of relying on the default user and group ID mapping range, idmap allows you to specify a unique mapping for each container, effectively isolating the permissions granted to specific user IDs and group IDs within that container.

Here's an example of how idmap could be used to achieve better permission isolation for a specific user ID and group ID:

Map a specific container user ID and group ID to dedicated host IDs:

lxc config set <container-name> raw.idmap "both 1001 1033"

This command maps both the user ID 1001 and the group ID 1001 inside the container to the host user ID 1033 and host group ID 1033, respectively. By using a distinct user ID and group ID (1033) on the host, you can avoid potential conflicts or confusion with existing IDs.

Create the user and group, and set permissions within the dedicated mapping:
Within the container, create the user with UID=1001 and the group with GID=1001, and set permissions accordingly. On the host system, ensure that the resources you want to grant access to have the appropriate permissions for the host user ID 1033 and host group ID 1033.

By using idmap with the "both" option and a distinct user ID and group ID on the host (1033), you can isolate the mapping for a specific user ID and group ID simultaneously, preventing permission overlaps for that user ID and group ID between containers. If you grant permissions to the user ID 1001 and group ID 1001 within one container, it will not affect or grant the same permissions to the corresponding user ID 1001 and group ID 1001 in other containers, as they are mapped to different host user IDs and group IDs.

This approach provides an additional layer of security and isolation for the specific user ID and group ID you want to isolate, ensuring that a compromised container cannot leverage those permissions to access resources across other containers.

Using a distinct and less commonly used user ID and group ID (such as 1033) on the host side can help avoid potential conflicts or confusion with existing IDs, making the configuration more clear and maintainable.

It's important to note that while idmap can mitigate permission overlaps for specific user IDs and group IDs, it should be used in conjunction with other security best practices, such as running services as non-root users, limiting resource sharing between containers, and following the principle of least privilege when granting permissions.