Resolving the Issue of Cannot Open VFIO Device in Docker Container as Non-Root User

Discover effective solutions for accessing VFIO devices in Docker containers as a non-root user. Learn how to configure Docker to avoid permission errors and enhance your virtualization experience.
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This video is based on the question https://stackoverflow.com/q/70842261/ asked by the user 'Gabe' ( https://stackoverflow.com/u/2097467/ ) and on the answer https://stackoverflow.com/a/70842386/ provided by the user 'larsks' ( https://stackoverflow.com/u/147356/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Issue of Cannot Open VFIO Device in Docker Container as Non-Root User

If you're trying to access a VFIO device within a Docker container as a non-root user and running into permission issues, you're not alone. This problem can occur due to Docker's default security settings, which limit device access for non-root users. In this guide, we'll dig deeper into the cause of this problem and present you with actionable solutions to resolve it.

Understanding the Problem

When running Docker containers, various security measures are put in place to protect the host system. One such measure is that Docker drops many privileges by default, including the ability to access most devices. This means even if you've set up your udev rules properly, as a non-root user inside a container, you may still face issues accessing the necessary resources.

Example Scenario

Consider that you have set up your environment like this:

Virtualization is enabled in the BIOS.

IOMMU is activated with the intel_iommu=on kernel command line.

A NIC is bound to the vfio-pci device, and the relevant udev rule has been added to allow access for your non-root user.

You might find that within the container, while you can see the device with proper permissions (/dev/vfio/35), trying to open it results in an IOError like so:

[[See Video to Reveal this Text or Code Snippet]]

However, this command works perfectly fine outside the container. So what can you do to fix this issue?

Solutions

Luckily, there are a couple of approaches you can take to resolve this permission issue. Below are the methods you can employ:

Method 1: Using the --device Flag

When you launch your Docker container, you can explicitly grant access to the specific device using the --device flag. It would look something like this:

[[See Video to Reveal this Text or Code Snippet]]

This command tells Docker to allow your container to access the specified VFIO device directly, circumventing the security measures that prevent permission issues for non-root users.

Method 2: Run Container as Privileged

If you need broader access within the container, you can run it with elevated privileges using the --privileged flag. This looks like:

[[See Video to Reveal this Text or Code Snippet]]

By using this option, you grant the container full access to the host's devices, including /dev, without having to specify them one by one.

Important Notes

In both methods described above, there is no need to explicitly bind-mount /dev.

In the first case, the devices you expose with --device will automatically be available, while in the second, the container will see the host's /dev by default.

Conclusion

Accessing VFIO devices in Docker containers as a non-root user can be troublesome, but with the right approaches, such as using the --device flag or running the container with --privileged, you can resolve the permission issues effectively. By applying these solutions, you should be able to continue working with your virtualization tasks smoothly. Remember to always consider the security implications of granting elevated privileges to your containers and use the most appropriate solution for your specific context.

Now you're all set to troubleshoot and resolve any VFIO device access issues in your Docker containers!

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