Docker: Race condition in docker cp allows bind mount redirection to host path

## Summary A race condition during `docker cp` mount setup allows a malicious container to redirect a bind mount target to an arbitrary host path, potentially overwriting host files or causing denial of service. ## Details When copying files into a container, the daemon sets up a temporary filesystem view by bind-mounting volumes into a private mount namespace. During this setup, the mount destination is created inside the container root and then a bind mount is attached using the container-relative path resolved to an absolute host path. Between mountpoint creation and the `mount()` syscall, a process running inside the container can replace the destination (or a parent path component) with a symlink pointing to an arbitrary location on the host. The `mount()` syscall follows the symlink, causing the volume to be bind-mounted onto an arbitrary host path instead of the intended container path. ## Impact A malicious container can redirect a volume bind mount to an arbitrary host path. The impact depends on the volume content and mount options: - If the volume is writable, arbitrary host files at the redirected path could be overwritten with the volume's contents. - If the volume is read-only, the host path is masked by the mount for the duration of the operation, causing denial of service. - In all cases the mount is temporary (torn down after the `docker cp` completes), but the effects of any writes persist. ### Conditions for exploitation - A container must have at least one volume mount. - A process inside the container must be able to rapidly create and swap symlinks at the volume mount destination path. - An operator must initiate a `docker cp` into that container, or call the `PUT /containers/{id}/archive` or `HEAD /containers/{id}/archive` API endpoints. ### Not affected - Containers that do not have volume mounts are not affected, as the race occurs during volume bind-mount setup. ## Workarounds - Only run containers from trusted images. - Avoid using `docker cp` with untrusted running containers. - Use authorization plugins to restrict access to the archive API endpoints (`PUT /containers/{id}/archive`, `HEAD /containers/{id}/archive`).