Motivation:
For pod-level user namespaces, it's impossible to force the container runtime
to join an existing network namespace after creating a new user namespace.
According to the capabilities section in [user_namespaces(7)][1], a network
namespace created by containerd is owned by the root user namespace. When the
container runtime (like runc or crun) creates a new user namespace, it becomes
a child of the root user namespace. Processes within this child user namespace
are not permitted to access resources owned by the parent user namespace.
If the network namespace is not owned by the new user namespace, the container
runtime will fail to mount /sys due to the [sysfs: Restrict mounting sysfs][2]
patch.
Referencing the [cap_capable][3] function in Linux, a process can access a
resource if:
* The resource is owned by the process's user namespace, and the process has
the required capability.
* The resource is owned by a child of the process's user namespace, and the
owner's user namespace was created by the process's UID.
In the context of pod-level user namespaces, the CRI plugin delegates the
creation of the network namespace to the container runtime when running the
pause container. After the pause container is initialized, the CRI plugin pins
the pause container's network namespace into `/run/netns` and then executes
the `CNI_ADD` command over it.
However, if the pause container is terminated during the pinning process, the
CRI plugin might encounter a PID cycle, leading to the `CNI_ADD` command
operating on an incorrect network namespace.
Moreover, rolling back the `RunPodSandbox` API is complex due to the delegation
of network namespace creation. As highlighted in issue #10363, the CRI plugin
can lose IP information after a containerd restart, making it challenging to
maintain robustness in the RunPodSandbox API.
Solution:
Allow containerd to create a new user namespace and then create the network
namespace within that user namespace. This way, the CRI plugin can force the
container runtime to join both the user namespace and the network namespace.
Since the network namespace is owned by the newly created user namespace,
the container runtime will have the necessary permissions to mount `/sys` on
the container's root filesystem. As a result, delegation of network namespace
creation is no longer needed.
NOTE:
* The CRI plugin does not need to pin the newly created user namespace as it
does with the network namespace, because the kernel allows retrieving a user
namespace reference via [ioctl_ns(2)][4]. As a result, the podsandbox
implementation can obtain the user namespace using the `netnsPath` parameter.
[1]: <https://man7.org/linux/man-pages/man7/user_namespaces.7.html>
[2]: <7dc5dbc879>
[3]: <2c85ebc57b/security/commoncap.c (L65)>
[4]: <https://man7.org/linux/man-pages/man2/ioctl_ns.2.html>
Signed-off-by: Wei Fu <fuweid89@gmail.com>
ee0ed75d64