Merge pull request #45178 from kinvolk/rata/userns-1.30

User namespaces doc changes for 1.30

Author: k8s-ci-robot

content/en/docs/concepts/workloads/pods/user-namespaces.md

@@ -7,7 +7,7 @@ min-kubernetes-server-version: v1.25
 ---
 
 <!-- overview -->
-{{< feature-state for_k8s_version="v1.25" state="alpha" >}}
+{{< feature-state for_k8s_version="v1.30" state="beta" >}}
 
 This page explains how user namespaces are used in Kubernetes pods. A user
 namespace isolates the user running inside the container from the one
@@ -46,7 +46,26 @@ tmpfs, Secrets use a tmpfs, etc.)
 Some popular filesystems that support idmap mounts in Linux 6.3 are: btrfs,
 ext4, xfs, fat, tmpfs, overlayfs.
 
-In addition, support is needed in the 
+In addition, the container runtime and its underlying OCI runtime must support
+user namespaces. The following OCI runtimes offer support:
+
+* [crun](https://github.com/containers/crun) version 1.9 or greater (it's recommend version 1.13+).
+
+<!-- ideally, update this if a newer minor release of runc comes out, whether or not it includes the idmap support -->
+{{< note >}}
+Many OCI runtimes do not include the support needed for using user namespaces in
+Linux pods. If you use a managed Kubernetes, or have downloaded it from packages
+and set it up, it's likely that nodes in your cluster use a runtime that doesn't
+include this support. For example, the most widely used OCI runtime is `runc`,
+and version `1.1.z` of runc doesn't support all the features needed by the
+Kubernetes implementation of user namespaces.
+
+If there is a newer release of runc than 1.1 available for use, check its
+documentation and release notes for compatibility (look for idmap mounts support
+in particular, because that is the missing feature).
+{{< /note >}}
+
+To use user namespaces with Kubernetes, you also need to use a CRI
 {{< glossary_tooltip text="container runtime" term_id="container-runtime" >}}
 to use this feature with Kubernetes pods:
 
@@ -137,20 +156,67 @@ use, see `man 7 user_namespaces`.
 
 ## Set up a node to support user namespaces
 
-It is recommended that the host's files and host's processes use UIDs/GIDs in
-the range of 0-65535.
+By default, the kubelet assigns pods UIDs/GIDs above the range 0-65535, based on
+the assumption that the host's files and processes use UIDs/GIDs within this
+range, which is standard for most Linux distributions. This approach prevents
+any overlap between the UIDs/GIDs of the host and those of the pods.
+
+Avoiding the overlap is important to mitigate the impact of vulnerabilities such
+as [CVE-2021-25741][CVE-2021-25741], where a pod can potentially read arbitrary
+files in the host. If the UIDs/GIDs of the pod and the host don't overlap, it is
+limited what a pod would be able to do: the pod UID/GID won't match the host's
+file owner/group.
+
+The kubelet can use a custom range for user IDs and group IDs for pods. To
+configure a custom range, the node needs to have:
+
+ * A user `kubelet` in the system (you cannot use any other username here)
+ * The binary `getsubids` installed (part of [shadow-utils][shadow-utils]) and
+   in the `PATH` for the kubelet binary.
+ * A configuration of subordinate UIDs/GIDs for the `kubelet` user (see
+   [`man 5 subuid`](https://man7.org/linux/man-pages/man5/subuid.5.html) and
+   [`man 5 subgid`](https://man7.org/linux/man-pages/man5/subgid.5.html)).
+
+This setting only gathers the UID/GID range configuration and does not change
+the user executing the `kubelet`.
+
+You must follow some constraints for the subordinate ID range that you assign
+to the `kubelet` user:
+
+* The subordinate user ID, that starts the UID range for Pods, **must** be a
+  multiple of 65536 and must also be greater than or equal to 65536. In other
+  words, you cannot use any ID from the range 0-65535 for Pods; the kubelet
+  imposes this restriction to make it difficult to create an accidentally insecure
+  configuration.
+
+* The subordinate ID count must be a multiple of 65536
+
+* The subordinate ID count must be at least `65536 x <maxPods>` where `<maxPods>`
+  is the maximum number of pods that can run on the node.
+
+* You must assign the same range for both user IDs and for group IDs, It doesn't
+  matter if other users have user ID ranges that don't align with the group ID
+  ranges.
+
+* None of the assigned ranges should overlap with any other assignment.
+
+* The subordinate configuration must be only one line. In other words, you can't
+  have multiple ranges.
 
-The kubelet will assign UIDs/GIDs higher than that to pods. Therefore, to
-guarantee as much isolation as possible, the UIDs/GIDs used by the host's files
-and host's processes should be in the range 0-65535.
+For example, you could define `/etc/subuid` and `/etc/subgid` to both have
+these entries for the `kubelet` user:
 
-Note that this recommendation is important to mitigate the impact of CVEs like
-[CVE-2021-25741][CVE-2021-25741], where a pod can potentially read arbitrary
-files in the hosts. If the UIDs/GIDs of the pod and the host don't overlap, it
-is limited what a pod would be able to do: the pod UID/GID won't match the
-host's file owner/group.
+```
+# The format is
+#   name:firstID:count of IDs
+# where
+# - firstID is 65536 (the minimum value possible)
+# - count of IDs is 110 (default limit for number of) * 65536
+kubelet:65536:7208960
+```
 
 [CVE-2021-25741]: https://github.com/kubernetes/kubernetes/issues/104980
+[shadow-utils]: https://github.com/shadow-maint/shadow
 
 ## Integration with Pod security admission checks
 

content/en/docs/reference/command-line-tools-reference/feature-gates/user-namespaces-support.md

@@ -6,8 +6,12 @@ _build:
   render: false
 
 stages:
-  - stage: alpha 
+  - stage: alpha
     defaultValue: false
     fromVersion: "1.28"
+    toVersion: "1.29"
+  - stage: beta
+    defaultValue: false
+    fromVersion: "1.30"
 ---
 Enable user namespace support for Pods.

content/en/docs/tasks/configure-pod-container/user-namespaces.md

@@ -7,7 +7,7 @@ min-kubernetes-server-version: v1.25
 ---
 
 <!-- overview -->
-{{< feature-state for_k8s_version="v1.25" state="alpha" >}}
+{{< feature-state for_k8s_version="v1.30" state="beta" >}}
 
 This page shows how to configure a user namespace for pods. This allows you to
 isolate the user running inside the container from the one in the host.
@@ -57,10 +57,6 @@ If you have a mixture of nodes and only some of the nodes provide user namespace
 Pods, you also need to ensure that the user namespace Pods are
 [scheduled](/docs/concepts/scheduling-eviction/assign-pod-node/) to suitable nodes.
 
-Please note that **if your container runtime doesn't support user namespaces, the
-`hostUsers` field in the pod spec will be silently ignored and the pod will be
-created without user namespaces.**
-
 <!-- steps -->
 
 ## Run a Pod that uses a user namespace {#create-pod}
@@ -82,27 +78,42 @@ to `false`. For example:
    kubectl attach -it userns bash
    ```
 
-And run the command. The output is similar to this:
+Run this command:
 
-```none
+```shell
 readlink /proc/self/ns/user
+```
+
+The output is similar to:
+
+```shell
 user:[4026531837]
+```
+
+Also run:
+
+```shell
 cat /proc/self/uid_map
-0          0 4294967295
 ```
 
-Then, open a shell in the host and run the same command.
+The output is similar to:
+```shell
+0  833617920      65536
+```
+
+Then, open a shell in the host and run the same commands.
+
+The `readlink` command shows the user namespace the process is running in. It
+should be different when it is run on the host and inside the container.
 
-The output must be different. This means the host and the pod are using a
-different user namespace. When user namespaces are not enabled, the host and the
-pod use the same user namespace.
+The last number of the `uid_map` file inside the container must be 65536, on the
+host it must be a bigger number.
 
 If you are running the kubelet inside a user namespace, you need to compare the
 output from running the command in the pod to the output of running in the host:
 
-```none
+```shell
 readlink /proc/$pid/ns/user
-user:[4026534732]
 ```
 
 replacing `$pid` with the kubelet PID.