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Projected Volumes
This document describes projected volumes in Kubernetes. Familiarity with volumes is suggested.
Introduction
A projected
volume maps several existing volume sources into the same directory.
Currently, the following types of volume sources can be projected:
All sources are required to be in the same namespace as the Pod. For more details, see the all-in-one volume design document.
Example configuration with a secret, a downwardAPI, and a configMap
apiVersion: v1
kind: Pod
metadata:
name: volume-test
spec:
containers:
- name: container-test
image: busybox:1.28
volumeMounts:
- name: all-in-one
mountPath: "/projected-volume"
readOnly: true
volumes:
- name: all-in-one
projected:
sources:
- secret:
name: mysecret
items:
- key: username
path: my-group/my-username
- downwardAPI:
items:
- path: "labels"
fieldRef:
fieldPath: metadata.labels
- path: "cpu_limit"
resourceFieldRef:
containerName: container-test
resource: limits.cpu
- configMap:
name: myconfigmap
items:
- key: config
path: my-group/my-config
Example configuration: secrets with a non-default permission mode set
apiVersion: v1
kind: Pod
metadata:
name: volume-test
spec:
containers:
- name: container-test
image: busybox:1.28
volumeMounts:
- name: all-in-one
mountPath: "/projected-volume"
readOnly: true
volumes:
- name: all-in-one
projected:
sources:
- secret:
name: mysecret
items:
- key: username
path: my-group/my-username
- secret:
name: mysecret2
items:
- key: password
path: my-group/my-password
mode: 511
Each projected volume source is listed in the spec under sources
. The
parameters are nearly the same with two exceptions:
- For secrets, the
secretName
field has been changed toname
to be consistent with ConfigMap naming. - The
defaultMode
can only be specified at the projected level and not for each volume source. However, as illustrated above, you can explicitly set themode
for each individual projection.
serviceAccountToken projected volumes
You can inject the token for the current service account into a Pod at a specified path. For example:
apiVersion: v1
kind: Pod
metadata:
name: sa-token-test
spec:
containers:
- name: container-test
image: busybox:1.28
volumeMounts:
- name: token-vol
mountPath: "/service-account"
readOnly: true
serviceAccountName: default
volumes:
- name: token-vol
projected:
sources:
- serviceAccountToken:
audience: api
expirationSeconds: 3600
path: token
The example Pod has a projected volume containing the injected service account
token. Containers in this Pod can use that token to access the Kubernetes API
server, authenticating with the identity of the pod's ServiceAccount.
The audience
field contains the intended audience of the
token. A recipient of the token must identify itself with an identifier specified
in the audience of the token, and otherwise should reject the token. This field
is optional and it defaults to the identifier of the API server.
The expirationSeconds
is the expected duration of validity of the service account
token. It defaults to 1 hour and must be at least 10 minutes (600 seconds). An administrator
can also limit its maximum value by specifying the --service-account-max-token-expiration
option for the API server. The path
field specifies a relative path to the mount point
of the projected volume.
subPath
volume mount will not receive updates for those volume sources.
SecurityContext interactions
The proposal for file permission handling in projected service account volume enhancement introduced the projected files having the correct owner permissions set.
Linux
In Linux pods that have a projected volume and RunAsUser
set in the Pod
SecurityContext
,
the projected files have the correct ownership set including container user
ownership.
When all containers in a pod have the same runAsUser
set in their
PodSecurityContext
or container
SecurityContext
,
then the kubelet ensures that the contents of the serviceAccountToken
volume are owned by that user,
and the token file has its permission mode set to 0600
.
Ephemeral containers added to a Pod after it is created do not change volume permissions that were set when the pod was created.
If a Pod's serviceAccountToken
volume permissions were set to 0600
because
all other containers in the Pod have the same runAsUser
, ephemeral
containers must use the same runAsUser
to be able to read the token.
Windows
In Windows pods that have a projected volume and RunAsUsername
set in the
Pod SecurityContext
, the ownership is not enforced due to the way user
accounts are managed in Windows. Windows stores and manages local user and group
accounts in a database file called Security Account Manager (SAM). Each
container maintains its own instance of the SAM database, to which the host has
no visibility into while the container is running. Windows containers are
designed to run the user mode portion of the OS in isolation from the host,
hence the maintenance of a virtual SAM database. As a result, the kubelet running
on the host does not have the ability to dynamically configure host file
ownership for virtualized container accounts. It is recommended that if files on
the host machine are to be shared with the container then they should be placed
into their own volume mount outside of C:\
.
By default, the projected files will have the following ownership as shown for an example projected volume file:
PS C:\> Get-Acl C:\var\run\secrets\kubernetes.io\serviceaccount\..2021_08_31_22_22_18.318230061\ca.crt | Format-List
Path : Microsoft.PowerShell.Core\FileSystem::C:\var\run\secrets\kubernetes.io\serviceaccount\..2021_08_31_22_22_18.318230061\ca.crt
Owner : BUILTIN\Administrators
Group : NT AUTHORITY\SYSTEM
Access : NT AUTHORITY\SYSTEM Allow FullControl
BUILTIN\Administrators Allow FullControl
BUILTIN\Users Allow ReadAndExecute, Synchronize
Audit :
Sddl : O:BAG:SYD:AI(A;ID;FA;;;SY)(A;ID;FA;;;BA)(A;ID;0x1200a9;;;BU)
This implies all administrator users like ContainerAdministrator
will have
read, write and execute access while, non-administrator users will have read and
execute access.
In general, granting the container access to the host is discouraged as it can open the door for potential security exploits.
Creating a Windows Pod with RunAsUser
in it's SecurityContext
will result in
the Pod being stuck at ContainerCreating
forever. So it is advised to not use
the Linux only RunAsUser
option with Windows Pods.