How to mount kerberised NFS on kubernetes?

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Is there a way to mount a Kerberos authenticated NFS server inside a Kubernetes pod as the user who created the pod?

We use FreeIPA for user management, and we have a Kubernetes cluster setup for training our deep learning models. We have our data on an NFS, which is authenticated using Kerberos. Here is what we are trying to achieve:

  1. Mount kerberized NFS in a pod.
  2. NFS should be mounted with the same permissions as that of the user who deployed the pod.
  3. Uses should not be able to exec into the pods deployed by other users and access their data.

We are using GKE for kubernetes and our NFS is in the same VPC.

kubernetes kerberos nfs authorization
1 Answers

This is how I do it.

For my approach you need:

  • Workers joined to the kerberso realm. (NFS is mounted bu the worker with the host TGT)
  • Keytabs stored in a safe place, I use vault and vault-agent-injector
  • Sidecar container that will manage the credentials
  • Shared KCM between server and pods sharing the KCM socket, avoiding other pods to have access to the stored TGTs
  • Krb5 stored as configmap in the namespace
  • Proper rights in the NFS exported data, owned by the users that exist in the IPA and which uids/gids will be used for running the containers in the pod.

The reasons for this approach are:

  • Information secured in vault
  • Secrets removed from storage and stored only in memory
  • Each container manages a single process or task.

With all this in mind you first write down your Dockerfile for the krb5-sidecar. command: ["/bin/sh"] args: ["-c", "/usr/bin/sleep 3600000"]

    FROM centos:centos7
    
    # install the kerberos client tools
    RUN yum install -y krb5-workstation && \
     mkdir /krb5 && chmod 755 /krb5
    
    # add resources, the kinit script and the default krb5 configuration
    ADD entrypoint.sh /entrypoint.sh
    RUN chmod +x /krb-sidecar-entrypoint.sh
    # Little trick here that will allow my container to remove 
    # the vault secrets without root
    RUN chmod u+s /usr/bin/rm
    ENTRYPOINT ["/entrypoint.sh"]

And this is the entrypoint script that manages

  1. Keytab load into KCM memory
  2. Removal from shared /vault/secrets file of the keytab
  3. Renewal of the kerberos ticket depending on your Krb5 policy

# Default value for renewing the TGT ticket
KERBEROS_RENEWAL_TIME=86400 # One day

# Move the keytab into keytabfile
echo  "Generating keytab file"
cat /vault/secrets/${USERNAME}.keytab  | cut -d' ' -f2 | base64 -d > /etc/${USERNAME}.keytab

# Get the TGT
echo "Loading keytab"
kinit -kt /etc/${USERNAME}.keytab ${USERNAME}@${REALM}

# Remove secrets for security reasons
rm -rf /vault/secrets/*
rm -rf /etc/${USERNAME}.keytab
echo "Secrets removed from tmpfs"
while :;
do
  kinit -R
  sleep ${KERBEROS_RENEWAL_TIME}
done

Of course you need to create PersistentVolumes and PersistentVolumeClaims for the deployment.

PersistentVolume

apiVersion: v1
kind: PersistentVolume
metadata:
  name: NFS-vol
spec:
  volumeMode: Filesystem
  accessModes:
    - ReadWriteMany
  persistentVolumeReclaimPolicy: Recycle
  storageClassName: slow
  mountOptions:
    - sec=krb5
  nfs:
    path: /exports
    server: nfs.server.test

PersistentVolumeClaim:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: nfsvol
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 3Gi

And finally the Deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: deployment-user
spec:
  selector:
    matchLabels:
      test: test
  template:
    metadata:
      labels:
        test: test
      annotations:
        vault.hashicorp.com/agent-inject: 'true'
        vault.hashicorp.com/agent-inject-secret-userKeytab: 'user/keytabs/user'
        vault.hashicorp.com/role: 'nfs'
        vault.hashicorp.com/ca-cert: 'certs/ca.crt'
        vault.hashicorp.com/tls-secret: 'tls-ca'
        vault.hashicorp.com/agent-pre-populate-only: "true"
    spec:
      securityContext:
        # Here we defined the user uid, this user must be present in the NFS server
        runAsUser: 2500
        runAsGroup: 2500
      # This may be needed or not depending on your DNS setup
      hostAliases:
        - ip: "192.168.111.130"
          hostnames:
            - "IPA"
            - "IPA.server"
        - ip: "192.168.111.131"
          hostnames:
            - "nfs"
            - "nfs.serer"
      restartPolicy: Always
      volumes:
      - name: nfs-user
        persistentVolumeClaim:
          claimName: nfs-vol
      - name: krb5
        configMap:
          name: keos-kerberos-config
      - name: kcmsocket
        hostPath:
          path: /var/run/.heim_org.h5l.kcm-socket
          type: File
      containers:
      - name: krb5-sidecar
        image: krb5-sidecar:0.1.0
        env:
        - name: KRB5CCNAME
          value: "KCM:"
        - name: USERNAME
          value: user
        - name: REALM
          value: server
        volumeMounts:
        - name: krb5
          mountPath: "/etc/krb5.conf"
          subPath: "krb5.conf"
        - name: kcmsocket
          mountPath: "/var/run/.heim_org.h5l.kcm-socket"
        lifecycle:
          preStop:
            exec:
              command: ["/usr/bin/kdestroy"]
      - name: mount-nfs-container
        image: nfs-centos:0.2.0
        env:
        - name: KRB5CCNAME
          value: "KCM:"
        volumeMounts:
        - name: nfs-user
          mountPath: "/nfs"
        - name: krb5
          mountPath: "/etc/krb5.conf"
          subPath: "krb5.conf"
        - name: kcmsocket
          mountPath: "/var/run/.heim_org.h5l.kcm-socket"
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