https://gitlab.com/abologna/kubevirt-and-kvm/-/blob/master/Isolation.md # Isolation How is the QEMU process isolated from the host and from other VMs? ## Traditional virtualization cgroups * managed by libvirt SELinux * libvirt is privileged and QEMU is protected by SELinux policies set by libvirt (SVirt) * QEMU runs with SELinux type `svirt_t` ## KubeVirt cgroups * Managed by kubelet * No involvement from libvirt * Memory limits * When using hard limits, the entire VM can be killed by Kubernetes * Memory consumption estimates are based on heuristics SELinux * KubeVirt is not using SVirt and there are no plans to do so * At the moment, the custom [KubeVirt SELinux policy][] is used to ensure libvirt has sufficient privilege to perform its own setup steps * The standard SELinux type used by containers is `container_t` * KubeVirt would like to eventually use the same for VMs as well Capabilities * The default set of capabilities is fairly conservative * Privileged operation should happen outside of the pod: in KubeVirt's case, a good candidate is `virt-handler`, the privileged components that runs at the node level * Additional capabilities can be requested for a pod * However, this is frowned upon and considered a liability from the security point of view * The cluster admin may even set a security policy that prevent pods from using certain capabilities * In such a scenario, KubeVirt workloads may be entirely unable to run ## Specific examples The following is a list of examples, either historical or current, of scenarios where libvirt's approach to isolation clashed with Kubernetes' and changes on either component were necessary. SELinux * libvirt use of hugetlbfs for hugepages config is disallowed by `container_t` * Possibly fixable by using memfd * [libvirt memoryBacking docs][] * [KubeVirt memfd issue][] * Use of libvirt+QEMU multiqueue tap support is disallowed by `container_t` * And there’s no way to pass in this setup from outside the existing stack * [KubeVirt multiqueue workaround][] extending their SELinux policy to allow `attach_queue` * Passing precreated tap devices to libvirt triggers relabelfrom+relabelto `tun_socket` SELinux access * This may not be virt stacks fault, seems to happen automatically when permissions aren’t correct Capabilities * libvirt performs memory locking for VFIO devices unconditionally * Previously KubeVirt had to grant `CAP_SYS_RESOURCE` to pods. KubeVirt worked around it by duplicating libvirt’s memory pinning calculations so the libvirt action would be a no-op, but that is fragile and may cause the issue to resurface if libvirt calculation logic changes. * References: [libvir-list memlock thread][], [KubeVirt memlock PR][], [libvirt qemuDomainGetMemLockLimitBytes][], [KubeVirt VMI.getMemlockSize][] * virtiofsd requires `CAP_SYS_ADMIN` capability to perform `unshare(CLONE_NEWPID|CLONE_NEWNS)` * This is required for certain use cases like running overlayfs in the VM on top of virtiofs, but is not a requirement for all usecases. * References: [KubeVirt virtiofs PR][], [RHEL virtiofs bug][] * KubeVirt uses libvirt for CPU pinning, which requires the pod to have `CAP_SYS_NICE`. * Long term, KubeVirt would like to handle that pinning in their privileged component virt-handler, so `CAP_SYS_NICE` can be dropped. * Sidenote: libvirt unconditionally requires `CAP_SYS_NICE` when any other running VM is using CPU pinning, however this sounds like a plain old bug. * References: [KubeVirt CPU pinning PR][], [KubeVirt CPU pinning workaround PR][], [RHEL CPU pinning bug][] * libvirt bridge usage used to require `CAP_NET_ADMIN` * This is a historical example for reference. libvirt usage of a bridge device always implied tap device creation, which required `CAP_NET_ADMIN` privileges for the pod * The fix was to teach libvirt to accept a precreated tap device and skip some setup operations on it * Example XML: `<interface type='ethernet'><target dev='mytap0' managed='no'/></interface>` * Kubevirt still hasn’t fully managed to drop `CAP_NET_ADMIN` though * References: [RHEL precreated TAP bug][], [libvirt precreated TAP patches][], [KubeVirt precreated TAP PR][], [KubeVirt NET_ADMIN issue][], [KubeVirt NET_ADMIN issue][] [KubeVirt CPU pinning PR]: https://github.com/kubevirt/kubevirt/pull/1381 [KubeVirt CPU pinning workaround PR]: https://github.com/kubevirt/kubevirt/pull/1648 [KubeVirt NET_ADMIN PR]: https://github.com/kubevirt/kubevirt/pull/3290 [KubeVirt NET_ADMIN issue]: https://github.com/kubevirt/kubevirt/issues/3085 [KubeVirt SELinux policy]: https://github.com/kubevirt/kubevirt/blob/master/cmd/virt-handler/virt_launcher.cil [KubeVirt VMI.getMemlockSize]: https://github.com/kubevirt/kubevirt/blob/f5ffba5f84365155c81d0e2cda4aa709da062230/pkg/virt-handler/isolation/isolation.go#L206 [KubeVirt memfd issue]: https://github.com/kubevirt/kubevirt/issues/3781 [KubeVirt memlock PR]: https://github.com/kubevirt/kubevirt/pull/2584 [KubeVirt multiqueue workaround]: https://github.com/kubevirt/kubevirt/pull/2941/commits/bc55cb916003c54f6cbf329112a4e36d0d874836 [KubeVirt precreated TAP PR]: https://github.com/kubevirt/kubevirt/pull/2837 [KubeVirt virtiofs PR]: https://github.com/kubevirt/kubevirt/pull/3493 [RHEL CPU pinning bug]: https://bugzilla.redhat.com/show_bug.cgi?id=1819801 [RHEL precreated TAP bug]: https://bugzilla.redhat.com/show_bug.cgi?id=1723367 [RHEL virtiofs bug]: https://bugzilla.redhat.com/show_bug.cgi?id=1854595 [libvir-list memlock thread]: https://www.redhat.com/archives/libvirt-users/2019-August/msg00046.html [libvirt memoryBacking docs]: https://libvirt.org/formatdomain.html#elementsMemoryBacking [libvirt precreated TAP patches]: https://www.redhat.com/archives/libvir-list/2019-August/msg01256.html [libvirt qemuDomainGetMemLockLimitBytes]: https://gitlab.com/libvirt/libvirt/-/blob/84bb5fd1ab2bce88e508d416f4bcea520c803ea8/src/qemu/qemu_domain.c#L8712
