Seccomp

A system call (syscall) is the formal interface through which a user-space process asks the kernel to perform a privileged operation on its behalf — opening a file, allocating memory, creating a process, establishing a network connection, sending a signal, or any other action that requires kernel mediation. User-space code runs at CPU privilege level 3 (ring 3) and cannot directly access hardware, manipulate kernel data structures, or perform I/O; the kernel runs at ring 0 with unrestricted access. A syscall is the crossing point: the process places its request in a defined register convention and issues a syscall instruction (on x86-64) that atomically switches the CPU to ring 0 and transfers control to the kernel’s syscall dispatch table. The kernel validates the request, performs the operation if permitted by standard Unix permissions and any active LSM hooks, and returns the result. From a security perspective, the syscall boundary is the complete list of what a process can ask the kernel to do — and therefore the complete list of operations that security controls like seccomp and BPF LSM can police.

KVM (Kernel-based Virtual Machine) and QEMU (Quick Emulator) solve different halves of the same problem and are almost always used together. KVM is a Linux kernel module that turns the kernel into a hypervisor: with Intel VT-x or AMD-V, guest CPUs run on real hardware at near-native speed and guest memory is managed through extended page tables. KVM has no device model of its own — no disk, network, or firmware emulation. QEMU supplies that in userspace (virtio, USB, VGA, ACPI) and controls KVM by issuing ioctl calls on /dev/kvm — creating VMs, mapping memory, running vCPUs via KVM_RUN — while QMP exposes external management over a Unix socket. libvirt, originally from Red Hat, sits above both: it translates domain XML into QEMU command lines and lifecycle operations. The stack is the default on Linux: RHEL ships qemu-kvm, OpenShift Virtualization runs KubeVirt (virt-launcher → libvirt → QEMU), and OpenShift Sandboxed Containers uses Kata Containers with the same hypervisor to isolate pods in micro-VMs.