Morphuzz: Bending (Input) Space to Fuzz Virtual Devices

The security of the entire cloud ecosystem crucially depends on the isolation guarantees that hypervisors provide between guest VMs and the host system. To allow VMs to communicate with their environment, hypervisors provide a slew of virtual-devices including network interface cards and performance-optimized VIRTIO-based SCSI adapters. As these devices sit directly on the hypervisor's isolation boundary and accept potentially attacker controlled input (e.g., from a malicious cloud tenant), bugs and vulnerabilities in the devices' implementations have the potential to render the hypervisor's isolation guarantees moot. Prior works applied fuzzing to simple virtual-devices, focusing on a narrow subset of the vast input-space and the state-of-the-art virtual-device fuzzer, Nyx, requires precise, manually-written, specifications to exercise complex devices. In this paper we present MORPHUZZ, a generic approach that leverages insights about hypervisor design combined with coverage-guided fuzzing to find bugs in virtual device implementations. Crucially MORPHUZZ does not rely on expert knowledge specific to each device. MORPHUZZ is the first approach that automatically elicits the complex I/O behaviors of the real-world virtual devices found in modern clouds. To demonstrate this capability, we implemented MORPHUZZ in QEMU and bhyve and fuzzed 33 different virtual devices (a superset of the 16 devices analyzed by prior work). Additionally, we show that MORPHUZZ is not tied to a specific CPU architecture, by fuzzing 3 additional ARM devices. MOR-PHUZZ matches or exceeds coverage obtained by Nyx, for 13/16 virtual devices, and identified a superset (110) of all crashes reported by Nyx (44). We reported all newly discovered bugs to the respective developers. Notably, MORPHUZZ achieves this without initial seed-inputs, or expert guidance.