CVE-2020-15564 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.13.x, allowing Arm guest OS users to cause a hypervisor crash because of a missing alignment check in VCPUOP_register_vcpu_info. The hypercall VCPUOP_register_vcpu_info is used by a guest to register a shared region with the hypervisor. The region will be mapped into Xen address space so it can be directly accessed. On Arm, the region is accessed with instructions that require a specific alignment. Unfortunately, there is no check that the address provided by the guest will be correctly aligned. As a result, a malicious guest could cause a hypervisor crash by passing a misaligned address. A malicious guest administrator may cause a hypervisor crash, resulting in a Denial of Service (DoS). All Xen versions are vulnerable. Only Arm systems are vulnerable. x86 systems are not affected.
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Analysis
by VulDB Data Team • 07/08/2020
The vulnerability CVE-2020-15564 represents a critical design flaw in the Xen hypervisor affecting versions through 4.13.x, specifically impacting Arm-based virtualization environments. This issue stems from insufficient validation mechanisms within the VCPUOP_register_vcpu_info hypercall implementation, which serves as a crucial interface for guest operating systems to register shared memory regions with the hypervisor. The vulnerability manifests when Arm guest users attempt to register memory regions that lack proper alignment requirements, creating a potential pathway for malicious exploitation that could compromise system stability and availability.
The technical root cause of this vulnerability lies in the absence of alignment validation within the hypercall processing logic. When guest operating systems invoke VCPUOP_register_vcpu_info, they provide a memory address that the hypervisor maps into its own address space for direct access. On Arm architectures, specific memory access instructions require precise alignment to function correctly, typically requiring addresses to be aligned to 8-byte boundaries or other architecture-specific requirements. However, the Xen hypervisor fails to validate that the guest-provided address meets these alignment requirements, creating a scenario where misaligned memory references can trigger unexpected behavior in the hypervisor's memory management subsystem.
The operational impact of this vulnerability extends beyond simple system instability, presenting significant security and availability concerns for virtualized environments. A malicious Arm guest administrator can exploit this weakness to deliberately cause hypervisor crashes through crafted misaligned address submissions, leading to immediate denial of service conditions that affect all virtual machines hosted on the compromised hypervisor. This vulnerability affects all Xen versions within the affected range and specifically targets Arm systems, with x86 implementations remaining unaffected due to their different memory access characteristics and alignment requirements. The implications are particularly severe in cloud computing and virtualization environments where multiple tenants share the same hypervisor infrastructure, as a single compromised guest could potentially disrupt services for all other virtual machines on the same host.
The vulnerability aligns with CWE-121, which addresses 'Stack-based Buffer Overflow' and related memory access issues, while also demonstrating characteristics consistent with ATT&CK technique T1499.001, 'Fragging', which involves the use of hypervisor-level attacks to cause system instability. Organizations deploying Xen hypervisors on Arm platforms should implement immediate mitigations including firmware updates from vendors, hypervisor version upgrades, and monitoring systems to detect anomalous memory access patterns from guest operating systems. Additionally, implementing guest isolation measures and restricting administrative privileges for guest users can help reduce the attack surface and limit potential exploitation of this vulnerability. The security community should also consider this vulnerability as part of broader hypervisor security assessments, particularly in environments where Arm-based virtualization is prevalent and where maintaining system availability is critical for business operations.