CVE-2015-7813 in Xen
Summary
by MITRE
Xen 4.4.x, 4.5.x, and 4.6.x does not limit the number of printk console messages when reporting unimplemented hypercalls, which allows local guests to cause a denial of service via a sequence of (1) HYPERVISOR_physdev_op hypercalls, which are not properly handled in the do_physdev_op function in arch/arm/physdev.c, or (2) HYPERVISOR_hvm_op hypercalls, which are not properly handled in the do_hvm_op function in arch/arm/hvm.c.
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Analysis
by VulDB Data Team • 06/25/2022
The vulnerability identified as CVE-2015-7813 affects Xen hypervisor versions 4.4.x through 4.6.x and represents a significant denial of service flaw that exploits improper handling of hypercalls within the ARM architecture implementation. This issue stems from the hypervisor's failure to implement proper rate limiting or message throttling mechanisms when processing unimplemented hypercalls, creating a condition where malicious guest operating systems can exploit this weakness to consume excessive system resources and potentially crash the host system. The vulnerability specifically targets the ARM-specific implementations of hypercall handling functions, namely do_physdev_op in arch/arm/physdev.c and do_hvm_op in arch/arm/hvm.c, where the hypervisor fails to adequately process or limit the number of console messages generated during error conditions.
The technical flaw manifests through the improper handling of two distinct hypercall types that are not fully implemented within the ARM architecture support. When guest operating systems attempt to execute HYPERVISOR_physdev_op and HYPERVISOR_hvm_op hypercalls that are not properly supported, the hypervisor generates an excessive number of printk console messages without implementing any form of rate limiting or message suppression. This behavior creates a resource exhaustion condition where the console output buffer fills up rapidly, potentially leading to system instability or complete denial of service. The vulnerability operates at the hypervisor level and requires local access from a guest operating system, making it a guest-to-host privilege escalation concern that can be exploited by malicious users within the virtualized environment.
From an operational impact perspective, this vulnerability enables local guest users to perform persistent denial of service attacks against the host system by continuously generating console messages through repeated hypercall invocations. The flaw affects systems running Xen hypervisors on ARM platforms and can result in complete system unresponsiveness, requiring manual intervention or system reboot to restore normal operation. Attackers can exploit this vulnerability to disrupt services, consume system resources, and potentially cause cascading failures in virtualized environments where multiple guests are running on the same host. The impact extends beyond simple service disruption as it can compromise the integrity of the virtualization layer, potentially affecting other guests sharing the same physical hardware. This vulnerability is particularly concerning in cloud computing environments where multiple tenants share the same infrastructure and could be exploited to degrade service quality or cause system-wide outages.
The vulnerability aligns with CWE-770, which describes the allocation of resources without proper limits or throttling, and represents a classic example of resource exhaustion through improper error handling. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, which covers network denial of service attacks, and T1565.001, which involves data manipulation through modification of system processes. The flaw demonstrates a critical weakness in the hypervisor's error handling mechanisms and proper resource management, where the system fails to implement basic safeguards against excessive logging or message generation. Effective mitigations include applying the vendor-provided patches that implement proper message rate limiting and console output controls, as well as implementing monitoring solutions to detect unusual logging patterns that may indicate exploitation attempts. System administrators should also consider implementing guest isolation measures and monitoring for abnormal hypercall sequences to prevent exploitation of this vulnerability in production environments.