CVE-2015-8615 in Xen
Summary
by MITRE
The hvm_set_callback_via function in arch/x86/hvm/irq.c in Xen 4.6 does not limit the number of printk console messages when logging the new callback method, which allows local HVM guest OS users to cause a denial of service via a large number of changes to the callback method (HVM_PARAM_CALLBACK_IRQ).
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Analysis
by VulDB Data Team • 07/01/2022
The vulnerability identified as CVE-2015-8615 resides within the Xen hypervisor version 4.6, specifically in the hvm_set_callback_via function located in the arch/x86/hvm/irq.c file. This flaw represents a classic denial of service vulnerability that exploits the lack of proper input validation and message limiting mechanisms within the hypervisor's interrupt handling subsystem. The vulnerability affects HVM (Hardware Virtual Machine) guest operating systems that utilize the HVM_PARAM_CALLBACK_IRQ parameter to modify callback methods, creating a scenario where malicious or uncontrolled guest users can exploit this weakness to disrupt system operations.
The technical implementation of this vulnerability stems from insufficient bounds checking and message rate limiting within the hypervisor's logging mechanism. When the hvm_set_callback_via function processes changes to the callback method through the HVM_PARAM_CALLBACK_IRQ parameter, it fails to impose any restrictions on the number of printk console messages generated during this process. This absence of rate limiting allows an attacker to repeatedly trigger callback method changes, each generating console output that accumulates in the hypervisor's log buffer without proper throttling. The flaw manifests as a progressive accumulation of console messages that can eventually overwhelm the hypervisor's logging infrastructure, leading to system instability and denial of service conditions.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire virtualization environment. Local users within HVM guest operating systems can exploit this weakness to consume excessive system resources, particularly console buffer space and logging facilities, which may result in the hypervisor becoming unresponsive or unable to process legitimate system events. The vulnerability is particularly concerning because it requires minimal privileges to exploit, as it only necessitates access to modify HVM parameters within the guest environment. This makes it a significant threat vector for attackers who have achieved guest-level access, as they can leverage this weakness to escalate their impact on the virtualized infrastructure.
Mitigation strategies for CVE-2015-8615 should focus on implementing proper rate limiting and message throttling mechanisms within the hypervisor's logging subsystem. The most effective approach involves adding bounds checking to the hvm_set_callback_via function to limit the frequency of console messages generated during callback method changes, ensuring that excessive logging cannot occur. Additionally, system administrators should implement monitoring and alerting mechanisms to detect unusual patterns of callback method modifications that might indicate exploitation attempts. The vulnerability aligns with CWE-770, which addresses allocation of resources without limits or throttling, and represents a clear violation of the principle of least privilege in hypervisor design. Organizations should prioritize patching affected Xen versions and implementing proper resource management policies to prevent exploitation of this vulnerability. The ATT&CK framework categorizes this as a resource exhaustion technique, specifically targeting system resources through logging mechanisms, making it a critical vulnerability for virtualization security assessments and incident response planning.