CVE-2015-7971 in Xen
Summary
by MITRE
Xen 3.2.x through 4.6.x does not limit the number of printk console messages when logging certain pmu and profiling hypercalls, which allows local guests to cause a denial of service via a sequence of crafted (1) HYPERCALL_xenoprof_op hypercalls, which are not properly handled in the do_xenoprof_op function in common/xenoprof.c, or (2) HYPERVISOR_xenpmu_op hypercalls, which are not properly handled in the do_xenpmu_op function in arch/x86/cpu/vpmu.c.
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Analysis
by VulDB Data Team • 06/25/2022
The vulnerability CVE-2015-7971 represents a significant denial of service flaw in the Xen hypervisor affecting versions 3.2.x through 4.6.x. This issue stems from inadequate input validation and resource management within the hypervisor's handling of specific performance monitoring unit operations. The vulnerability specifically targets the xenoprof and xenpmu hypercalls that are designed to provide performance profiling capabilities to guest operating systems while maintaining system stability. When these hypercalls are improperly handled by the hypervisor's kernel components, particularly in the do_xenoprof_op function located in common/xenoprof.c and the do_xenpmu_op function in arch/x86/cpu/vpmu.c, the system becomes susceptible to resource exhaustion attacks.
The technical implementation of this vulnerability exploits the lack of proper bounds checking and message limiting mechanisms in the hypervisor's console logging infrastructure. When local guest operating systems execute crafted sequences of HYPERCALL_xenoprof_op and HYPERVISOR_xenpmu_op hypercalls, the hypervisor's response mechanism fails to enforce limits on the number of printk console messages generated during processing. This allows an attacker to flood the hypervisor's console output buffer with excessive logging messages, ultimately leading to system resource exhaustion and denial of service conditions. The flaw demonstrates poor adherence to secure coding practices as outlined in CWE-770, which addresses improper resource management and excessive resource consumption vulnerabilities. The vulnerability's impact is particularly concerning because it affects the core hypervisor functionality and can be exploited by local guest users who may not have elevated privileges.
The operational impact of CVE-2015-7971 extends beyond simple service disruption as it can effectively render the entire virtualization platform unusable. Attackers can leverage this vulnerability to cause sustained denial of service conditions that may require manual intervention to resolve, including hypervisor restarts or system reboots. The vulnerability creates a scenario where guest operating systems can indirectly attack the host hypervisor infrastructure, violating fundamental security principles of virtualization isolation. From an attack methodology perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, although in this case the attack vector operates through hypervisor-level hypercalls rather than network protocols. The flaw essentially allows privilege escalation in terms of system availability impact, as local guests can cause system-wide service disruption.
Mitigation strategies for CVE-2015-7971 must focus on both immediate patching and architectural improvements to prevent similar vulnerabilities. Organizations should prioritize upgrading to Xen versions that contain fixes for this vulnerability, typically those released after the patching timeline established by the Xen Project security team. The fix implemented in subsequent versions typically involves adding proper message limiting mechanisms and bounds checking to the hypercall processing functions, ensuring that printk operations are properly controlled and do not consume excessive system resources. Additionally, system administrators should implement monitoring solutions to detect unusual patterns of hypercall usage that might indicate exploitation attempts. The vulnerability highlights the importance of adhering to secure coding practices and proper resource management in hypervisor development, particularly when handling guest-generated operations that could potentially impact system stability and availability.