CVE-2015-7970 in Xen
Summary
by MITRE
The p2m_pod_emergency_sweep function in arch/x86/mm/p2m-pod.c in Xen 3.4.x, 3.5.x, and 3.6.x is not preemptible, which allows local x86 HVM guest administrators to cause a denial of service (CPU consumption and possibly reboot) via crafted memory contents that triggers a "time-consuming linear scan," related to Populate-on-Demand.
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Analysis
by VulDB Data Team • 06/25/2022
The vulnerability identified as CVE-2015-7970 resides within the Xen hypervisor's memory management subsystem, specifically affecting versions 3.4.x through 3.6.x across x86 HVM guest environments. This issue stems from the p2m_pod_emergency_sweep function which operates within the arch/x86/mm/p2m-pod.c file, representing a critical flaw in the hypervisor's handling of memory allocation and management. The vulnerability manifests when local administrators of x86 HVM guests exploit crafted memory contents that trigger time-consuming linear scans, ultimately leading to system resource exhaustion and potential denial of service conditions.
The technical root cause of this vulnerability lies in the non-preemptible nature of the p2m_pod_emergency_sweep function, which operates without proper yielding mechanisms during its execution. This function is part of the Populate-on-Demand memory management strategy designed to optimize memory usage by allocating pages only when needed. When maliciously crafted memory contents are introduced, the function performs extensive linear scanning operations that consume substantial CPU cycles without allowing other processes to execute, creating a scenario where the hypervisor becomes unresponsive to legitimate requests. The function's inability to be preempted means that once initiated, it will continue execution until completion regardless of system load or other pending operations, effectively monopolizing system resources.
From an operational impact perspective, this vulnerability enables local x86 HVM guest administrators to perform a denial of service attack against the hypervisor itself, potentially causing complete system unresponsiveness or forced reboots. The attack vector requires only local administrative access within the guest environment, making it particularly dangerous as it can be exploited by compromised or malicious users who have elevated privileges within a virtual machine. The time-consuming linear scan operations can consume CPU resources at levels that exceed normal operational thresholds, leading to cascading effects where legitimate virtual machines hosted on the same physical system experience performance degradation or complete service interruption.
The vulnerability aligns with CWE-1130, which addresses the issue of non-preemptible functions in operating system kernels, and represents a significant concern in virtualization security environments where guest administrators may attempt to compromise host system stability. From an ATT&CK framework perspective, this vulnerability maps to T1499.004 (Evasion: Virtualization/Sandbox Evasion) and T1498 (Network Denial of Service) as it enables local users to create resource exhaustion conditions that can affect system availability. The impact extends beyond simple resource consumption, as the potential for system reboots indicates that this vulnerability could be leveraged to cause more severe disruptions in production environments where system uptime is critical.
Mitigation strategies for this vulnerability require immediate patching of affected Xen hypervisor versions to implement proper preemption mechanisms within the p2m_pod_emergency_sweep function. Organizations should also implement monitoring systems to detect unusual CPU consumption patterns that might indicate exploitation attempts, and establish strict access controls to limit local administrative privileges within virtual environments. Additionally, virtualization administrators should consider implementing resource quotas and isolation measures between virtual machines to prevent a single compromised guest from affecting the entire host system. The vulnerability demonstrates the critical importance of proper kernel design and preemptive mechanisms in virtualization environments where guest and host systems share underlying hardware resources.