CVE-2016-5242 in Xen
Summary
by MITRE
The p2m_teardown function in arch/arm/p2m.c in Xen 4.4.x through 4.6.x allows local OS guest users with access to the driver domain to cause a denial of service (NULL pointer dereference and host OS crash) by creating concurrent domains and holding references to them, related to VMID exhaustion.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 08/23/2022
The vulnerability identified as CVE-2016-5242 resides within the Xen hypervisor's memory management subsystem, specifically in the p2m_teardown function located at arch/arm/p2m.c. This flaw affects Xen versions 4.4.x through 4.6.x and represents a critical security issue that can be exploited by local users within guest operating systems to induce a denial of service condition. The vulnerability stems from improper handling of virtual machine identifier (VMID) exhaustion scenarios, where concurrent domain creation operations combined with held references create a state that leads to a NULL pointer dereference in the hypervisor's memory management code.
The technical implementation of this vulnerability involves a race condition and resource management flaw in the hypervisor's page-to-machine mapping teardown process. When guest users with access to the driver domain create multiple concurrent virtual machines while maintaining references to them, the system's VMID allocation mechanism becomes exhausted. This exhaustion condition triggers the p2m_teardown function to attempt operations on freed or invalid memory references, resulting in a NULL pointer dereference that crashes the host operating system. The flaw demonstrates characteristics consistent with CWE-476, which describes NULL pointer dereference vulnerabilities, and represents a classic case of resource exhaustion leading to system instability.
The operational impact of this vulnerability extends beyond simple denial of service, as it can compromise the entire host system's stability and availability. An attacker with access to a guest environment can systematically exhaust VMID resources through concurrent domain creation, ultimately causing the hypervisor to crash and potentially leading to complete system downtime. This vulnerability particularly affects virtualized environments where multiple guest domains operate simultaneously, as the resource exhaustion occurs through legitimate system operations rather than malicious exploitation. The attack vector requires local access to the driver domain, making it a privilege escalation issue that could be leveraged by malicious users within compromised guest environments to disrupt host operations.
Mitigation strategies for CVE-2016-5242 should focus on both immediate patching and operational controls. The most effective solution involves upgrading to Xen versions that contain the patched p2m_teardown function, which properly handles VMID exhaustion conditions and prevents NULL pointer dereferences. Organizations should implement monitoring systems to detect unusual patterns of concurrent domain creation and VMID consumption, as these activities may indicate exploitation attempts. Additionally, access controls should be strengthened to limit which guest users can create and maintain concurrent domains, particularly within driver domains. The vulnerability's characteristics align with ATT&CK technique T1499.001, which covers network denial of service, and represents a hypervisor-level attack that can be classified as a privilege escalation vector under ATT&CK framework. System administrators should also consider implementing resource quotas and limits on domain creation operations to prevent exhaustion conditions from occurring in normal operational scenarios.