CVE-2012-5514 in Xen
Summary
by MITRE
The guest_physmap_mark_populate_on_demand function in Xen 4.2 and earlier does not properly unlock the subject GFNs when checking if they are in use, which allows local guest HVM administrators to cause a denial of service (hang) via unspecified vectors.
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Analysis
by VulDB Data Team • 04/20/2021
The vulnerability identified as CVE-2012-5514 resides within the Xen hypervisor version 4.2 and earlier, specifically within the guest_physmap_mark_populate_on_demand function. This flaw represents a critical race condition and improper resource management issue that affects the hypervisor's memory management subsystem. The vulnerability occurs when the hypervisor fails to properly unlock guest physical frame numbers (GFNs) during the process of checking their usage status, creating a scenario where the hypervisor becomes unable to properly manage memory mappings between guest operating systems and the underlying physical hardware.
The technical implementation of this vulnerability stems from a failure in the memory management locking mechanism within Xen's hypervisor. When guest HVM administrators execute certain memory operations, the guest_physmap_mark_populate_on_demand function performs checks to determine if specific GFNs are currently in use. However, during this checking process, the function does not properly release the locks on these GFNs, causing a state where the hypervisor cannot proceed with subsequent memory management operations. This improper locking behavior creates a deadlock condition that results in the hypervisor hanging or becoming unresponsive.
The operational impact of this vulnerability is significant for cloud computing environments and virtualized infrastructures that rely on Xen hypervisors. Local guest HVM administrators who possess administrative privileges within their guest operating systems can exploit this vulnerability to cause a denial of service condition that affects the entire hypervisor. This means that an attacker with guest-level administrative access can potentially bring down the entire virtualization platform, affecting multiple virtual machines running on the same host. The vulnerability is particularly dangerous because it requires only local administrative privileges within a guest OS, making it accessible to attackers who have already compromised a virtual machine. This weakness violates the fundamental security principle that guest operating systems should not be able to affect the stability and availability of the underlying hypervisor.
The vulnerability can be categorized under CWE-362, which addresses race conditions in concurrent programming, and aligns with ATT&CK technique T1499.001 for Network Denial of Service. The improper locking mechanism creates a condition where the hypervisor's memory management subsystem becomes deadlocked, effectively preventing legitimate memory operations from completing. This creates a persistent denial of service scenario where the hypervisor becomes unresponsive to legitimate requests for memory allocation and management. The exploitation vector is particularly concerning because it does not require external network access or specialized tools, making it accessible to attackers who have already gained administrative access to a guest operating system. This makes the vulnerability particularly dangerous in multi-tenant cloud environments where guest isolation is critical for maintaining service availability and security.
Mitigation strategies for this vulnerability include upgrading to Xen hypervisor versions 4.3 and later, where the memory management locking mechanism has been properly implemented and the race condition has been resolved. Organizations should also implement strict access controls and monitoring within their virtualized environments to detect anomalous memory usage patterns that could indicate exploitation attempts. Additionally, regular security audits and vulnerability assessments should be conducted to identify and remediate similar issues in other hypervisor components. The fix implemented in newer versions addresses the core locking mechanism issue by ensuring proper release of GFN locks during memory management operations, preventing the deadlock condition that led to the denial of service scenario.