CVE-2018-5244 in Xen
Summary
by MITRE
In Xen 4.10, new infrastructure was introduced as part of an overhaul to how MSR emulation happens for guests. Unfortunately, one tracking structure isn't freed when a vcpu is destroyed. This allows guest OS administrators to cause a denial of service (host OS memory consumption) by rebooting many times.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 01/28/2021
The vulnerability identified as CVE-2018-5244 resides within the Xen hypervisor version 4.10, specifically impacting the management of Model Specific Registers (MSR) emulation infrastructure. This flaw emerged during a significant architectural overhaul of how MSR operations are handled for guest virtual machines, introducing new tracking mechanisms that were intended to improve system functionality. The root cause of the vulnerability lies in the improper memory management of a specific tracking structure that is utilized during MSR emulation processes. When a virtual cpu (vcpu) is destroyed as part of normal system operations or guest reboots, the system fails to properly release the associated tracking structure back to the host operating system's memory pool. This memory leak occurs because the cleanup routine responsible for deallocating resources does not account for this particular tracking structure, creating a persistent memory reservation that accumulates over time.
The operational impact of this vulnerability manifests as a denial of service condition that specifically targets host OS memory consumption. Attackers or malicious actors with administrative privileges within a guest operating system can exploit this flaw by repeatedly rebooting the guest virtual machine multiple times. Each reboot operation triggers the creation of a new tracking structure that is subsequently not freed when the vcpu is destroyed, leading to progressive memory consumption on the host system. This memory leak becomes increasingly problematic as the number of guest reboots increases, potentially exhausting available host memory resources and causing system instability or complete system crashes. The vulnerability particularly affects environments where guest virtual machines are frequently rebooted or where automated processes might trigger such operations, making it a significant concern for cloud providers and virtualization environments that host multiple guest instances.
This vulnerability aligns with CWE-401: "Improper Release of Memory Before Removing Last Reference" which categorizes memory management flaws where resources are not properly deallocated even when they are no longer needed. The flaw also maps to ATT&CK technique T1499.001: "Network Denial of Service" and T1070.004: "File Deletion" in the context of resource exhaustion attacks that consume system resources to prevent legitimate operations. The memory consumption pattern described in CVE-2018-5244 represents a classic example of a resource exhaustion attack where the attacker leverages legitimate system operations to accumulate memory leaks. The vulnerability demonstrates a fundamental flaw in the hypervisor's resource management protocols and highlights the critical importance of proper memory cleanup routines in virtualized environments where guest operations can directly impact host system stability. Organizations utilizing Xen hypervisor version 4.10 should implement immediate mitigations including patching to newer versions, monitoring memory consumption patterns, and implementing resource limits on guest virtual machines to prevent excessive memory accumulation.
The exploitation of this vulnerability requires guest OS administrative privileges and demonstrates the inherent security implications of hypervisor memory management. The attack vector is relatively simple and accessible, making it a significant concern for virtualization security. The cumulative nature of the memory leak means that even small amounts of memory per reboot can compound into substantial resource exhaustion over time. This vulnerability underscores the critical need for comprehensive testing of resource management code during hypervisor development and the importance of maintaining strict memory hygiene practices in virtualized environments. Security practitioners should consider implementing automated monitoring systems that detect unusual memory consumption patterns and establish baseline measurements to identify potential exploitation attempts. The remediation approach should focus on patch management and ensuring that all Xen installations are updated to versions that properly address this memory cleanup issue, while also implementing operational procedures to monitor and limit guest reboot frequency in production environments.