CVE-2017-10915 in Xen
Summary
by MITRE
The shadow-paging feature in Xen through 4.8.x mismanages page references and consequently introduces a race condition, which allows guest OS users to obtain Xen privileges, aka XSA-219.
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Analysis
by VulDB Data Team • 10/22/2019
The vulnerability identified as CVE-2017-10915 represents a critical security flaw within the Xen hypervisor's shadow paging implementation that affects versions 4.8.x and earlier. This issue stems from improper management of page references within the shadow paging mechanism, creating a race condition that can be exploited by malicious guest operating system users to escalate their privileges and gain unauthorized access to the underlying Xen hypervisor. The vulnerability specifically impacts the hypervisor's memory management subsystem where shadow page tables are maintained to provide guest operating systems with isolated memory views while preserving system integrity.
The technical root cause of this vulnerability lies in the improper handling of reference counting mechanisms within the shadow paging feature. When guest operating systems access memory pages, the hypervisor maintains shadow page tables to track memory mappings and ensure proper isolation between virtual machines. However, the race condition occurs during the process of updating page reference counts and managing the lifecycle of shadow page table entries. This flaw allows malicious guest users to manipulate the reference counting logic in such a way that they can cause the hypervisor to release memory resources prematurely or maintain references to memory that should have been freed. The race condition specifically manifests when multiple threads or processes attempt to access and modify shadow page table entries simultaneously, creating a window where the hypervisor's internal state becomes inconsistent.
The operational impact of this vulnerability is severe and potentially catastrophic for virtualized environments that rely on Xen hypervisors. An attacker with access to a guest operating system can exploit this vulnerability to escalate privileges from guest user level to hypervisor root level, effectively breaking the fundamental security isolation that virtualization is designed to provide. This privilege escalation allows the malicious guest to access memory spaces of other virtual machines, potentially obtaining sensitive data, credentials, or system information from neighboring virtual environments. The vulnerability essentially undermines the entire virtualization security model by enabling a guest operating system to escape its isolated environment and gain control over the hypervisor itself, which could lead to complete system compromise and data breaches across multiple virtual machines.
The exploitation of this vulnerability aligns with several tactics described in the ATT&CK framework, particularly those related to privilege escalation and defense evasion. Attackers can leverage this flaw to move laterally within virtualized environments and potentially establish persistent access to the hypervisor level. The vulnerability also relates to CWE-362, which describes race conditions in software systems where multiple threads or processes access shared resources concurrently without proper synchronization. Organizations using affected Xen versions face significant risk, as this vulnerability can be exploited remotely through compromised guest operating systems, potentially affecting cloud infrastructure, data centers, and enterprise virtualization deployments. The security implications extend beyond individual virtual machines to threaten the entire virtualized infrastructure, making this a critical vulnerability that requires immediate attention and remediation.
Mitigation strategies for this vulnerability include applying the official security patches released by Xen Project for versions 4.8.x and earlier, which address the reference counting and race condition issues within the shadow paging implementation. Organizations should also implement additional monitoring and access controls to detect potential exploitation attempts, as well as consider implementing hypervisor-level security features such as hardware virtualization extensions that provide additional isolation mechanisms. Regular vulnerability assessments and security audits of virtualized environments are essential to identify and remediate similar issues before they can be exploited by malicious actors. The patching process should be prioritized and tested thoroughly to ensure that the security fixes do not introduce compatibility issues with existing virtual machine configurations or applications running within the virtualized environment.