CVE-2017-15596 in Xen
Summary
by MITRE
An issue was discovered in Xen 4.4.x through 4.9.x allowing ARM guest OS users to cause a denial of service (prevent physical CPU usage) because of lock mishandling upon detection of an add-to-physmap error.
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Analysis
by VulDB Data Team • 01/17/2021
The vulnerability identified as CVE-2017-15596 represents a critical denial of service flaw affecting Xen hypervisor versions 4.4.x through 4.9.x on ARM platforms. This issue stems from improper lock handling mechanisms within the hypervisor's memory management subsystem, specifically when processing add-to-physmap operations. The flaw manifests when guest operating systems running on ARM architecture attempt to manipulate physical memory mappings, triggering error conditions that are not properly managed by the hypervisor's locking mechanisms. The root cause can be categorized under CWE-664, which describes improper control of a resource through multiple access points, and more specifically aligns with CWE-362, which addresses concurrent execution issues including race conditions and lock handling problems.
The technical implementation of this vulnerability occurs within the ARM-specific memory management code path of the Xen hypervisor where the add-to-physmap functionality fails to properly acquire or release locks during error handling scenarios. When an error condition is detected during the mapping process, the hypervisor's lock state becomes inconsistent, leading to a situation where physical CPU resources become effectively unavailable for legitimate use. This lock mishandling creates a scenario where subsequent memory operations cannot proceed correctly, resulting in the complete prevention of physical CPU usage by the affected system. The flaw demonstrates characteristics consistent with the ATT&CK technique T1499.004, which involves network denial of service attacks through resource exhaustion or lock manipulation, though in this case the resource exhaustion occurs at the hypervisor level rather than network bandwidth.
The operational impact of CVE-2017-15596 extends beyond simple service disruption, as it fundamentally compromises the hypervisor's ability to manage physical CPU resources effectively. ARM guest operating systems that encounter this condition will experience complete system unresponsiveness or severe performance degradation, as the hypervisor becomes unable to properly schedule or allocate physical CPU time to guest VMs. This vulnerability affects virtualized environments where multiple ARM-based guests are running simultaneously, potentially causing cascading failures across the entire virtualization infrastructure. The flaw is particularly concerning in cloud computing environments where hypervisor stability directly impacts service availability and customer experience. Organizations running Xen hypervisors on ARM platforms must understand that this vulnerability can be exploited by malicious or compromised guest operating systems to effectively render the underlying physical hardware unusable, creating a significant risk to system availability and service continuity.
Mitigation strategies for CVE-2017-15596 should focus on immediate patch application to Xen hypervisor versions 4.10.x and later, which contain the necessary fixes for the lock handling mechanism. System administrators should implement monitoring solutions to detect abnormal lock behavior or resource allocation patterns that might indicate exploitation attempts. The vulnerability highlights the importance of proper lock management in hypervisor code and serves as a reminder of the critical nature of concurrency control in virtualization environments. Organizations should also consider implementing guest isolation measures and resource quotas to limit the potential impact of a single compromised guest on the overall system. Additionally, regular security audits of hypervisor code and proper implementation of the principle of least privilege should be enforced to minimize the attack surface and prevent exploitation of similar concurrency-related vulnerabilities in the future.