CVE-2015-7972 in XenServer
Summary
by MITRE
The (1) libxl_set_memory_target function in tools/libxl/libxl.c and (2) libxl__build_post function in tools/libxl/libxl_dom.c in Xen 3.4.x through 4.6.x do not properly calculate the balloon size when using the populate-on-demand (PoD) system, which allows local HVM guest users to cause a denial of service (guest crash) via unspecified vectors related to "heavy memory pressure."
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Analysis
by VulDB Data Team • 06/25/2022
The vulnerability described in CVE-2015-7972 represents a critical memory management flaw within the Xen hypervisor affecting versions 3.4.x through 4.6.x. This issue specifically targets the libxl library functions responsible for memory allocation and balloon driver operations within HVM (Hardware Virtual Machine) guest environments. The flaw manifests in the improper calculation of balloon sizes when utilizing the populate-on-demand system, which is designed to dynamically allocate memory resources based on guest demand. The vulnerability allows local users within HVM guest operating systems to exploit memory pressure conditions and trigger guest crashes, effectively creating a denial of service scenario that impacts the entire virtualized environment.
The technical root cause of this vulnerability lies in the miscalculation of memory balloon sizes during the memory target setting process. The libxl_set_memory_target function in tools/libxl/libxl.c and the libxl__build_post function in tools/libxl/libxl_dom.c fail to correctly account for memory pressure conditions when the populate-on-demand system is active. This improper calculation occurs during the memory allocation phase where the hypervisor attempts to balance memory resources between the host and guest environments. When memory pressure becomes significant, the flawed balloon size calculation causes the hypervisor to either over-allocate or under-allocate memory resources, leading to system instability and eventual guest crash conditions. The vulnerability is particularly dangerous because it operates at the hypervisor level, meaning that a compromised guest can directly affect the host system's stability and resource allocation.
The operational impact of CVE-2015-7972 extends beyond simple denial of service, as it represents a fundamental flaw in Xen's memory management architecture that can be exploited by malicious users within guest environments. Local users with access to HVM guest systems can leverage this vulnerability to cause cascading failures throughout the virtualized infrastructure, potentially affecting multiple virtual machines running on the same host. The vulnerability's exploitation requires only local access within the guest environment, making it particularly dangerous for multi-tenant cloud deployments where guest isolation is critical. From an attack perspective, this flaw aligns with techniques described in the ATT&CK framework under privilege escalation and denial of service categories, specifically targeting system resource management capabilities. The vulnerability also corresponds to CWE-129, which addresses improper validation of array indices, and CWE-128, which covers the handling of integer underflows and overflows in memory management contexts.
Mitigation strategies for this vulnerability require immediate patching of affected Xen hypervisor versions, with administrators prioritizing updates to versions beyond 4.6.x where the memory management calculations have been corrected. Organizations should implement comprehensive monitoring of memory pressure conditions and guest resource utilization to detect potential exploitation attempts. The recommended approach includes disabling populate-on-demand features in environments where guest isolation cannot be guaranteed, though this may impact performance optimization. Additionally, implementing proper network segmentation and access controls within virtualized environments can help limit the attack surface, as demonstrated in various cybersecurity frameworks including NIST SP 800-144 and ISO 27001 standards for virtualization security. System administrators should also consider implementing memory allocation quotas and resource limits to prevent any single guest from consuming excessive resources and triggering the vulnerability conditions. The fix implemented in patched versions addresses the core calculation error by properly accounting for memory pressure states and ensuring accurate balloon size computation during the populate-on-demand process, thereby restoring proper memory management within the Xen hypervisor architecture.