CVE-2012-4535 in Xen
Summary
by MITRE
Xen 3.4 through 4.2, and possibly earlier versions, allows local guest OS administrators to cause a denial of service (Xen infinite loop and physical CPU consumption) by setting a VCPU with an "inappropriate deadline."
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Analysis
by VulDB Data Team • 04/19/2021
The vulnerability identified as CVE-2012-4535 affects the Xen hypervisor version 3.4 through 4.2, with potential impacts on earlier releases. This issue represents a significant security flaw that enables local guest operating system administrators to execute denial of service attacks against the hypervisor itself. The vulnerability specifically targets the virtual CPU scheduling mechanism within Xen, creating a scenario where malicious actions by guest administrators can lead to severe performance degradation and system instability.
The technical flaw manifests when a guest OS administrator sets a Virtual CPU (VCPU) with an "inappropriate deadline" parameter. This improper configuration triggers an infinite loop within the Xen hypervisor's scheduling code, causing the physical CPU resources to be consumed continuously without proper termination. The vulnerability exploits the hypervisor's handling of VCPU deadlines, which are used to manage virtual processor scheduling and ensure fair resource allocation among virtual machines. When an inappropriate deadline value is set, the hypervisor enters a loop where it continuously attempts to process the invalid scheduling information, leading to excessive CPU consumption that can impact the entire physical host system.
The operational impact of this vulnerability extends beyond simple denial of service, as it can cause substantial performance degradation that affects all virtual machines running on the compromised host. Guest administrators who understand the hypervisor internals can exploit this weakness to consume excessive CPU cycles, potentially leading to system instability, resource exhaustion, and disruption of services across multiple virtual environments. This vulnerability is particularly dangerous because it allows privilege escalation from guest administrator level to system-level impact, enabling attackers to affect the availability of the entire virtualization infrastructure.
Mitigation strategies for CVE-2012-4535 involve implementing proper access controls and monitoring within virtualized environments to prevent guest administrators from setting inappropriate VCPU deadlines. System administrators should ensure that Xen hypervisor versions are updated to patched releases that address this scheduling loop vulnerability. The fix typically involves adding proper validation checks for deadline parameters in the VCPU scheduling code, preventing the infinite loop condition from occurring. Organizations should also implement monitoring solutions that can detect unusual CPU consumption patterns and alert administrators to potential exploitation attempts. This vulnerability aligns with CWE-835 which addresses infinite loops in software implementations, and represents a specific instance of how improper input validation can lead to resource exhaustion attacks. The ATT&CK framework categorizes this as a privilege escalation technique where local users leverage hypervisor weaknesses to gain broader system control and availability impact.