CVE-2012-3495 in XenServer
Summary
by MITRE
The physdev_get_free_pirq hypercall in arch/x86/physdev.c in Xen 4.1.x and Citrix XenServer 6.0.2 and earlier uses the return value of the get_free_pirq function as an array index without checking that the return value indicates an error, which allows guest OS users to cause a denial of service (invalid memory write and host crash) and possibly gain privileges via unspecified vectors.
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Analysis
by VulDB Data Team • 04/12/2021
The vulnerability described in CVE-2012-3495 represents a critical security flaw in the Xen hypervisor architecture that affects versions 4.1.x and Citrix XenServer 6.0.2 and earlier. This issue resides within the physdev_get_free_pirq hypercall implementation in the x86 architecture specific code file arch/x86/physdev.c. The flaw demonstrates a classic improper error handling pattern that can be exploited to compromise system integrity and availability. The vulnerability specifically targets the interaction between guest operating systems and the hypervisor's physical device management subsystem, creating a pathway for malicious exploitation that can escalate beyond simple denial of service to potential privilege escalation.
The technical implementation of this vulnerability stems from a fundamental flaw in how the hypervisor processes the return value from the get_free_pirq function. This function is designed to allocate a free interrupt request line (IRQ) for physical device management, but the physdev_get_free_pirq hypercall fails to properly validate the return value before using it as an array index. When get_free_pirq encounters an error condition, it returns a negative value indicating failure, but the calling code treats this error return as a valid index. This improper validation creates a scenario where an attacker-controlled guest OS can manipulate the system into performing invalid memory operations, specifically targeting memory locations that should remain protected from guest access. The vulnerability operates under the CWE-252 improper check or handling of exceptional conditions, which falls under the broader category of weak error handling in system components.
The operational impact of this vulnerability manifests in two primary ways: denial of service and potential privilege escalation. When exploited, the vulnerability causes invalid memory writes that can corrupt critical hypervisor data structures, leading to host crashes and complete system unavailability. This denial of service affects all virtual machines running on the compromised hypervisor, effectively bringing down entire virtualized environments. The privilege escalation aspect emerges from the fact that guest OS users can leverage this flaw to execute arbitrary code with elevated privileges, potentially allowing them to escape the virtual machine sandbox and gain access to the underlying host system. This escalation capability makes the vulnerability particularly dangerous in multi-tenant environments where multiple users share the same physical infrastructure, as it could enable one malicious tenant to compromise others or gain administrative access to the entire virtualization platform. The attack vectors are unspecified but typically involve guest OS code that makes the physdev_get_free_pirq hypercall with malicious parameters, potentially through device drivers or system management interfaces.
Mitigation strategies for this vulnerability require immediate patching of affected systems to the latest stable versions of Xen hypervisor where the error handling has been corrected. Organizations should implement comprehensive monitoring for suspicious hypercall patterns and system behavior that might indicate exploitation attempts. The fix involves adding proper validation checks to ensure that error return values from get_free_pirq are not used as array indices, which aligns with ATT&CK technique T1059 for executing commands and T1068 for privilege escalation. System administrators should also consider implementing hypervisor-level access controls and monitoring to detect unauthorized attempts to make privileged hypercalls, as this vulnerability demonstrates the importance of maintaining strict boundaries between guest operating systems and hypervisor components. Additionally, virtualization administrators should regularly audit their hypervisor configurations to ensure that unnecessary or overly permissive device access permissions are not granted to guest operating systems, thereby reducing the attack surface available to potential exploiters.