CVE-2011-2901 in Xen
Summary
by MITRE
Off-by-one error in the __addr_ok macro in Xen 3.3 and earlier allows local 64 bit PV guest administrators to cause a denial of service (host crash) via unspecified hypercalls that ignore virtual-address bits.
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Analysis
by VulDB Data Team • 01/08/2022
The vulnerability identified as CVE-2011-2901 represents a critical off-by-one error within the __addr_ok macro implementation in Xen hypervisor versions 3.3 and earlier. This flaw exists at the kernel level of the virtualization platform, specifically affecting 64-bit paravirtualized guest administrators who possess local administrative privileges. The issue stems from improper validation of virtual address boundaries during hypercall processing, creating a condition where maliciously crafted hypercalls can trigger unexpected behavior in the host system. The vulnerability manifests when virtual-address bits are ignored during address validation, allowing attackers to manipulate memory access patterns that should otherwise be restricted.
The technical implementation of this vulnerability leverages the fundamental design flaw in the __addr_ok macro which is responsible for validating memory addresses within the hypervisor's memory management subsystem. When processing certain hypercalls, the macro fails to properly account for the full address space, particularly in the context of 64-bit virtual addressing where the most significant bits play a crucial role in determining valid memory regions. This off-by-one error effectively creates a boundary condition where addresses that should be rejected as invalid are incorrectly accepted, potentially leading to memory corruption or invalid memory access attempts. The flaw operates at the intersection of virtualization layer security and memory management integrity, where guest administrators can exploit the boundary condition to bypass intended access controls.
The operational impact of CVE-2011-2901 extends beyond simple denial of service, as it represents a potential pathway for more severe compromise scenarios. While the immediate effect is a host crash that results in denial of service, the underlying vulnerability demonstrates a weakness in the hypervisor's address validation mechanisms that could theoretically be extended to other attack vectors. The vulnerability is particularly concerning because it affects local 64-bit PV guest administrators, meaning that any malicious user with administrative privileges within a guest operating system can leverage this flaw to crash the host hypervisor. This creates a scenario where guest isolation is compromised, potentially allowing for escalation attacks or service disruption across multiple virtual machines sharing the same physical host. The vulnerability aligns with CWE-129, which addresses improper validation of input boundaries, and could be categorized under ATT&CK technique T1068 for local privilege escalation through kernel vulnerabilities.
Mitigation strategies for CVE-2011-2901 primarily focus on upgrading to patched versions of the Xen hypervisor where the __addr_ok macro has been corrected to properly handle virtual address boundaries. System administrators should immediately implement patches released by Xen Project that address this specific off-by-one error in the address validation logic. Additionally, implementing strict access controls and limiting administrative privileges within guest operating systems can reduce the attack surface for this vulnerability. Network segmentation and monitoring for unusual hypercall patterns may help detect exploitation attempts, though the vulnerability's nature makes detection challenging as it operates within the legitimate virtualization interface. Organizations should also consider implementing hypervisor hardening measures that restrict the capabilities available to paravirtualized guests, particularly those with elevated privileges, to minimize the potential impact of such boundary condition exploits. The fix for this vulnerability demonstrates the importance of rigorous address validation in hypervisor implementations and the critical need for thorough testing of memory management components in virtualization environments.