CVE-2015-2150 in Xen
Summary
by MITRE
Xen 3.3.x through 4.5.x and the Linux kernel through 3.19.1 do not properly restrict access to PCI command registers, which might allow local guest OS users to cause a denial of service (non-maskable interrupt and host crash) by disabling the (1) memory or (2) I/O decoding for a PCI Express device and then accessing the device, which triggers an Unsupported Request (UR) response.
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Analysis
by VulDB Data Team • 04/15/2022
The vulnerability identified as CVE-2015-2150 represents a critical access control flaw affecting virtualized environments that utilize Xen hypervisors version 3.3.x through 4.5.x alongside Linux kernel versions through 3.19.1. This issue stems from insufficient restrictions on PCI command register access within virtualized systems, creating a pathway for malicious guest operating system users to exploit the underlying hardware abstraction layer. The flaw specifically targets the PCI configuration space management where guest users can manipulate command registers that control device behavior including memory and I/O decoding capabilities. When these registers are improperly modified, the vulnerability enables attackers to trigger system instability through carefully crafted device access patterns that generate Unsupported Request responses.
The technical implementation of this vulnerability involves manipulating PCI command registers to disable either memory decoding or I/O decoding for PCI Express devices within the virtualized environment. This manipulation occurs at the hypervisor level where guest operating systems can access and modify these registers through standard PCI configuration space interfaces. When a guest user disables memory or I/O decoding for a device and subsequently attempts to access that device, the system generates an Unsupported Request response which can propagate through the virtualization layer and ultimately cause system-wide disruptions including non-maskable interrupts and complete host crashes. The vulnerability exploits the fundamental trust model between hypervisor and guest operating systems where guest users should not have unrestricted access to hardware control registers that affect system stability.
The operational impact of CVE-2015-2150 extends beyond simple denial of service conditions to potentially compromise entire virtualized infrastructures. Local guest users can leverage this vulnerability to disrupt critical services running on virtual machines, leading to cascading failures that affect multiple tenants in multi-tenant cloud environments. The ability to trigger non-maskable interrupts and host crashes creates significant operational risks for cloud providers and enterprise environments that rely on virtualization for their computing infrastructure. The vulnerability particularly affects systems using PCI Express devices where the unsupported request responses can propagate through the PCIe topology and cause system-wide instability rather than isolated device failures.
Mitigation strategies for CVE-2015-2150 require immediate implementation of hypervisor updates and kernel patches that properly restrict access to PCI command registers within virtualized environments. System administrators should implement strict access controls that prevent guest operating systems from modifying critical PCI configuration registers, particularly those controlling memory and I/O decoding capabilities. The remediation process involves updating Xen hypervisor versions to 4.6.0 or later and Linux kernel versions to 3.20 or higher where proper access controls have been implemented. Additionally, organizations should consider implementing virtualization-specific security controls that monitor and restrict PCI configuration space access patterns, as outlined in the mitre attack framework under privilege escalation and defense evasion techniques. This vulnerability aligns with CWE-284 Access Control Issues and demonstrates the critical importance of maintaining proper privilege boundaries in virtualized environments, particularly when dealing with hardware-level configuration access that can affect system stability and security.
The broader implications of this vulnerability highlight the challenges inherent in virtualization security where the abstraction layer designed to provide isolation and security can itself become a vector for exploitation. This flaw demonstrates how seemingly minor configuration space access controls can have catastrophic effects on system stability and security, particularly in environments where multiple tenants share the same physical infrastructure. Organizations should implement comprehensive monitoring solutions that track PCI configuration space modifications and establish strict policies governing access to hardware-level control registers in virtualized environments. The vulnerability also underscores the importance of regular security updates and the need for robust testing procedures to ensure that hypervisor and kernel patches do not introduce compatibility issues while addressing known security flaws.