CVE-2015-8552 in Xen
Summary
by MITRE
The PCI backend driver in Xen, when running on an x86 system and using Linux 3.1.x through 4.3.x as the driver domain, allows local guest administrators to generate a continuous stream of WARN messages and cause a denial of service (disk consumption) by leveraging a system with access to a passed-through MSI or MSI-X capable physical PCI device and XEN_PCI_OP_enable_msi operations, aka "Linux pciback missing sanity checks."
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Analysis
by VulDB Data Team • 07/01/2022
The vulnerability identified as CVE-2015-8552 resides within the PCI backend driver implementation of the Xen hypervisor, specifically affecting systems where the driver domain operates on Linux kernel versions 3.1.x through 4.3.x. This flaw manifests when a local guest administrator possesses access to a physically passed-through PCI device that supports MSI or MSI-X interrupt mechanisms. The core issue stems from insufficient validation within the XEN_PCI_OP_enable_msi operations, which allows malicious actors to exploit a critical gap in input sanitization. The vulnerability operates through the Xen PCI backend subsystem, which serves as the interface between virtualized guest operating systems and physical PCI hardware, creating a pathway for improper resource handling that directly impacts system stability and availability.
The technical exploitation of this vulnerability leverages the absence of proper sanity checks within the pciback driver module, which is responsible for managing PCI device passthrough operations in Xen environments. When a guest administrator executes XEN_PCI_OP_enable_msi operations against a passed-through PCI device, the system fails to validate the parameters provided, particularly those related to MSI configuration. This lack of input validation enables an attacker to repeatedly trigger WARN message generation through malformed MSI enable requests. The system's response to these malformed operations creates a continuous stream of kernel warning messages that are logged to disk storage, ultimately leading to rapid disk space exhaustion. This mechanism operates at the kernel level within the hypervisor's PCI subsystem, making it particularly dangerous as it can be exploited without requiring elevated privileges beyond those of a local guest administrator.
The operational impact of CVE-2015-8552 represents a significant denial of service vulnerability that can severely compromise system availability and performance. The continuous generation of WARN messages creates a sustained load on system logging mechanisms and disk I/O operations, with each warning message consuming filesystem space and potentially causing the affected system to become unresponsive. In virtualized environments where multiple guest domains share underlying physical resources, this vulnerability can affect not only the specific guest domain exploiting it but potentially impact the entire hypervisor host. The attack requires minimal privileges and can be executed repeatedly, making it particularly dangerous for production systems where disk space management and system logging are critical components of operational stability. The vulnerability's impact is exacerbated by the fact that the warning messages are logged to persistent storage, meaning the denial of service continues even after the initial attack vector is closed.
Mitigation strategies for CVE-2015-8552 should focus on both immediate system-level protections and long-term architectural improvements. The most direct approach involves applying kernel patches that implement proper input validation for XEN_PCI_OP_enable_msi operations, ensuring that all MSI configuration parameters are thoroughly checked before processing. System administrators should also consider implementing disk space monitoring and alerting mechanisms to detect rapid consumption of storage resources, which can serve as early warning indicators of exploitation attempts. Additionally, the vulnerability highlights the importance of proper privilege isolation in virtualized environments, where guest administrators should not be granted access to operations that can impact system stability. Organizations should implement network segmentation and access controls to limit which guest domains can interact with physical PCI devices, reducing the attack surface. From a security standards perspective, this vulnerability aligns with CWE-129 and CWE-131 categories related to input validation and improper handling of buffer sizes, while also mapping to ATT&CK techniques involving privilege escalation and denial of service through system resource exhaustion. The remediation process should include comprehensive testing of patched systems to ensure that legitimate PCI passthrough operations continue to function correctly while preventing the specific exploit patterns that lead to resource consumption issues.