CVE-2015-8554 in Xen
Summary
by MITRE
Buffer overflow in hw/pt-msi.c in Xen 4.6.x and earlier, when using the qemu-xen-traditional (aka qemu-dm) device model, allows local x86 HVM guest administrators to gain privileges by leveraging a system with access to a passed-through MSI-X capable physical PCI device and MSI-X table entries, related to a "write path."
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Analysis
by VulDB Data Team • 07/01/2022
The vulnerability described in CVE-2015-8554 represents a critical buffer overflow flaw within the Xen hypervisor's hardware virtualization subsystem. This issue specifically affects versions 4.6.x and earlier, where the problem manifests in the hw/pt-msi.c component responsible for handling passthrough MSI-X capabilities. The vulnerability occurs when utilizing the qemu-xen-traditional (qemu-dm) device model, which serves as the traditional device model for Xen virtual machines. The flaw is particularly dangerous because it allows local administrators within an x86 HVM guest to escalate their privileges, creating a significant security risk for virtualized environments.
The technical implementation of this vulnerability stems from improper bounds checking during the processing of MSI-X table entries within the passthrough device model. When a physical PCI device with MSI-X capabilities is passed through to a guest virtual machine, the hypervisor must manage the communication between the guest and the physical device through the MSI-X mechanism. The buffer overflow occurs in what is termed the "write path" of the MSI-X handling code, where insufficient validation of input data leads to memory corruption. This particular write path vulnerability allows an attacker to overwrite adjacent memory locations, potentially corrupting critical data structures or executing arbitrary code within the hypervisor context.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally undermines the security isolation that virtualization environments are designed to provide. An attacker with administrative access to an x86 HVM guest can exploit this flaw to elevate their privileges to that of the hypervisor itself, effectively breaking out of the guest environment and gaining control over the host system. This scenario enables attackers to compromise other virtual machines running on the same host, potentially leading to widespread system compromise within a virtualized infrastructure. The vulnerability is particularly concerning because it requires only local access within the guest environment, making it accessible to users who might otherwise have limited privileges.
This vulnerability aligns with CWE-121, which describes "Stack-based Buffer Overflow" conditions, and more specifically relates to improper input validation in memory management operations. From an ATT&CK framework perspective, this represents a privilege escalation technique that can be categorized under T1068, "Exploitation for Privilege Escalation," and potentially T1059, "Command and Scripting Interpreter," as the exploitation may involve executing commands within the compromised hypervisor environment. The attack vector requires the presence of a passed-through MSI-X capable physical PCI device, making it particularly relevant in environments where hardware passthrough is utilized for performance optimization or specialized device access.
Mitigation strategies for CVE-2015-8554 should prioritize immediate patching of affected Xen hypervisor versions, with administrators upgrading to versions 4.7.0 or later where the vulnerability has been addressed. Additionally, organizations should consider implementing strict access controls and monitoring for unusual behavior within virtual machines that have access to passthrough devices. The implementation of hypervisor security modules and regular security audits can help identify potential exploitation attempts. For environments where immediate patching is not feasible, administrators should consider disabling MSI-X passthrough capabilities for virtual machines that do not require such functionality, thereby reducing the attack surface. The vulnerability also underscores the importance of maintaining up-to-date virtualization security practices and implementing comprehensive monitoring solutions that can detect anomalous behavior patterns indicative of privilege escalation attempts.