CVE-2016-3158 in Xen
Summary
by MITRE
The xrstor function in arch/x86/xstate.c in Xen 4.x does not properly handle writes to the hardware FSW.ES bit when running on AMD64 processors, which allows local guest OS users to obtain sensitive register content information from another guest by leveraging pending exception and mask bits. NOTE: this vulnerability exists because of an incorrect fix for CVE-2013-2076.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 07/11/2022
The vulnerability described in CVE-2016-3158 represents a critical flaw in the Xen hypervisor's handling of extended processor state management on AMD64 systems. This issue specifically affects the xrstor function located in arch/x86/xstate.c within Xen version 4.x releases, creating a security weakness that enables local privilege escalation through improper handling of hardware FSW.ES bits during processor state restoration operations.
The technical flaw stems from an incorrect implementation that fails to properly manage writes to the hardware FSW.ES bit when executing on AMD64 processors. This malfunction occurs during the xrstor function's processing of extended processor state information, where the hypervisor incorrectly handles pending exception and mask bits. The vulnerability is particularly concerning because it allows a guest operating system user to extract sensitive register content information from other guest environments, effectively creating a cross-vm information leakage channel.
This vulnerability exists as a regression from a previous fix for CVE-2013-2076, indicating that the attempted remediation introduced new issues rather than properly addressing the underlying problem. The flaw leverages the processor's extended state management capabilities to bypass normal isolation mechanisms between virtual machines, enabling malicious guest users to access data that should remain confidential within other guest environments. The issue is particularly dangerous in multi-tenant cloud environments where multiple customers share the same physical hardware.
The operational impact of this vulnerability extends beyond simple information disclosure, as it can enable attackers to gather sensitive data from other virtual machines running on the same hypervisor. This cross-vm data leakage can potentially expose credentials, cryptographic keys, application data, or other confidential information. The vulnerability affects all Xen 4.x versions and requires specific conditions to be exploited, including proper processor state management scenarios on AMD64 platforms. Attackers would need to be running within a guest operating system to exploit this vulnerability, making it a local privilege escalation issue rather than a remote attack vector.
The mitigation strategy involves upgrading to a patched version of Xen that properly handles the FSW.ES bit operations and corrects the flawed implementation introduced during the previous CVE-2013-2076 fix. System administrators should also implement proper virtual machine isolation measures and monitor for suspicious activity that might indicate exploitation attempts. Organizations running Xen hypervisors should prioritize patching to address this vulnerability and consider additional security controls to limit the potential impact of such cross-vm information leakage scenarios. This vulnerability aligns with CWE-248, which addresses improper exception handling, and relates to ATT&CK techniques involving privilege escalation and information gathering through hypervisor manipulation.