CVE-2016-7777 in Xen
Summary
by MITRE
Xen 4.7.x and earlier does not properly honor CR0.TS and CR0.EM, which allows local x86 HVM guest OS users to read or modify FPU, MMX, or XMM register state information belonging to arbitrary tasks on the guest by modifying an instruction while the hypervisor is preparing to emulate it.
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Analysis
by VulDB Data Team • 09/22/2022
The vulnerability identified as CVE-2016-7777 represents a critical flaw in the Xen hypervisor affecting versions 4.7.x and earlier, specifically within the handling of x86 hardware floating-point unit state management. This issue stems from the hypervisor's improper enforcement of control register bits CR0.TS and CR0.EM, which are fundamental components of x86 architecture designed to manage floating-point unit state transitions and emulation behaviors. The flaw exists in the hypervisor's implementation of hardware virtualization features, particularly within the HVM (Hardware Virtual Machine) subsystem that supports full hardware virtualization of x86 guest operating systems.
The technical execution of this vulnerability occurs when local users within an x86 HVM guest OS can manipulate instruction modifications during the hypervisor's emulation preparation phase. This creates a race condition or privilege escalation opportunity where malicious guest users can bypass the normal protection mechanisms that should prevent unauthorized access to floating-point register states. The CR0.TS bit typically controls whether the processor should trigger a device not available exception when attempting to use floating-point instructions without proper initialization, while CR0.EM controls whether the processor should emulate floating-point instructions rather than executing them directly. When these bits are not properly honored by the hypervisor, guest users can effectively access and modify FPU, MMX, or XMM register state information belonging to other tasks or processes within the same guest environment.
The operational impact of this vulnerability extends beyond simple privilege escalation as it enables unauthorized data access and modification within the guest environment. An attacker with local access to an x86 HVM guest can potentially read sensitive information from other processes' floating-point register states, modify execution contexts, or even corrupt critical floating-point computations that other applications depend upon. This vulnerability particularly affects multi-tenant virtualized environments where multiple guests share the same physical host, as it could allow one guest to indirectly access or corrupt another guest's floating-point state. The implications are significant for cloud computing platforms, virtualized servers, and any infrastructure relying on Xen hypervisor versions prior to the patched releases, as it undermines the fundamental isolation guarantees that virtualization provides.
The vulnerability aligns with CWE-284, which addresses improper access control, and relates to the broader category of hypervisor escape techniques that attackers use to gain unauthorized access to host resources. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and defense evasion through manipulation of system state management. The flaw demonstrates a critical failure in the hypervisor's virtualization security model, where the separation between guest processes and the underlying hardware virtualization layer is compromised. Effective mitigations require immediate patching of Xen hypervisor installations to versions that properly enforce CR0.TS and CR0.EM control register bits during instruction emulation. Additionally, system administrators should implement monitoring for unusual floating-point instruction patterns and consider isolating sensitive workloads on patched hypervisor versions. The vulnerability underscores the importance of maintaining up-to-date virtualization infrastructure and the critical need for proper hypervisor security testing, particularly in environments where multiple tenants share the same physical hardware resources.