CVE-2016-4480 in Xen
Summary
by MITRE
The guest_walk_tables function in arch/x86/mm/guest_walk.c in Xen 4.6.x and earlier does not properly handle the Page Size (PS) page table entry bit at the L4 and L3 page table levels, which might allow local guest OS users to gain privileges via a crafted mapping of memory.
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Analysis
by VulDB Data Team • 08/21/2022
The vulnerability identified as CVE-2016-4480 resides within the Xen hypervisor's memory management subsystem, specifically in the guest_walk_tables function located at arch/x86/mm/guest_walk.c. This flaw affects Xen versions 4.6.x and earlier, representing a critical security weakness that could be exploited by malicious guest operating system users to escalate privileges. The issue stems from improper handling of the Page Size (PS) bit within page table entries at the L4 and L3 levels of the x86 page table hierarchy, creating a potential attack vector that undermines the hypervisor's memory isolation guarantees.
The technical root cause of this vulnerability lies in the improper validation of page table entry bits during guest memory walk operations. When the guest_walk_tables function processes page table entries, it fails to correctly account for the PS bit's presence at the L4 and L3 table levels, which indicates that a page table entry represents a large page rather than a regular page. This oversight allows attackers to craft specific memory mappings that bypass normal page table validation mechanisms, potentially enabling them to manipulate memory access permissions and create unauthorized mappings within the guest's virtual address space.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally compromises the security model that hypervisors provide. Local guest users can exploit this weakness to gain elevated privileges within their guest environment, potentially allowing them to execute arbitrary code with the privileges of the guest OS or even access other guest VMs running on the same host. This represents a significant bypass of the hypervisor's isolation properties, as the vulnerability enables attackers to manipulate memory mappings in ways that should be restricted to hypervisor-level operations. The attack requires local access within a guest OS but could lead to complete compromise of the guest environment and potential lateral movement to other VMs on the same physical host.
From a cybersecurity perspective, this vulnerability maps directly to CWE-119 Improper Access Control and CWE-200 Information Exposure, as it allows unauthorized access to memory regions that should remain protected. The ATT&CK framework categorizes this under T1059 Command and Scripting Interpreter and T1068 Exploitation for Privilege Escalation, as attackers would need to leverage the memory manipulation capabilities to escalate privileges within the guest environment. Organizations running affected Xen versions should immediately implement mitigations including patching to newer Xen releases, implementing strict memory access controls, and monitoring for anomalous memory mapping patterns. The vulnerability underscores the critical importance of proper page table validation in hypervisor implementations and highlights the need for comprehensive security testing of memory management subsystems in virtualization platforms.
This vulnerability demonstrates the complex security challenges inherent in virtualization environments where the hypervisor must maintain strict isolation between guest operating systems while providing necessary memory management functionality. The flaw represents a failure in the hypervisor's memory management validation logic, where the Page Size bit handling creates a path for privilege escalation that should be prevented by the hypervisor's security model. The impact extends to all systems utilizing affected Xen versions, making it a critical vulnerability that requires immediate attention from system administrators and security teams responsible for virtualized environments.