CVE-2016-7092 in Xen
Summary
by MITRE
The get_page_from_l3e function in arch/x86/mm.c in Xen allows local 32-bit PV guest OS administrators to gain host OS privileges via vectors related to L3 recursive pagetables.
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Analysis
by VulDB Data Team • 04/12/2019
The vulnerability identified as CVE-2016-7092 represents a critical privilege escalation flaw within the Xen hypervisor's memory management subsystem. This issue affects 32-bit paravirtualized guest operating systems running on Xen hypervisors, where local administrators of these guest environments can potentially elevate their privileges to gain full control over the host operating system. The vulnerability resides in the get_page_from_l3e function located within the arch/x86/mm.c file, which handles the management of level 3 recursive pagetables in the x86 architecture. This flaw demonstrates the inherent risks associated with hypervisor security, where guest-level privileges can be leveraged to compromise the isolation guarantees that hypervisors are designed to provide.
The technical root cause of this vulnerability stems from improper validation and handling of recursive pagetable entries within the Xen hypervisor's memory management code. When processing level 3 pagetable entries, the get_page_from_l3e function fails to adequately verify the integrity of the recursive pointers, allowing malicious guest administrators to manipulate these entries in ways that bypass normal privilege boundaries. This particular implementation flaw enables attackers to construct specially crafted pagetable structures that, when processed by the vulnerable function, cause the hypervisor to incorrectly map memory regions, effectively allowing guest code to access host kernel memory spaces. The vulnerability specifically targets the x86 architecture's recursive pagetable mechanism, which is designed to allow the kernel to access its own page tables through a special mapping. The weakness occurs when this mechanism is improperly handled in the context of paravirtualized guests, where the guest's ability to control certain memory mappings can be exploited to escalate privileges.
The operational impact of CVE-2016-7092 is severe and far-reaching within virtualized environments that utilize Xen hypervisors. A successful exploitation of this vulnerability allows a local administrator of a 32-bit paravirtualized guest to achieve host OS privileges, effectively breaking the fundamental security isolation that separates guest operating systems from the host. This privilege escalation can lead to complete compromise of the virtualized environment, as the attacker can then access all other virtual machines running on the same host, extract sensitive data, modify system configurations, or even establish persistent backdoors. The implications extend beyond individual guest compromise, as this vulnerability can enable attackers to escalate their access to the entire hypervisor infrastructure, potentially affecting multiple tenants in multi-tenant cloud environments. Organizations relying on Xen-based virtualization platforms face significant risk, particularly those running 32-bit guest operating systems where this vulnerability can be exploited.
Mitigation strategies for CVE-2016-7092 primarily focus on patching the affected Xen hypervisor versions with the security fixes provided by the Xen Project. Organizations should immediately apply the relevant security updates that address the improper validation of recursive pagetable entries in the get_page_from_l3e function. Additionally, system administrators should consider implementing additional security controls such as disabling unnecessary guest privileges, monitoring for anomalous memory access patterns, and ensuring that all virtual machines are running updated and patched guest operating systems. The vulnerability aligns with CWE-191, which describes integer underflow conditions, and can be mapped to ATT&CK technique T1055.011 for privilege escalation through kernel exploits. Organizations should also consider implementing hypervisor hardening measures, including disabling unused hypervisor features, implementing strict access controls, and regularly auditing virtual machine configurations to prevent exploitation of similar vulnerabilities in the future. The fix typically involves strengthening input validation and ensuring proper bounds checking when processing recursive pagetable entries, thereby preventing malicious manipulation of the memory management subsystem.