CVE-2016-6258 in Xen
Summary
by MITRE
The PV pagetable code in arch/x86/mm.c in Xen 4.7.x and earlier allows local 32-bit PV guest OS administrators to gain host OS privileges by leveraging fast-paths for updating pagetable entries.
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Analysis
by VulDB Data Team • 03/13/2019
The vulnerability identified as CVE-2016-6258 represents a critical privilege escalation flaw within the Xen hypervisor affecting versions 4.7.x and earlier. This vulnerability specifically targets the paravirtualized (PV) pagetable implementation located in the arch/x86/mm.c file, where local administrators of 32-bit guest operating systems can exploit a design weakness to escalate their privileges to host system level access. The flaw exploits fast-path mechanisms intended for performance optimization in pagetable entry updates, creating an avenue for malicious code execution that bypasses normal security boundaries between guest and host environments.
The technical implementation of this vulnerability stems from insufficient validation within the pagetable update code paths, particularly when handling fast-paths for page table modifications. When a 32-bit PV guest administrator executes malicious code that manipulates pagetable entries, the hypervisor's pagetable code fails to properly validate the modifications against host security constraints. This validation gap allows the guest to modify critical pagetable structures in ways that should be restricted to the host kernel itself, effectively enabling a form of privilege escalation attack that can result in complete host compromise. The vulnerability operates through a combination of memory management manipulation and hypervisor privilege boundary violations, making it particularly dangerous in virtualized environments where guest isolation is paramount.
The operational impact of CVE-2016-6258 extends far beyond simple privilege escalation, as it fundamentally undermines the security model of Xen virtualization platforms. Attackers leveraging this vulnerability can gain complete control over the host system, potentially leading to data theft, system compromise, or use of the compromised host as a launch point for further attacks within the virtualized infrastructure. This vulnerability is particularly concerning in multi-tenant environments where multiple guests share the same physical host, as it allows one guest to potentially compromise all other guests and the underlying host system. The attack vector requires local administrative access within the guest environment, making it more accessible than some other hypervisor vulnerabilities but still representing a significant threat to virtualized infrastructure security.
Mitigation strategies for CVE-2016-6258 primarily involve upgrading to Xen versions that contain the patched implementation of the pagetable code, specifically versions beyond 4.7.x where the vulnerability has been addressed. Organizations should implement comprehensive patch management procedures to ensure all Xen hypervisors are updated promptly, as this vulnerability can be exploited by malicious actors with local access to a 32-bit PV guest environment. Additional defensive measures include monitoring for suspicious pagetable modifications and implementing strict access controls within guest environments to limit the potential impact of compromised guest systems. From a cybersecurity perspective, this vulnerability aligns with CWE-284, which addresses improper access control in software implementations, and maps to ATT&CK technique T1068, which involves local privilege escalation through exploitation of system vulnerabilities. The fix implemented by Xen developers involved strengthening the validation checks within the pagetable update mechanisms and removing or restricting the dangerous fast-path behaviors that enabled the privilege escalation.