CVE-2015-7835 in Xen
Summary
by MITRE
The mod_l2_entry function in arch/x86/mm.c in Xen 3.4 through 4.6.x does not properly validate level 2 page table entries, which allows local PV guest administrators to gain privileges via a crafted superpage mapping.
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Analysis
by VulDB Data Team • 06/25/2022
The vulnerability identified as CVE-2015-7835 resides within the Xen hypervisor's memory management subsystem, specifically in the mod_l2_entry function located in arch/x86/mm.c. This flaw affects Xen versions ranging from 3.4 through 4.6.x, representing a significant portion of the hypervisor's release history. The issue manifests as insufficient validation of level 2 page table entries, which are critical components in the virtual memory management architecture that govern how guest operating systems interact with physical memory through the hypervisor's abstraction layer.
The technical flaw stems from inadequate input validation within the page table manipulation routines that handle superpage mappings in paravirtualized (PV) guest environments. When a privileged local user within a PV guest system constructs a malicious superpage mapping, the hypervisor fails to properly validate the level 2 page table entries, allowing for potential privilege escalation. This vulnerability operates at the kernel level within the hypervisor's memory management code, making it particularly dangerous as it can be exploited by users who already have access to a guest operating system but lack administrative privileges.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it enables local PV guest administrators to potentially gain unauthorized access to system resources and escalate their privileges to root level. The attack vector requires local access within a paravirtualized guest environment, but once exploited, the consequences can be severe as the attacker could potentially access other guest VMs, compromise host system integrity, or extract sensitive data from the hypervisor environment. This represents a critical security weakness that undermines the fundamental isolation guarantees that hypervisors are designed to provide between virtual machines and the host system.
The vulnerability aligns with CWE-121, which addresses stack-based buffer overflow conditions, and demonstrates characteristics consistent with privilege escalation flaws that can be leveraged to bypass access controls. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques, specifically T1068, which involves exploiting vulnerabilities to gain higher-level privileges. The remediation strategy involves patching the affected Xen versions with proper input validation routines that ensure level 2 page table entries are thoroughly validated before being processed. Organizations should immediately update to patched versions of Xen, with particular attention to versions 4.7 and later, which contain the necessary fixes to address this memory management validation flaw. The fix implements proper validation checks that prevent malicious superpage mappings from being processed, thereby maintaining the integrity of the hypervisor's memory management system and preserving the security boundaries between virtual machines and the host system.