CVE-2021-28697 in Xen
Summary
by MITRE • 08/28/2021
grant table v2 status pages may remain accessible after de-allocation Guest get permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, get de-allocated when a guest switched (back) from v2 to v1. The freeing of such pages requires that the hypervisor know where in the guest these pages were mapped. The hypervisor tracks only one use within guest space, but racing requests from the guest to insert mappings of these pages may result in any of them to become mapped in multiple locations. Upon switching back from v2 to v1, the guest would then retain access to a page that was freed and perhaps re-used for other purposes.
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Analysis
by VulDB Data Team • 09/01/2021
The vulnerability described in CVE-2021-28697 represents a critical memory management flaw within the Xen hypervisor's grant table v2 implementation that exposes guest virtual machines to potential privilege escalation and information disclosure attacks. This issue specifically affects the transition mechanism between grant table versions where v2 status pages are de-allocated when a guest switches back from v2 to v1 mode. The fundamental problem lies in the hypervisor's tracking mechanism which maintains awareness of only a single mapping location for these critical pages, creating a race condition scenario that can lead to memory corruption and unauthorized access. The vulnerability stems from inadequate synchronization and tracking of memory mappings during the grant table version switching process, which is a core component of Xen's memory sharing and inter-vm communication mechanisms.
The technical flaw manifests when multiple racing requests from a guest VM attempt to map the same grant table v2 status page at different locations within guest memory space. During normal operation, the hypervisor maintains a single reference to where these pages are mapped, but when concurrent mapping operations occur, the hypervisor may not properly account for all mappings. When the guest transitions from v2 to v1 mode, the hypervisor frees the status pages based on its single tracked location, while other mappings may still exist in different memory locations. This creates a scenario where the guest retains access to pages that have been deallocated and potentially re-assigned to different purposes, effectively allowing the guest to access memory that should be restricted to hypervisor operations. The flaw aligns with CWE-362, which describes a race condition in the context of concurrent access to shared resources, and specifically relates to improper handling of memory deallocation in virtualized environments.
The operational impact of this vulnerability extends beyond simple memory access issues, as it provides a potential pathway for privilege escalation attacks that could allow guest VMs to access sensitive hypervisor memory regions and potentially extract confidential information or manipulate system state. Attackers could exploit this vulnerability to read hypervisor data structures, inject malicious code into hypervisor memory, or disrupt system integrity by accessing pages that were intended to be exclusively available to the hypervisor. The vulnerability affects the fundamental security model of Xen virtualization, where guest isolation is paramount, and could enable attackers to break out of their designated virtual machine boundaries. This issue particularly impacts cloud environments and virtualized infrastructure where multiple tenants share the same physical hardware, as a compromised guest could potentially access or manipulate other guests' memory or hypervisor operations. The vulnerability also relates to ATT&CK technique T1055, which covers privilege escalation through exploitation of system vulnerabilities, and T1068, which involves local privilege escalation through the exploitation of a system's software vulnerabilities.
Mitigation strategies for this vulnerability require immediate patching of affected Xen hypervisor versions, as the fix typically involves implementing proper synchronization mechanisms to track all mappings of grant table status pages during the version switching process. Organizations should also implement monitoring for suspicious memory access patterns and ensure proper hypervisor version management to prevent exploitation. The fix typically includes enhanced tracking of all guest memory mappings before deallocation and proper synchronization between hypervisor memory management and guest mapping operations. Additional mitigations include implementing strict access controls on hypervisor memory regions, regular security audits of virtualization environments, and maintaining up-to-date hypervisor versions with security patches. System administrators should also consider implementing network segmentation and access controls to limit the potential impact of a successful exploitation, while monitoring for unusual behavior patterns that might indicate attempted exploitation of this vulnerability.