CVE-2017-10921 in Xen
Summary
by MITRE
The grant-table feature in Xen through 4.8.x does not ensure sufficient type counts for a GNTMAP_device_map and GNTMAP_host_map mapping, which allows guest OS users to cause a denial of service (count mismanagement and memory corruption) or obtain privileged host OS access, aka XSA-224 bug 2.
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Analysis
by VulDB Data Team • 12/30/2020
The CVE-2017-10921 vulnerability represents a critical flaw in the Xen hypervisor's grant table implementation that affects versions 4.8.x and earlier. This vulnerability resides within the grant-table feature responsible for managing memory mappings between guest operating systems and the host hypervisor. The issue stems from insufficient type counting mechanisms when handling GNTMAP_device_map and GNTMAP_host_map mappings, creating a fundamental weakness in the hypervisor's memory management architecture. The vulnerability is classified under CWE-129 as an insufficient type count validation, which directly impacts the integrity of memory allocation processes within the virtualization layer. From an operational perspective, this flaw enables malicious guest users to manipulate the grant table's internal counters, leading to cascading effects that can compromise both system stability and security boundaries.
The technical exploitation of this vulnerability occurs through improper validation of type counts during grant table operations, specifically when mapping device and host memory regions. Attackers can manipulate the internal counter mechanisms that track the number of active mappings, causing the hypervisor to mismanage memory resources and potentially corrupt memory structures. This mismanagement manifests as either denial of service conditions where the hypervisor becomes unstable due to incorrect counter values, or more severely, privilege escalation allowing guest users to execute code with host-level privileges. The vulnerability's impact aligns with ATT&CK technique T1055 for privilege escalation and T1499 for denial of service, demonstrating how hypervisor-level flaws can be leveraged across multiple attack vectors. The flaw essentially allows attackers to bypass normal memory management controls that should prevent guest operating systems from directly accessing or corrupting host memory structures.
The operational consequences of CVE-2017-10921 extend beyond simple service disruption to encompass potential complete system compromise. When exploited, the vulnerability can lead to memory corruption that may result in kernel panics, system crashes, or more insidiously allow attackers to execute arbitrary code with the privileges of the host operating system. This represents a severe escalation from guest-level privileges to host-level control, effectively breaking the fundamental isolation that virtualization environments are designed to provide. Organizations running Xen hypervisors in production environments face significant risk, as this vulnerability could be exploited by malicious actors to gain unauthorized access to host systems, potentially compromising entire virtualized infrastructures. The vulnerability particularly affects cloud environments and virtualized deployments where multiple tenants share the same physical hardware, as guest users could exploit this flaw to attack other virtual machines or the underlying host system.
Mitigation strategies for CVE-2017-10921 require immediate patching of affected Xen hypervisor versions to address the grant table type counting implementation. Organizations should prioritize updating to Xen 4.9 or later versions where the vulnerability has been resolved through improved counter validation mechanisms. Additionally, implementing strict monitoring of grant table operations and memory allocation patterns can help detect potential exploitation attempts. Network segmentation and access controls should be reinforced to limit guest user privileges and reduce the attack surface. The fix addresses the root cause by implementing proper type count validation that prevents guest users from manipulating the internal counters used for mapping operations. Security teams should also conduct thorough vulnerability assessments of their virtualized environments to ensure all Xen installations are properly updated and that no legacy systems remain vulnerable to this specific class of memory management flaw.