CVE-2018-10901 in Linux
Summary
by MITRE
A flaw was found in Linux kernel's KVM virtualization subsystem. The VMX code does not restore the GDT.LIMIT to the previous host value, but instead sets it to 64KB. With a corrupted GDT limit a host's userspace code has an ability to place malicious entries in the GDT, particularly to the per-cpu variables. An attacker can use this to escalate their privileges.
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Analysis
by VulDB Data Team • 04/27/2023
The vulnerability identified as CVE-2018-10901 resides within the Linux kernel's KVM virtualization subsystem, specifically affecting the VMX (Intel Virtual Machine Extensions) code implementation. This flaw represents a critical privilege escalation vulnerability that exploits the improper handling of Global Descriptor Table (GDT) limits during virtual machine context switches. The issue manifests when the VMX code fails to correctly restore the GDT.LIMIT field to its original host value, instead defaulting to a fixed 64KB limit regardless of the actual host configuration.
The technical implementation of this vulnerability stems from a memory management oversight in the virtualization layer where the kernel does not properly maintain the integrity of the GDT structure during hypervisor transitions. When a virtual machine executes, the VMX code modifies the GDT.LIMIT to 64KB, but fails to restore the original host GDT.LIMIT value upon returning to the host environment. This creates a persistent state where the host operating system's memory management is compromised, as the GDT limit is artificially constrained to an insufficient value.
The operational impact of this vulnerability is severe and directly enables local privilege escalation attacks. An attacker with access to a guest virtual machine can exploit this flaw to manipulate the host's GDT structure, particularly targeting per-cpu variables that are crucial for system stability and security. The corrupted GDT limit allows malicious code in the guest environment to insert unauthorized entries into the host's GDT, effectively creating backdoors or modifying critical system memory mappings. This vulnerability is particularly dangerous in multi-tenant environments where guest VMs are not properly isolated, as it provides a direct path for one VM to compromise the host and potentially other VMs running on the same physical hardware.
From a cybersecurity perspective, this vulnerability aligns with CWE-284 (Improper Access Control) and CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) categories, as it involves improper privilege handling and shared memory management across virtualization boundaries. The attack vector follows ATT&CK techniques related to privilege escalation through kernel exploits and virtualization escape methods. The vulnerability is classified as a hypervisor-level flaw that can be leveraged by attackers to bypass traditional security boundaries, making it particularly concerning for cloud environments and virtualized infrastructure deployments. The 64KB GDT limit restriction creates a specific attack surface where malicious code can manipulate memory layouts to gain unauthorized access to system resources, potentially leading to complete system compromise and data exfiltration.
Mitigation strategies for CVE-2018-10901 require immediate kernel updates and patches from Linux kernel maintainers, as well as careful monitoring of virtualization environments for suspicious activity. Organizations should implement strict VM isolation policies and consider disabling unnecessary virtualization features. The patch addresses the root cause by ensuring proper GDT.LIMIT restoration during VMX context switches, maintaining the integrity of the host's memory management structure. Security teams should also conduct thorough vulnerability assessments of their virtualization infrastructure and implement continuous monitoring for potential exploitation attempts.