CVE-2018-18021 in Linux
Summary
by MITRE
arch/arm64/kvm/guest.c in KVM in the Linux kernel before 4.18.12 on the arm64 platform mishandles the KVM_SET_ON_REG ioctl. This is exploitable by attackers who can create virtual machines. An attacker can arbitrarily redirect the hypervisor flow of control (with full register control). An attacker can also cause a denial of service (hypervisor panic) via an illegal exception return. This occurs because of insufficient restrictions on userspace access to the core register file, and because PSTATE.M validation does not prevent unintended execution modes.
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Analysis
by VulDB Data Team • 05/23/2023
The vulnerability described in CVE-2018-18021 represents a critical flaw in the Linux kernel's KVM (Kernel-based Virtual Machine) implementation specifically affecting the arm64 platform. This issue resides within the arch/arm64/kvm/guest.c file and affects kernel versions prior to 4.18.12, making it a significant concern for systems utilizing ARM64-based virtualization environments. The vulnerability stems from improper handling of the KVM_SET_ON_REG ioctl command which allows attackers with VM creation privileges to manipulate hypervisor execution flow through arbitrary register control, effectively compromising the integrity of the virtualization layer.
The technical root cause of this vulnerability lies in insufficient restrictions on userspace access to the core register file within the KVM subsystem. When attackers can create virtual machines, they exploit the lack of proper validation mechanisms that should prevent unauthorized modification of critical processor state registers. The flaw specifically involves inadequate PSTATE.M (Processor State Mode) validation which fails to prevent unintended execution modes that could lead to privilege escalation or arbitrary code execution within the hypervisor context. This represents a classic case of insufficient input validation and privilege checking, aligning with CWE-284 access control vulnerabilities that allow unauthorized privilege escalation.
The operational impact of this vulnerability extends beyond simple privilege escalation to include potential denial of service conditions through hypervisor panics caused by illegal exception returns. Attackers can effectively redirect the hypervisor's flow of control with full register manipulation capabilities, potentially enabling them to execute arbitrary code at the hypervisor level or cause system instability. This creates a severe threat landscape where a compromised virtual machine could be used as a launching point for attacks against the host system, violating fundamental virtualization security principles. The vulnerability essentially undermines the isolation guarantees that virtualization environments are designed to provide.
Mitigation strategies for CVE-2018-18021 primarily involve upgrading to Linux kernel versions 4.18.12 or later where the vulnerability has been patched. Organizations should also implement strict access controls to prevent unauthorized VM creation and execution, particularly in multi-tenant environments where guest VMs might be created by untrusted users. Additional defensive measures include monitoring for suspicious KVM ioctl usage patterns, implementing hypervisor hardening techniques, and ensuring proper privilege separation between host and guest environments. The ATT&CK framework would categorize this vulnerability under privilege escalation techniques and hypervisor abuse, while the CWE classification aligns with improper access control and insufficient validation of privilege levels.