CVE-2021-3653 in Linux
Summary
by MITRE • 09/30/2021
A flaw was found in the KVM's AMD code for supporting SVM nested virtualization. The flaw occurs when processing the VMCB (virtual machine control block) provided by the L1 guest to spawn/handle a nested guest (L2). Due to improper validation of the "int_ctl" field, this issue could allow a malicious L1 to enable AVIC support (Advanced Virtual Interrupt Controller) for the L2 guest. As a result, the L2 guest would be allowed to read/write physical pages of the host, resulting in a crash of the entire system, leak of sensitive data or potential guest-to-host escape. This flaw affects Linux kernel versions prior to 5.14-rc7.
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Analysis
by VulDB Data Team • 06/20/2025
The vulnerability described in CVE-2021-3653 represents a critical security flaw within the Linux kernel's KVM hypervisor implementation, specifically affecting AMD processors that support SVM (Secure Virtual Machine) nested virtualization. This issue resides in the virtual machine control block processing mechanism that governs how nested virtual machines operate within the hypervisor environment. The flaw manifests when the hypervisor processes VMCB structures provided by the first-level guest operating system to manage and spawn second-level guest virtual machines, creating a complex multi-layered virtualization scenario that introduces significant attack surface considerations.
The technical root cause of this vulnerability stems from inadequate validation of the "int_ctl" field within the VMCB structure during nested virtualization operations. This field controls interrupt handling mechanisms and when improperly validated, allows malicious first-level guests to manipulate the Advanced Virtual Interrupt Controller functionality for second-level guests. The improper validation creates a pathway where an attacker can enable AVIC support for nested guests without proper authorization, fundamentally compromising the isolation guarantees that virtualization environments are designed to maintain. This vulnerability specifically impacts systems using AMD processors with SVM capabilities and affects Linux kernel versions prior to 5.14-rc7, making it a version-specific issue that required targeted patching.
The operational impact of this vulnerability extends far beyond simple system instability, representing a severe escalation of privileges and potential data compromise within virtualized environments. When exploited, the flaw enables malicious first-level guests to perform unauthorized read and write operations against physical memory pages of the host system, creating opportunities for complete system compromise. This capability allows for arbitrary code execution on the host, potential information disclosure, and complete bypass of virtualization security boundaries. The vulnerability essentially enables guest-to-host escape scenarios that undermine the fundamental security model of virtualization, where guest operating systems should remain isolated from the host system and other guests. The potential for system crashes, data leaks, and complete host compromise makes this vulnerability particularly dangerous in multi-tenant cloud environments or enterprise virtualization deployments.
Mitigation strategies for CVE-2021-3653 primarily focus on kernel version upgrades to 5.14-rc7 or later releases that contain the necessary patches addressing the improper validation of the int_ctl field. Organizations should prioritize updating their Linux kernel implementations across all virtualized environments, particularly those running AMD-based systems with SVM support. Additional defensive measures include implementing strict hypervisor access controls, monitoring for anomalous virtualization behavior, and ensuring proper network segmentation between guest operating systems. The vulnerability aligns with CWE-20 (Improper Input Validation) and represents a classic example of how virtualization security boundaries can be compromised through insufficient validation of hypervisor control structures. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation and defense evasion techniques, as it allows attackers to bypass hypervisor protections and gain unauthorized access to host resources, potentially enabling further attacks within the compromised environment.