CVE-2021-3501 in Linux
Summary
by MITRE • 05/06/2021
A flaw was found in the Linux kernel in versions before 5.12. The value of internal.ndata, in the KVM API, is mapped to an array index, which can be updated by a user process at anytime which could lead to an out-of-bounds write. The highest threat from this vulnerability is to data integrity and system availability.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2021-3501 represents a critical out-of-bounds write flaw within the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem, specifically affecting versions prior to 5.12. This issue resides in the internal.ndata parameter handling mechanism that governs virtual machine configuration within the kernel's virtualization framework. The flaw manifests when user processes can manipulate the internal.ndata value, which serves as an array index for internal kernel data structures, creating a scenario where arbitrary memory locations can be overwritten during virtual machine operations. This vulnerability operates at the intersection of kernel-level memory management and virtualization interfaces, making it particularly dangerous for systems running virtualized workloads.
The technical exploitation of this vulnerability stems from insufficient input validation and bounds checking within the KVM API implementation. When user processes interact with the KVM subsystem through the internal.ndata parameter, they can manipulate this value to reference memory locations beyond the intended array boundaries. This creates a classic out-of-bounds write condition that can be leveraged to corrupt kernel memory structures, potentially leading to system crashes, data corruption, or privilege escalation. The vulnerability's impact is amplified because it operates within the kernel's virtualization layer, where a compromised user process could gain elevated privileges or cause system-wide instability. This flaw aligns with CWE-129, which describes improper validation of array indices, and represents a direct violation of memory safety principles in kernel space operations.
The operational impact of CVE-2021-3501 extends beyond simple system availability concerns to encompass data integrity and system stability across virtualized environments. Systems utilizing KVM for virtual machine management face potential compromise when running untrusted user code, as the vulnerability allows for arbitrary memory corruption that could be exploited to gain unauthorized access or cause denial of service conditions. The threat model particularly affects cloud environments, containerized platforms, and any infrastructure relying on kernel-based virtualization where multiple users share the same physical host. Attackers could leverage this vulnerability to execute arbitrary code with kernel privileges, potentially leading to complete system compromise, or to cause persistent availability issues through repeated exploitation attempts. The vulnerability's classification under the ATT&CK framework would align with techniques involving privilege escalation and system compromise through kernel-level vulnerabilities.
Mitigation strategies for CVE-2021-3501 primarily focus on immediate kernel version upgrades to 5.12 or later, which contain the necessary patches to address the out-of-bounds write condition. System administrators should prioritize patching all affected systems, particularly those running virtualized workloads, as the vulnerability can be exploited remotely through user processes interacting with KVM interfaces. Additional protective measures include implementing strict access controls for KVM interfaces, monitoring for anomalous ndata parameter usage patterns, and deploying kernel hardening techniques such as stack canaries and address space layout randomization. Organizations should also conduct thorough vulnerability assessments of their virtualization infrastructure to identify any potential exploitation vectors and ensure that all virtual machine configurations properly validate input parameters before processing. The vulnerability serves as a reminder of the critical importance of maintaining up-to-date kernel versions in virtualized environments where multiple users or processes interact with kernel subsystems.