CVE-2021-3501: Linux Kernel Privilege Escalation Flaw

CVE-2021-3501 Overview

A critical out-of-bounds write vulnerability has been discovered in the Linux kernel's KVM (Kernel-based Virtual Machine) API affecting versions before 5.12. The flaw exists in how the internal.ndata value is mapped to an array index, which can be modified by a user process at any time, creating a race condition that leads to an out-of-bounds write. This vulnerability poses a significant threat to data integrity and system availability on affected virtualization hosts.

Critical Impact

Local attackers with low privileges can exploit this KVM vulnerability to corrupt kernel memory, potentially causing system crashes or modifying critical system data on virtualization hosts.

Affected Products

• Linux Kernel (versions before 5.12)
• Red Hat Enterprise Linux 8.0
• Red Hat Enterprise Linux for Real Time 8
• Red Hat Virtualization 4.0
• Red Hat Virtualization Host 4.0
• Fedora 33
• NetApp Cloud Backup
• NetApp SolidFire Baseboard Management Controller Firmware
• NetApp H-Series Storage Systems (H300S, H500S, H700S, H300E, H500E, H700E, H410S, H410C)

Discovery Timeline

• 2021-05-06 - CVE-2021-3501 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3501

Vulnerability Analysis

This vulnerability (CWE-787: Out-of-Bounds Write) affects the KVM hypervisor subsystem in the Linux kernel. The core issue lies in how the KVM API handles the internal.ndata value, which is used as an array index for internal data structures. Because this value can be updated by a user process asynchronously, a time-of-check to time-of-use (TOCTOU) race condition exists that allows an attacker to manipulate the index value between validation and use.

When successfully exploited, an attacker can write data outside the bounds of the intended array, corrupting adjacent kernel memory. This can lead to denial of service through system crashes, or potentially allow modification of sensitive kernel data structures. The vulnerability requires local access and low privileges, but does not require user interaction to exploit.

Root Cause

The root cause is insufficient synchronization in the KVM API when handling the internal.ndata value. The kernel fails to properly protect this value from concurrent modification by user-space processes after validation has occurred. This creates a window where an attacker can race to change the array index to an out-of-bounds value after the bounds check has passed but before the value is used for array access.

Attack Vector

The attack requires local access to a system running a vulnerable Linux kernel with KVM enabled. An attacker with low-level privileges (such as access to create virtual machines or interact with the KVM API) can exploit this vulnerability through the following sequence:

1. The attacker initiates a legitimate KVM API call that uses the internal.ndata value
2. The kernel performs bounds checking on the current value
3. In a separate thread, the attacker rapidly modifies the internal.ndata value to an out-of-bounds index
4. The kernel uses the now-corrupted index for array access, resulting in an out-of-bounds write

The vulnerability is particularly concerning for multi-tenant virtualization environments where untrusted users may have access to create or manage virtual machines. Exploitation does not require any user interaction beyond the attacker having local system access.

Detection Methods for CVE-2021-3501

Indicators of Compromise

• Unexpected kernel panics or system crashes on KVM-enabled hosts
• Anomalous behavior in virtual machine operations or KVM API calls
• Kernel log entries indicating memory corruption or out-of-bounds access errors
• Unexplained modifications to kernel memory structures

Detection Strategies

• Monitor kernel logs for KVM-related error messages, particularly those involving memory access violations
• Implement kernel integrity monitoring to detect unexpected modifications to kernel data structures
• Deploy runtime application self-protection (RASP) solutions that can detect exploitation attempts
• Use hardware-based memory protection features where available to detect out-of-bounds writes

Monitoring Recommendations

• Enable kernel auditing for KVM-related system calls and API interactions
• Configure alerting for system crashes or kernel panics on virtualization hosts
• Monitor for unusual patterns in KVM usage that may indicate exploitation attempts
• Implement centralized logging for all virtualization infrastructure to correlate suspicious events

How to Mitigate CVE-2021-3501

Immediate Actions Required

• Update the Linux kernel to version 5.12 or later immediately on all affected systems
• If immediate patching is not possible, consider temporarily disabling KVM functionality on critical systems
• Review and restrict access to KVM APIs to only authorized users and processes
• Implement enhanced monitoring on systems that cannot be immediately patched

Patch Information

The vulnerability has been addressed in Linux kernel version 5.12 and later. The fix is available in the upstream kernel repository. Organizations using affected distributions should apply vendor-specific patches:

• Linux Kernel: Update to version 5.12 or apply the specific commit 04c4f2ee3f68c9a4bf1653d15f1a9a435ae33f7a from the upstream repository. See Linux Kernel Commit Details for details.
• Red Hat Enterprise Linux: Apply security updates as documented in Red Hat Bug Report #1950136
• NetApp Products: Refer to NetApp Security Advisory NTAP-20210618-0008 for affected products and remediation guidance

Workarounds

• Restrict access to KVM functionality to only trusted administrators until patches can be applied
• Consider running virtual machines on non-KVM hypervisors temporarily if the risk is deemed critical
• Implement network segmentation to limit exposure of vulnerable virtualization hosts
• Use containerization instead of full virtualization where feasible as an interim measure

# Check current kernel version
uname -r

# Verify if KVM modules are loaded
lsmod | grep kvm

# Temporarily unload KVM modules if not in use (requires stopping all VMs first)
sudo modprobe -r kvm_intel  # For Intel processors
sudo modprobe -r kvm_amd    # For AMD processors
sudo modprobe -r kvm

# Check for available kernel updates (RHEL/CentOS)
sudo yum check-update kernel

# Apply kernel updates (RHEL/CentOS)
sudo yum update kernel -y && sudo reboot