CVE-2021-3753: Linux Kernel Race Condition Vulnerability

CVE-2021-3753 Overview

CVE-2021-3753 is a race condition vulnerability affecting the Linux kernel's virtual terminal (VT) subsystem. The flaw exists in the vt_k_ioctl function within drivers/tty/vt/vt_ioctl.c, where write access to vc_mode is not protected by proper locking mechanisms during the KDSETMDE ioctl operation. This race condition can lead to an out-of-bounds read in the VT subsystem, potentially exposing sensitive kernel memory data to local attackers.

Critical Impact

Local attackers with low privileges can exploit this race condition to read sensitive kernel memory, compromising data confidentiality on affected Linux systems and enterprise deployments.

Affected Products

• Linux Kernel (multiple versions)
• Red Hat Enterprise Linux 7.0 and 8.0
• NetApp Active IQ Unified Manager for VMware vSphere
• NetApp Element Software, SolidFire, and HCI Management Node
• NetApp H-Series Hardware (H300S, H500S, H700S, H410S, H410C) and associated firmware

Discovery Timeline

• September 1, 2021 - Vulnerability disclosed on the Openwall OSS-Security mailing list
• February 16, 2022 - CVE-2021-3753 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-3753

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read), stemming from a race condition in the Linux kernel's virtual terminal ioctl handler. The vt_kdsetmode function processes KDSETMDE ioctl requests to change the console mode, but the write access to the vc_mode variable was not properly synchronized with console locking mechanisms.

When multiple threads or processes access the VT ioctl interface concurrently, a Time-of-Check Time-of-Use (TOCTOU) condition can occur. An attacker exploiting this race window can trigger an out-of-bounds read operation, potentially leaking kernel memory contents. The attack requires local access and low privileges, but does not require user interaction.

The primary impact is to data confidentiality—sensitive information stored in kernel memory could be disclosed to an unprivileged local user. While the vulnerability does not directly enable code execution or system modification, leaked kernel memory could contain cryptographic keys, credentials, or memory layout information useful for subsequent attacks.

Root Cause

The root cause is insufficient synchronization in the vt_kdsetmode function. The vc_mode variable was being modified without holding the appropriate console lock, creating a race window where concurrent access could result in inconsistent state and out-of-bounds memory reads. The kernel's VT subsystem manages virtual consoles and their display modes, and this particular code path lacked the necessary locking to prevent concurrent modifications.

Attack Vector

Exploitation requires local access to the system with the ability to invoke ioctl calls on virtual terminal devices. An attacker would need to create a race condition by rapidly issuing KDSETMDE ioctl requests from multiple threads or processes simultaneously. The timing window for successful exploitation requires high attack complexity, making reliable exploitation challenging but not impossible on systems with predictable timing characteristics.

The fix adds proper console locking to the vt_kdsetmode function, ensuring that mode changes are atomic with respect to other VT operations:

  *
  * XXX It should at least call into the driver, fbdev's definitely need to
  * restore their engine state. --BenH
+ *
+ * Called with the console lock held.
  */
 static int vt_kdsetmode(struct vc_data *vc, unsigned long mode)
 {

Source: GitHub Linux Commit

Detection Methods for CVE-2021-3753

Indicators of Compromise

• Unusual ioctl system calls targeting /dev/tty* or /dev/console devices with rapid, repeated patterns
• Multiple concurrent processes attempting KDSETMDE operations on virtual terminal devices
• Kernel log messages indicating memory access anomalies in the VT subsystem
• Suspicious local user activity involving VT ioctl operations

Detection Strategies

• Monitor system calls for unusual patterns of ioctl operations on TTY devices using auditd or SentinelOne's behavioral analysis
• Deploy kernel-level monitoring to detect race condition exploitation attempts in the VT subsystem
• Implement file integrity monitoring on critical kernel modules related to TTY/VT functionality
• Use SentinelOne's Singularity platform to correlate suspicious process behavior with VT ioctl activity

Monitoring Recommendations

• Enable audit logging for ioctl system calls on virtual terminal devices (/dev/tty*, /dev/console)
• Configure SentinelOne agents to alert on anomalous kernel memory access patterns
• Monitor for processes spawning multiple threads that simultaneously access VT ioctl interfaces
• Review kernel logs regularly for VT-related errors or warnings that could indicate exploitation attempts

How to Mitigate CVE-2021-3753

Immediate Actions Required

• Update the Linux kernel to a patched version containing commit 2287a51ba822384834dafc1c798453375d1107c7
• Apply vendor-specific patches from Red Hat or NetApp for affected enterprise products
• Restrict local access to the system to trusted users only as a compensating control
• Review and limit permissions on /dev/tty* and /dev/console devices where feasible

Patch Information

The vulnerability has been addressed in the upstream Linux kernel through commit 2287a51ba822384834dafc1c798453375d1107c7, which extends console locking to the vt_kdsetmode function. Enterprise distributions have released corresponding patches:

• Red Hat: Refer to Red Hat Bug Report 1999589 for RHEL patch information
• NetApp: Consult the NetApp Security Advisory NTAP-20221028-0003 for affected product updates

Workarounds

• Limit local user access to systems running vulnerable kernel versions
• Implement mandatory access control (SELinux/AppArmor) policies to restrict ioctl operations on VT devices
• Consider disabling unused virtual console functionality where operationally feasible
• Deploy SentinelOne endpoint protection to detect and prevent exploitation attempts

# Configuration example - Audit ioctl calls on VT devices
# Add to /etc/audit/audit.rules
-a always,exit -F arch=b64 -S ioctl -F path=/dev/tty0 -F perm=w -k vt_ioctl_monitor
-a always,exit -F arch=b64 -S ioctl -F path=/dev/console -F perm=w -k vt_ioctl_monitor