CVE-2021-20261: Linux Kernel Race Condition Vulnerability

CVE-2021-20261 Overview

A race condition vulnerability was discovered in the Linux kernel's implementation of the floppy disk drive controller driver software. This flaw exists due to improper synchronization in the floppy disk controller code, which could allow an attacker with local access to exploit the timing window between operations. The impact of this issue is lessened by the fact that the default permissions on the floppy device (/dev/fd0) are restricted to root. However, if the permissions on the device have been changed, the impact changes significantly. In the default configuration, root (or equivalent) permissions are required to attack this flaw.

Critical Impact

Local attackers with elevated privileges or access to a misconfigured floppy device could exploit this race condition to potentially achieve high impact on confidentiality, integrity, and availability of the affected system.

Affected Products

• Linux Kernel (multiple versions including 4.5.x series)
• Red Hat Enterprise Linux 7.0
• Linux Kernel versions with floppy disk controller driver enabled

Discovery Timeline

• 2021-03-11 - CVE-2021-20261 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-20261

Vulnerability Analysis

This vulnerability is classified as CWE-362 (Race Condition). The flaw resides in the floppy disk drive controller driver implementation within the Linux kernel. Race conditions occur when the behavior of software depends on the sequence or timing of uncontrollable events, such as the execution order of threads or processes accessing shared resources.

In this case, the floppy controller driver software contains a timing vulnerability where concurrent operations can interfere with each other, potentially leading to memory corruption or other undefined behavior. The attack requires local access and elevated privileges under default system configurations, as the floppy device (/dev/fd0) is typically restricted to root access only.

The vulnerability affects systems where the floppy disk controller driver is enabled and in use. While floppy drives are largely obsolete in modern computing environments, the driver may still be loaded in some Linux distributions for legacy hardware support or compatibility reasons.

Root Cause

The root cause of this vulnerability is improper synchronization in the floppy disk controller driver code. The driver fails to properly lock or synchronize access to shared resources during concurrent operations, creating a Time-of-Check to Time-of-Use (TOCTOU) scenario. When multiple operations attempt to access the floppy controller state simultaneously, the lack of proper locking mechanisms allows the race condition to manifest.

Attack Vector

The attack vector for this vulnerability is local, requiring the attacker to have access to the target system. Under default configurations, the attacker would need root or equivalent privileges to interact with the floppy device. However, if system administrators have relaxed permissions on /dev/fd0 to allow non-root users access, the attack surface expands significantly.

An attacker would need to:

1. Have local access to a system with the floppy controller driver loaded
2. Have appropriate permissions to access /dev/fd0 (root by default)
3. Trigger concurrent operations that exploit the race condition timing window

The vulnerability exploitation requires precise timing to hit the race condition window, making it complex to exploit reliably in practice.

Detection Methods for CVE-2021-20261

Indicators of Compromise

• Unexpected crashes or kernel panics related to the floppy controller driver (floppy.ko)
• Anomalous access patterns to /dev/fd0 device file
• Multiple concurrent processes attempting to access floppy disk resources
• System instability when floppy-related operations are performed

Detection Strategies

• Monitor kernel logs for floppy driver-related errors or warnings using dmesg or system journal
• Implement file integrity monitoring on /dev/fd0 permissions to detect unauthorized changes
• Use audit rules to track access attempts to floppy device files: auditctl -w /dev/fd0 -p rwxa -k floppy_access
• Deploy kernel-level monitoring tools to detect anomalous driver behavior

Monitoring Recommendations

• Configure system logging to capture all kernel driver events
• Set up alerts for permission changes on device files under /dev/
• Monitor for multiple rapid succession accesses to legacy hardware interfaces
• Review loaded kernel modules periodically to identify unnecessary drivers like floppy that could be disabled

How to Mitigate CVE-2021-20261

Immediate Actions Required

• Apply the latest kernel security patches from your distribution vendor
• Verify that /dev/fd0 permissions are restricted to root only (default configuration)
• Consider blacklisting the floppy driver module if not required: echo "blacklist floppy" >> /etc/modprobe.d/blacklist.conf
• Unload the floppy module if currently loaded and not needed: modprobe -r floppy

Patch Information

The fix for this vulnerability is available through the official Linux kernel repository. The patch is documented in Linux Kernel Commit a0c80ef. For Red Hat Enterprise Linux users, additional tracking and patching information is available via Red Hat Bug Report #1932150.

System administrators should apply vendor-specific kernel updates that incorporate this fix. For Red Hat Enterprise Linux 7.0 systems, ensure you have applied the latest security errata from Red Hat.

Workarounds

• Disable the floppy driver module by blacklisting it in /etc/modprobe.d/blacklist.conf
• Ensure strict permissions on /dev/fd0 remain at default (root-only access)
• Remove physical floppy hardware if present and not required for operations
• Use SELinux or AppArmor policies to restrict access to floppy device interfaces

# Configuration example - Blacklist and disable floppy driver
echo "blacklist floppy" | sudo tee /etc/modprobe.d/blacklist-floppy.conf
echo "install floppy /bin/true" | sudo tee -a /etc/modprobe.d/blacklist-floppy.conf
sudo modprobe -r floppy 2>/dev/null || true
sudo update-initramfs -u