CVE-2020-25668: Linux Kernel Use-After-Free Vulnerability

CVE-2020-25668 Overview

A use-after-free vulnerability was discovered in the Linux Kernel due to improper synchronization of access to the global variable fg_console within the con_font_op function. This race condition vulnerability allows a local attacker with low privileges to potentially exploit the race window between console switching operations and font manipulation, leading to memory corruption that could enable privilege escalation, arbitrary code execution, or system crashes.

Critical Impact

Local attackers can exploit this race condition to achieve privilege escalation or execute arbitrary code in kernel context by manipulating the timing of console font operations.

Affected Products

• Linux Kernel (multiple versions)
• Debian Linux 9.0
• NetApp Cloud Backup
• NetApp SolidFire & HCI Management Node
• NetApp SolidFire Baseboard Management Controller
• NetApp H410C, H300S, H500S, H700S, H300E, H500E, H700E, H410S
• NetApp A250 and 500F storage systems

Discovery Timeline

• 2020-10-30 - Initial disclosure via Openwall OSS Security mailing list
• 2020-11-04 - Additional details published via Openwall OSS Security Update
• 2021-05-26 - CVE-2020-25668 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25668

Vulnerability Analysis

This vulnerability exists in the Linux Kernel's virtual console subsystem, specifically in the con_font_op function responsible for handling console font operations. The root cause is a race condition (CWE-362) combined with improper locking (CWE-662) when accessing the fg_console global variable.

The vulnerability requires local access to exploit, and while the attack complexity is high due to the need to win the race condition, successful exploitation can result in complete compromise of confidentiality, integrity, and availability of the affected system. An attacker must have local access with low privileges to trigger the vulnerable code path.

Root Cause

The vulnerability stems from inadequate synchronization mechanisms protecting the fg_console global variable. When multiple operations access this variable concurrently—particularly during console switching while font operations are in progress—a use-after-free condition can occur. The console structure may be deallocated by one thread while another thread still holds a reference and attempts to operate on the freed memory, leading to memory corruption.

Attack Vector

The attack requires local access to the system with the ability to interact with the virtual console subsystem. An attacker would need to:

1. Trigger font operations on a console through appropriate system calls or ioctl commands
2. Simultaneously trigger a console switch operation to create a race condition
3. Win the race to cause the use-after-free, potentially gaining kernel-level code execution

The vulnerability is exploited locally through manipulation of console-related operations. The attacker must carefully time their operations to exploit the narrow race window between the check and use of the fg_console variable. Successful exploitation could allow an unprivileged local user to escalate privileges to root or cause system instability.

Detection Methods for CVE-2020-25668

Indicators of Compromise

• Unexpected kernel crashes or panics related to console subsystem operations
• Unusual activity in console font manipulation system calls (ioctl operations)
• Memory corruption errors in kernel logs referencing con_font_op or console-related functions
• Signs of privilege escalation from unprivileged user accounts

Detection Strategies

• Monitor kernel logs for KASAN (Kernel Address Sanitizer) reports indicating use-after-free conditions in console-related code paths
• Implement system call auditing for console-related ioctl operations that could indicate exploitation attempts
• Deploy kernel-level integrity monitoring to detect unauthorized privilege changes
• Review audit logs for suspicious patterns of console manipulation by non-administrative users

Monitoring Recommendations

• Enable and monitor kernel crash dump analysis for patterns consistent with use-after-free exploitation
• Implement runtime kernel integrity verification to detect memory corruption
• Monitor for unusual patterns of console switching combined with font operations
• Deploy SentinelOne Singularity Platform for real-time kernel-level threat detection and behavioral analysis

How to Mitigate CVE-2020-25668

Immediate Actions Required

• Update the Linux Kernel to a patched version containing commit 90bfdeef83f1d6c696039b6a917190dcbbad3220
• Apply vendor-specific patches from Debian, NetApp, or other affected distributions
• Restrict local access to systems running vulnerable kernel versions
• Monitor systems for signs of exploitation while patches are being applied

Patch Information

The vulnerability has been addressed in the upstream Linux Kernel. The fix is available in the Linux Kernel Git commit which implements proper synchronization for the fg_console variable access.

Debian has released patches through their LTS announcements (see Debian LTS Announcement and Debian LTS Security Notice). NetApp has also published security guidance in their Security Advisory NTAP-20210702-0005.

Workarounds

• Restrict local console access to trusted users only while awaiting kernel updates
• Disable or limit virtual console functionality where not required for operations
• Implement mandatory access control policies (SELinux/AppArmor) to restrict console-related system calls
• Consider kernel live patching solutions to apply fixes without system reboot where available

# Check current kernel version for vulnerability
uname -r

# Update kernel on Debian-based systems
sudo apt update && sudo apt upgrade linux-image-$(uname -r)

# Restrict console access permissions (temporary workaround)
chmod 600 /dev/tty[0-9]*