CVE-2023-3567: Linux Kernel Use-After-Free Vulnerability

CVE-2023-3567 Overview

A use-after-free vulnerability has been identified in the Linux Kernel's virtual console subsystem, specifically in the vcs_read function within drivers/tty/vt/vc_screen.c. This memory corruption flaw allows a local attacker with user-level access to potentially crash the system or leak sensitive kernel information, compromising both system availability and confidentiality.

Critical Impact

Local attackers can exploit this use-after-free condition to cause system crashes (denial of service) or extract sensitive internal kernel memory contents, potentially exposing privileged information.

Affected Products

• Linux Kernel (multiple versions including 6.2.0 release candidates)
• Red Hat Enterprise Linux 8.0 and 9.0
• Canonical Ubuntu Linux 14.04 ESM, 16.04 ESM, 18.04 LTS, 20.04 LTS, and 22.04 LTS

Discovery Timeline

• July 24, 2023 - CVE-2023-3567 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-3567

Vulnerability Analysis

This use-after-free vulnerability exists in the virtual console screen reading functionality of the Linux Kernel. The flaw occurs when the vcs_read function in vc_screen.c attempts to access memory that has already been freed, creating a dangerous condition where the kernel may operate on stale or reallocated memory regions.

Use-after-free vulnerabilities in kernel space are particularly concerning because they can lead to arbitrary memory access with kernel-level privileges. In this case, the vulnerability enables two primary attack outcomes: information disclosure through leaked kernel memory contents, and denial of service via system crashes triggered by accessing invalid memory.

The vulnerability requires local access to exploit, meaning an attacker must already have user-level access to the affected system. However, once exploited, the impact can be significant as kernel memory may contain sensitive data including credentials, encryption keys, or other privileged information.

Root Cause

The root cause of CVE-2023-3567 lies in improper memory lifecycle management within the vc_screen component. When a virtual console structure is freed while a concurrent read operation via vcs_read is in progress, the function continues to reference the deallocated memory. This race condition between memory deallocation and access creates the use-after-free condition, classified as CWE-416 (Use After Free).

Attack Vector

The attack vector for this vulnerability is local, requiring the attacker to have existing user-level access to the target system. The exploitation process involves:

1. An attacker with local access triggers a read operation on the virtual console screen device
2. Simultaneously, conditions are created that cause the virtual console structure to be freed
3. The vcs_read function continues to operate on the freed memory
4. Depending on timing and memory state, this results in either kernel memory disclosure or a system crash

The vulnerability can be triggered through the /dev/vcs* or /dev/vcsa* device files, which provide access to virtual console screen contents.

Detection Methods for CVE-2023-3567

Indicators of Compromise

• Unexpected system crashes or kernel panics referencing vcs_read or vc_screen in kernel logs
• Unusual access patterns to /dev/vcs* or /dev/vcsa* device files
• Kernel oops messages indicating memory corruption in the tty/vt subsystem
• Anomalous processes repeatedly accessing virtual console devices

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for use-after-free errors or memory corruption warnings
• Implement file access auditing on /dev/vcs* and /dev/vcsa* devices using auditd
• Deploy kernel-level memory corruption detection tools such as KASAN (Kernel Address Sanitizer) on test systems
• Use SentinelOne Singularity Platform to detect anomalous kernel-level behavior and memory access patterns

Monitoring Recommendations

• Enable kernel crash dump collection to capture diagnostic information during exploitation attempts
• Configure auditd rules to log access to virtual console device files
• Implement real-time monitoring of kernel log entries for memory-related errors
• Deploy endpoint detection solutions capable of identifying kernel exploitation attempts

How to Mitigate CVE-2023-3567

Immediate Actions Required

• Update the Linux Kernel to a patched version that addresses CVE-2023-3567
• Apply vendor-specific security updates from Red Hat, Canonical, or your distribution maintainer
• Restrict access to /dev/vcs* and /dev/vcsa* devices to only essential users and processes
• Consider deploying kernel live patches if available to address the vulnerability without rebooting

Patch Information

Multiple vendors have released security patches addressing this vulnerability. Red Hat has issued several security advisories including RHSA-2024:0412, RHSA-2024:0431, RHSA-2024:0432, RHSA-2024:0439, RHSA-2024:0448, RHSA-2024:0575, RHSA-2024:2394, RHSA-2024:2950, and RHSA-2024:3138. Kernel live patches are also available through LSN-0098-1 and LSN-0099-1. The upstream kernel fix is documented in the Spinics stable commits notification.

Workarounds

• Restrict access to virtual console devices using filesystem permissions or SELinux/AppArmor policies
• Disable unused virtual consoles if not required for system operation
• Implement mandatory access control policies to limit which processes can access /dev/vcs* devices
• Monitor and audit access to virtual console devices while awaiting patch deployment

# Restrict access to virtual console devices
chmod 600 /dev/vcs*
chmod 600 /dev/vcsa*

# Add audit rule to monitor virtual console access
auditctl -w /dev/vcs1 -p rwa -k vcs_access
auditctl -w /dev/vcsa1 -p rwa -k vcs_access