CVE-2026-31618: Linux Kernel DOS Vulnerability

CVE-2026-31618 Overview

CVE-2026-31618 is a divide-by-zero vulnerability in the Linux kernel's tdfxfb framebuffer driver. The flaw resides in the handling of the FBIOPUT_VSCREENINFO ioctl, where the driver uses the user-supplied pixclock value directly as a divisor without validating that it is non-zero. A local user with access to the framebuffer device can trigger a kernel crash by supplying a zero pixclock value, producing a denial-of-service condition. The fix mirrors the approach taken in commit 19f953e74356 ("fbdev: fb_pm2fb: Avoid potential divide by zero error"), adding an input validation check before the division operation. The vulnerability is tracked under [CWE-369: Divide By Zero].

Critical Impact
A local, low-privileged user can trigger a kernel-level divide-by-zero, crashing the system and causing a denial of service on affected Linux hosts.

Affected Products

Linux Kernel (multiple stable branches affected; see kernel.org commit references)
Systems with the tdfxfb framebuffer driver enabled
Linux Kernel versions back through 2.6.12 and earlier stable trees referenced by the patch series

Discovery Timeline

2026-04-24 - CVE-2026-31618 published to NVD
2026-04-28 - Last updated in NVD database

Technical Details for CVE-2026-31618

Vulnerability Analysis

The tdfxfb driver implements a framebuffer interface for 3dfx graphics hardware in the Linux kernel. When userspace issues an FBIOPUT_VSCREENINFO ioctl, the driver consumes the pixclock field from the supplied fb_var_screeninfo structure. This field represents the pixel clock period and is later used as a divisor when computing timing parameters.

Because pixclock is taken directly from userspace without a zero check, supplying a value of 0 causes an arithmetic division by zero inside kernel context. The resulting fault terminates kernel execution paths and produces a crash, denying service to all users on the host. The fix introduces a guard that rejects a zero pixclock value before any division, aligning the driver's behavior with the earlier fb_pm2fb fix referenced by the commit message.

Root Cause

The root cause is missing input validation on attacker-controlled numeric input. The driver trusts the pixclock field from FBIOPUT_VSCREENINFO and performs division using it without bounds checking, violating defensive coding practices for kernel ioctl handlers [CWE-369].

Attack Vector

Exploitation requires local access and the ability to open the /dev/fb* device node associated with the tdfxfb driver. An attacker with read/write access to the framebuffer device issues an FBIOPUT_VSCREENINFO ioctl carrying a crafted fb_var_screeninfo structure where pixclock is set to 0. The ioctl handler subsequently performs a division using this value, generating a kernel divide-by-zero fault. No remote vector, user interaction, or memory corruption primitive is involved. The impact is limited to availability, with no confidentiality or integrity loss.

No public proof-of-concept code has been released for this issue, and it is not listed in the CISA Known Exploited Vulnerabilities catalog. The EPSS score remains low, consistent with a local denial-of-service issue requiring device access.

Detection Methods for CVE-2026-31618

Indicators of Compromise

Kernel Oops or divide error messages in dmesg referencing tdfxfb or framebuffer ioctl call paths
Unexpected system crashes or reboots following userspace interaction with /dev/fb* device nodes
Audit log entries showing non-administrative processes issuing FBIOPUT_VSCREENINFO ioctls on framebuffer devices

Detection Strategies

Inventory hosts running kernels predating the patches at commits 53cb4e7, 63dfb0b, 6567d3e, 859a239, 8f98b81, and fc386da to identify exposure
Monitor auditd rules covering ioctl syscalls on /dev/fb* devices for processes outside expected display stacks
Correlate kernel ring buffer crash signatures containing divide_error with the responsible process and uid

Monitoring Recommendations

Forward kernel logs and audit events to a centralized logging or SIEM platform for retention and search
Alert on repeated kernel divide-by-zero traps originating from the same uid or session
Track package and kernel versions across the fleet to confirm patch rollout completeness

How to Mitigate CVE-2026-31618

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in the kernel.org commits and reboot affected hosts
Restrict access to /dev/fb* device nodes to trusted users and display server accounts only
Unload or blacklist the tdfxfb module on systems that do not require 3dfx framebuffer support

Patch Information

The fix has been merged across multiple stable branches. Refer to the official kernel.org commits: Kernel Patch 53cb4e7, Kernel Patch 63dfb0b, Kernel Patch 6567d3e, Kernel Patch 859a239, Kernel Patch 8f98b81, and Kernel Patch fc386da. Each patch adds a validation check that rejects a zero pixclock before the division.

Workarounds

Blacklist the tdfxfb kernel module on hosts without legacy 3dfx hardware to remove the attack surface
Tighten permissions on /dev/fb* so only the display server user can issue ioctls
Apply mandatory access control profiles (SELinux, AppArmor) limiting framebuffer ioctl access to authorized processes

bash

# Blacklist the tdfxfb module to remove the vulnerable code path
echo "blacklist tdfxfb" | sudo tee /etc/modprobe.d/blacklist-tdfxfb.conf
sudo rmmod tdfxfb 2>/dev/null || true

# Restrict framebuffer device access to the video group
sudo chown root:video /dev/fb0
sudo chmod 0660 /dev/fb0