CVE-2026-31605: Linux Kernel fbdev Divide-by-Zero DoS

CVE-2026-31605 Overview

CVE-2026-31605 is a divide-by-zero vulnerability in the Linux kernel's udlfb 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 it against zero. A local user with access to the framebuffer device can submit a crafted fb_var_screeninfo structure that triggers a kernel crash. The issue mirrors an earlier fix in the fb_pm2fb driver (commit 19f953e74356) and is classified as [CWE-369] Divide By Zero. Linux maintainers have merged patches across multiple stable branches to add the missing input validation.

Critical Impact
A local, low-privileged user with access to a DisplayLink USB framebuffer device can crash the kernel through a malformed FBIOPUT_VSCREENINFO ioctl, causing a denial-of-service condition on affected systems.

Affected Products

Linux kernel — drivers/video/fbdev/udlfb.c (DisplayLink USB framebuffer driver)
Multiple stable kernel branches prior to the backported fixes
Systems exposing /dev/fb* device nodes to non-root users with DisplayLink hardware attached

Discovery Timeline

2026-04-24 - CVE-2026-31605 published to NVD
2026-04-29 - Last updated in NVD database

Technical Details for CVE-2026-31605

Vulnerability Analysis

The udlfb driver supports DisplayLink USB display adapters and exposes a standard Linux framebuffer interface. When userspace issues the FBIOPUT_VSCREENINFO ioctl, the driver receives a fb_var_screeninfo structure containing display timing parameters, including pixclock. The driver uses pixclock directly as a divisor while computing display timing values. When pixclock is set to zero by the caller, the division triggers a CPU divide-by-zero exception. On Linux, this results in a kernel oops and a denial-of-service condition for the affected process context, and may destabilize the system depending on configuration. The bug is identical in shape to the previously addressed fb_pm2fb issue resolved in commit 19f953e74356, indicating an incomplete sweep of similar patterns across framebuffer drivers.

Root Cause

The root cause is missing input validation on attacker-controlled data. The udlfb driver trusts the pixclock field supplied by user space without checking for the zero case before using it as the denominator. [CWE-369] describes exactly this class of defect, where a divisor is not validated prior to use.

Attack Vector

Exploitation is local and requires the attacker to have permission to open the /dev/fb* node associated with a udlfb device. The attacker opens the framebuffer device, constructs a fb_var_screeninfo structure with pixclock set to 0, and issues the FBIOPUT_VSCREENINFO ioctl. The kernel reaches the unguarded division and faults. No user interaction or network access is required, and confidentiality and integrity are not affected — only availability.

No public proof-of-concept exploit is referenced in the advisory. See the upstream patches for the exact code paths and validation added: Kernel Patch Commit 03797cd and Kernel Patch Commit cce24f7.

Detection Methods for CVE-2026-31605

Indicators of Compromise

Kernel oops or panic entries in dmesg or /var/log/kern.log referencing divide error within udlfb functions.
Unexpected termination of processes that recently invoked FBIOPUT_VSCREENINFO on a /dev/fb* node bound to a DisplayLink device.
Repeated open/ioctl activity against udlfb framebuffer devices from non-administrative users.

Detection Strategies

Audit kernel crash telemetry for divide_error exceptions originating in framebuffer driver call stacks.
Use Linux audit rules (auditctl) to log ioctl calls against /dev/fb* devices and correlate with crash events.
Inventory hosts running vulnerable kernel versions with udlfb loaded by checking lsmod | grep udlfb across the fleet.

Monitoring Recommendations

Forward kernel logs to a centralized logging or SIEM platform and alert on divide error strings tied to framebuffer modules.
Track loaded kernel modules over time and flag systems where udlfb is present alongside non-root users with access to /dev/fb*.
Monitor user-to-device permission changes on framebuffer nodes through file integrity monitoring on /dev/fb* and udev rules.

How to Mitigate CVE-2026-31605

Immediate Actions Required

Apply the upstream stable kernel patches that add pixclock validation in udlfb before division.
Restrict access to /dev/fb* device nodes so only trusted system accounts can issue framebuffer ioctls.
If DisplayLink USB displays are not in use, unload and blacklist the udlfb module to remove the attack surface.

Patch Information

The Linux kernel maintainers have merged fixes across multiple stable branches. Refer to the following commits for the exact code changes: Kernel Patch Commit 03797cd, Kernel Patch Commit 6de048d, Kernel Patch Commit a31e451, Kernel Patch Commit afaaaa3, Kernel Patch Commit cccbf9b, and Kernel Patch Commit cce24f7. Update to a distribution kernel that includes one of these commits.

Workarounds

Blacklist the udlfb module on systems that do not require DisplayLink USB display support.
Tighten permissions on framebuffer device nodes so only the video group or root can open /dev/fb*.
Where possible, prevent untrusted local users from interactive shell access on hosts with udlfb loaded.

bash

# Blacklist the udlfb module and tighten framebuffer permissions
echo 'blacklist udlfb' | sudo tee /etc/modprobe.d/blacklist-udlfb.conf
sudo rmmod udlfb 2>/dev/null || true

# Restrict access to framebuffer device nodes
sudo chown root:video /dev/fb*
sudo chmod 0660 /dev/fb*