CVE-2026-89817: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-89817 Overview

CVE-2026-89817 is an out-of-bounds read vulnerability in the Linux kernel's drm/gud (Generic USB Display) driver. The flaw resides in gud_connector_add_tv_mode(), which reads a buffer of fixed-size TV mode names from a USB device. Pointers into that buffer are passed to drm_mode_create_tv_properties_legacy(), which invokes strlen() on each name. Because nothing guarantees the device NUL-terminates a name, strlen() can read past the end of a slot and, for the final mode, past the end of the allocation.

Critical Impact
A malicious or malformed USB display device can trigger an out-of-bounds read in kernel memory, leading to potential information disclosure or kernel instability.

Affected Products

Linux kernel — drm/gud (Generic USB Display) driver
Distributions shipping vulnerable kernel versions prior to the referenced stable fixes
Systems that permit connection of USB display devices using the GUD protocol

Discovery Timeline

2026-09-16 - CVE-2026-89817 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-89817

Vulnerability Analysis

The drm/gud driver supports USB-attached displays that expose display metadata over the USB bus. During connector setup, gud_connector_add_tv_mode() retrieves an array of fixed-size character slots representing legacy TV mode names. Each slot has a bounded length, but the driver treats the contents as C strings by passing raw pointers to drm_mode_create_tv_properties_legacy().

The consumer function calls strlen() on each pointer. When the device omits the terminating NUL byte within a slot, strlen() continues past the slot boundary, reading adjacent memory. For the last slot, the read extends past the end of the driver's allocation. This is a classic out-of-bounds read [CWE-125] driven by untrusted device-supplied data.

Root Cause

The driver assumes that USB device-supplied mode names are NUL-terminated within their fixed-size slot. No validation enforces termination before the buffer is consumed by string-processing routines. The fix explicitly terminates each name at the end of its slot before passing pointers downstream, ensuring strlen() remains within bounds.

Attack Vector

Exploitation requires an attacker-controlled USB device that identifies as a GUD display. When the device is attached to a vulnerable host, the malicious mode descriptor omits NUL termination, causing kernel-side reads beyond the intended buffer. Impact depends on adjacent allocations and may include kernel memory disclosure, oops-level crashes, or use as a primitive within a larger exploitation chain. Physical or supply-chain access to the USB port is the practical prerequisite.

No public proof-of-concept code is referenced in the advisory. The fix is distributed across multiple stable kernel branches via the commits listed under Linux Kernel Commit 08c8ec2, Linux Kernel Commit 500cb24, Linux Kernel Commit 6ea61f1, Linux Kernel Commit 89210cb, Linux Kernel Commit 9096edf, Linux Kernel Commit b86438a, and Linux Kernel Commit d0f3312.

Detection Methods for CVE-2026-89817

Indicators of Compromise

Kernel oops, KASAN reports, or slab-out-of-bounds warnings referencing gud_connector_add_tv_mode or drm_mode_create_tv_properties_legacy.
Unexpected USB display device enumeration events on endpoints that do not normally use GUD-class peripherals.
Crashes or connector-registration failures immediately following attachment of an unknown USB display.

Detection Strategies

Enable KASAN on test kernels to surface out-of-bounds reads in the drm/gud code path during regression testing.
Correlate dmesg output containing gud driver messages with USB device attach events in host telemetry.
Audit installed kernel versions against the stable release trees carrying the referenced fix commits.

Monitoring Recommendations

Ingest kernel logs (journald, /var/log/kern.log) into centralized logging and alert on BUG, KASAN, or WARN entries naming drm/gud symbols.
Monitor USB device attach and driver bind events on high-value Linux endpoints using udev or auditd rules.
Track kernel package versions across the fleet to identify hosts still running pre-patch releases.

How to Mitigate CVE-2026-89817

Immediate Actions Required

Update Linux kernels to a stable release that includes one of the referenced fix commits for the drm/gud driver.
Restrict physical and logical access to USB ports on servers, kiosks, and other systems that do not require USB display peripherals.
Inventory endpoints that load the gud module and prioritize them for patching.

Patch Information

The upstream fix modifies gud_connector_add_tv_mode() to NUL-terminate each mode name at the end of its slot before passing pointers to drm_mode_create_tv_properties_legacy(). Patched commits are available across multiple stable branches. Refer to the upstream references, including Linux Kernel Commit d0f3312 and Linux Kernel Commit 08c8ec2, and apply the vendor-provided kernel update matching your distribution.

Workarounds

Blacklist the gud kernel module on systems that do not require USB display support using /etc/modprobe.d/ configuration.
Apply USB device access controls (for example, USBGuard) to allow only known-good peripherals to bind to display drivers.
Enforce physical port controls and disable unused USB host controllers in firmware where feasible.

bash

# Configuration example: prevent the vulnerable driver from loading
echo "blacklist gud" | sudo tee /etc/modprobe.d/blacklist-gud.conf
sudo depmod -a
sudo update-initramfs -u