CVE-2026-31732: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31732 Overview

CVE-2026-31732 is a medium severity vulnerability in the Linux kernel GPIO (General Purpose Input/Output) subsystem. The flaw resides in the gpiochip_add_data_with_key() function within the kernel's GPIO chip registration path. Error handling paths fail to drop the reference count on gdev->dev, producing resource leaks each time chip registration fails. The issue traces back to commit aab5c6f20023 ("gpio: set device type for GPIO chips"), which left gdev->dev.release unset. The vulnerability affects Linux kernel 7.0 release candidates rc1 through rc6 and is tracked under [CWE-401] (Missing Release of Memory after Effective Lifetime).

Critical Impact

A local authenticated user can trigger repeated GPIO chip registration failures to leak kernel memory and exhaust system resources, leading to denial of service.

Affected Products

• Linux Kernel 7.0-rc1 through 7.0-rc6
• Linux Kernel builds containing commit aab5c6f20023
• Distributions shipping pre-release 7.0 kernels for GPIO-enabled hardware

Discovery Timeline

• 2026-05-01 - CVE-2026-31732 published to NVD
• 2026-05-07 - Last updated in NVD database

Technical Details for CVE-2026-31732

Vulnerability Analysis

The vulnerability lives in the GPIO chip registration path inside drivers/gpio/gpiolib.c. After device_initialize() runs on gdev->dev, ownership of the device structure transfers to gpiodev_release(). However, because commit aab5c6f20023 left gdev->dev.release unset, the device's release callback never fires when error paths bail out. Subsequent error handlers therefore neither drop the device reference nor free the associated resources.

The fix reorders initialization steps in gpiochip_add_data_with_key() and introduces two distinct error zones. ERR ZONE 1 covers failures before device_initialize(), where allocations are freed directly. ERR ZONE 2 covers failures after device_initialize(), where gpio_device_put() correctly drops the reference and triggers gpiodev_release(). The reorder also prevents double-free conditions that could occur if both zones executed for the same allocation.

Root Cause

The root cause is a missing release function pointer on a kernel device structure combined with error handling paths that assumed direct kfree() semantics. When device_initialize() runs, the kernel device model takes responsibility for the structure's lifetime through reference counting. Unsetting gdev->dev.release breaks this contract and prevents proper cleanup on the error path.

Attack Vector

Exploitation requires local access with the ability to trigger GPIO chip registration. A local user with permissions to load GPIO drivers, attach GPIO-capable hardware, or invoke kernel interfaces that call gpiochip_add_data_with_key() can repeatedly induce registration failures. Each failed registration leaks the gdev->dev structure and any associated allocations, gradually exhausting kernel memory and degrading system availability.

No synthetic exploitation code is provided. The vulnerability mechanism is documented in the upstream kernel commits referenced under Git Kernel Commit c/16fdabe, Git Kernel Commit c/f0cf9c7, and Git Kernel Commit c/fb4584d.

Detection Methods for CVE-2026-31732

Indicators of Compromise

• Steadily increasing kernel slab memory usage attributable to device and gpio_device allocations without corresponding frees
• Repeated GPIO chip registration failure messages in dmesg or /var/log/kern.log
• Unexpected loads or unloads of GPIO driver modules from non-administrative users

Detection Strategies

• Inspect kernel version output from uname -r to identify hosts running Linux 7.0-rc1 through 7.0-rc6
• Audit /proc/slabinfo for unbounded growth in device-related slab caches over time
• Correlate kernel logs for recurring GPIO chip registration errors paired with low-privilege user activity

Monitoring Recommendations

• Track kernel memory consumption trends and alert on sustained growth without workload changes
• Enable kernel audit rules for init_module and finit_module syscalls to catch GPIO driver load patterns
• Monitor process activity that interacts with /dev/gpiochip* or sysfs GPIO interfaces for anomalous repetition

How to Mitigate CVE-2026-31732

Immediate Actions Required

• Upgrade affected systems to a Linux kernel build that includes commits 16fdabe143fc, f0cf9c7b7c28, or fb4584d2b324
• Avoid deploying Linux 7.0 release candidates rc1 through rc6 in production environments
• Restrict the ability of unprivileged users to load kernel modules that register GPIO chips

Patch Information

The upstream fix reorders initialization in gpiochip_add_data_with_key() and ensures gpio_device_put() is called on error paths after device_initialize(). Patches are available at Git Kernel Commit c/16fdabe, Git Kernel Commit c/f0cf9c7, and Git Kernel Commit c/fb4584d. Distribution maintainers should incorporate these commits into their stable kernel branches.

Workarounds

• Limit physical and logical access to GPIO hardware interfaces such as /dev/gpiochip* through filesystem permissions
• Disable GPIO driver modules on systems that do not require GPIO functionality using modprobe.blacklist
• Apply mandatory access control policies (SELinux, AppArmor) restricting which users can interact with GPIO subsystems

# Configuration example: blacklist GPIO modules where unused
echo "blacklist gpio_generic" | sudo tee /etc/modprobe.d/blacklist-gpio.conf
sudo chmod 600 /dev/gpiochip*
sudo update-initramfs -u