CVE-2026-31759: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31759 Overview

CVE-2026-31759 is a double free vulnerability in the Linux kernel USB ULPI (UTMI+ Low Pin Interface) subsystem. The flaw resides in the ulpi_register_interface() error path. When device_register() fails, ulpi_register() calls put_device() on ulpi->dev, which triggers the ulpi_dev_release() callback that drops the OF node reference and frees the ulpi structure. The current error path then calls kfree(ulpi) a second time, producing a double free condition. The fix removes the redundant free and lets put_device() handle cleanup through ulpi_dev_release().

Critical Impact
A double free in kernel memory can lead to heap corruption, denial of service through kernel panic, or potential local privilege escalation if an attacker can influence the surrounding allocator state.

Affected Products

Linux kernel versions containing the vulnerable ulpi_register_interface() implementation in drivers/usb/common/ulpi.c
Distributions shipping the affected stable kernel branches referenced in the upstream fix commits
Embedded and server systems using USB ULPI PHY drivers

Discovery Timeline

2026-05-01 - CVE-2026-31759 published to NVD
2026-05-01 - Last updated in NVD database

Technical Details for CVE-2026-31759

Vulnerability Analysis

The vulnerability is a classic double free [CWE-415] in the Linux kernel USB ULPI registration logic. The ULPI subsystem provides an interface for USB PHY transceivers. During interface registration, the kernel allocates a ulpi structure, initializes its embedded struct device, and calls device_register() to attach it to the device model.

When device_register() fails, the kernel invokes put_device() to release the partially initialized device. The reference counting machinery then calls the registered release callback, ulpi_dev_release(), which is responsible for dropping the OF (Open Firmware) node reference and freeing the ulpi structure with kfree().

The defect is that the surrounding error path in ulpi_register_interface() also calls kfree(ulpi) after put_device() returns. Because the release callback already freed the same allocation, the second kfree() operates on memory that may have been reallocated or returned to the slab allocator, corrupting kernel heap metadata.

Root Cause

The root cause is duplicated cleanup responsibility between the device release callback and the function-level error handler. Once a struct device has been initialized and put_device() is called, ownership of any embedding allocation transfers to the release callback. Mixing manual kfree() with reference-counted release semantics is the canonical pattern that produces double frees in the kernel device model.

Attack Vector

Reaching the vulnerable error path requires device_register() to fail during ULPI interface registration. This typically requires local kernel-level interaction such as loading a malformed device tree, attaching a crafted USB PHY device, or triggering allocation failures during driver probe. Remote exploitation is not applicable. The practical risk is denial of service through kernel panic, with potential for memory corruption primitives in advanced exploitation scenarios.

The vulnerability mechanism is described in the upstream commit messages. See the Kernel Git Commit 272a9b2 and related stable backports for the exact source diff.

Detection Methods for CVE-2026-31759

Indicators of Compromise

Kernel panic or oops messages referencing ulpi_register_interface, ulpi_dev_release, or kfree in the call stack
SLUB or SLAB allocator warnings such as double free or corruption, kernel BUG at mm/slub.c, or Object already free
Unexpected USB PHY driver probe failures followed by system instability

Detection Strategies

Inventory running kernel versions with uname -r and compare against the patched stable releases referenced in the kernel.org commits
Enable CONFIG_SLUB_DEBUG and CONFIG_DEBUG_PAGEALLOC on test systems to surface double free conditions during driver probe
Monitor dmesg and /var/log/kern.log for ULPI registration failures and slab corruption signatures

Monitoring Recommendations

Forward kernel ring buffer logs to a centralized logging platform and alert on slab corruption keywords
Track unexpected reboots and kernel crashes on systems using USB ULPI PHY drivers, common on embedded ARM and SoC platforms
Apply file integrity monitoring to /boot/vmlinuz* and /lib/modules/ to detect kernel package state changes

How to Mitigate CVE-2026-31759

Immediate Actions Required

Identify systems running affected Linux kernel versions, prioritizing hosts that load USB ULPI PHY drivers
Apply vendor-supplied kernel updates that incorporate the upstream fix from the referenced kernel.org commits
Reboot affected systems after patching to ensure the new kernel image is loaded

Patch Information

The fix removes the duplicate kfree(ulpi) call from ulpi_register_interface() and relies on put_device() to invoke ulpi_dev_release() for cleanup. The patch has been backported across multiple stable branches. Reference commits include Kernel Git Commit 01af542, Kernel Git Commit 38c28fe, Kernel Git Commit 8763f83, Kernel Git Commit a6e5461, Kernel Git Commit aaeae65, Kernel Git Commit ee248e6, Kernel Git Commit 2f70ba9, and Kernel Git Commit 272a9b2.

Workarounds

Where the patch cannot be deployed immediately, blacklist USB ULPI PHY modules on systems that do not require them using /etc/modprobe.d/ configuration
Restrict physical and local access to systems that expose USB PHY interfaces, since exploitation requires triggering the registration error path
Track distribution security advisories from Red Hat, SUSE, Ubuntu, and Debian for backported kernel updates aligned to your supported kernel branch

bash

# Verify running kernel and check for ULPI module presence
uname -r
lsmod | grep -i ulpi

# Optionally prevent the module from loading until patched
echo 'blacklist ulpi' | sudo tee /etc/modprobe.d/blacklist-ulpi.conf
sudo update-initramfs -u