CVE-2026-31726: Linux Kernel Race Condition Vulnerability

CVE-2026-31726 Overview

CVE-2026-31726 is a NULL pointer dereference vulnerability in the Linux kernel's USB gadget UVC (USB Video Class) function driver. The flaw exists in uvc_function_unbind() and is triggered by a race condition during power management (PM) suspend transitions. When the PM subsystem freezes user space processes, wait_event_interruptible_timeout() is aborted early, allowing the unbind thread to nullify cdev->gadget. Subsequent V4L2 release operations then access the nullified pointer through usb_gadget_deactivate(), triggering a kernel panic. The vulnerability is classified under [CWE-476] (NULL Pointer Dereference) and affects multiple Linux kernel branches, including 7.0 release candidates.

Critical Impact

A local user with the ability to trigger USB gadget unbind during PM suspend can cause a kernel panic, resulting in a denial-of-service condition on affected Linux systems.

Affected Products

• Linux Kernel (multiple stable branches)
• Linux Kernel 7.0-rc1 through 7.0-rc6
• Systems using configfs-gadget with the UVC function driver

Discovery Timeline

• 2026-05-01 - CVE-2026-31726 published to NVD
• 2026-05-07 - Last updated in NVD database

Technical Details for CVE-2026-31726

Vulnerability Analysis

The vulnerability stems from a race condition between USB gadget unbind operations and V4L2 release callbacks during power management transitions. Commit b81ac4395bbe previously introduced a 1500ms two-stage synchronization wait in uvc_function_unbind() to prevent kernel panics during clean shutdown sequences. This timing-based approach proved insufficient when the PM subsystem initiates a suspend cycle.

When PM begins freezing user space processes, the wait_event_interruptible_timeout() call in the unbind path is interrupted early. The unbind thread then proceeds to set cdev->gadget = NULL. When tasks resume after suspend completes or aborts, the V4L2 release path executes uvc_v4l2_release(), which calls uvc_function_disconnect() and ultimately usb_gadget_deactivate() against the nullified gadget pointer.

Root Cause

The root cause is improper state synchronization between the unbind thread and the V4L2 release callback. The original timeout-based design assumed user space would complete cleanup before unbind nullified the gadget pointer. PM-initiated freezing breaks that assumption by aborting the timeout wait. There is no mutex or completion guarding access to cdev->gadget after it is nullified, allowing a use-after-nullification access pattern.

Attack Vector

Exploitation requires local access with privileges to trigger USB gadget unbind operations and influence PM suspend cycles. An attacker with these capabilities can race the unbind operation against a suspend transition to deterministically reach the nullified pointer access in usb_gadget_deactivate+0x14/0xf4. The result is a kernel NULL pointer dereference at virtual address 0x30, producing a kernel panic and denial of service. The fix introduces a func_unbound flag in struct uvc_device protected by a new uvc->lock mutex, plus a completion to synchronize uvc_function_unbind() with uvc_vdev_release() to prevent both NULL dereference and use-after-free conditions.

Detection Methods for CVE-2026-31726

Indicators of Compromise

• Kernel panic logs referencing usb_gadget_deactivate, usb_function_deactivate, uvc_function_disconnect, or uvc_v4l2_release in the call trace.
• Dmesg entries showing uvc_function_unbind no clean disconnect, wait for release immediately followed by PM suspend entry messages.
• Unexpected system crashes correlating with USB gadget configuration changes during sleep or hibernate cycles.

Detection Strategies

• Monitor /var/log/kern.log and journald output for NULL pointer dereference traces involving the UVC gadget call chain.
• Audit kernel versions across Linux fleets to identify hosts running affected stable branches or 7.0 release candidates.
• Correlate PM suspend/resume events with USB gadget configfs activity to identify systems at elevated risk.

Monitoring Recommendations

• Forward kernel ring buffer logs to a centralized SIEM for crash signature analysis.
• Track frequency of UVC gadget bind/unbind operations on edge devices, embedded systems, and Android-derived platforms.
• Alert on repeated kernel oops events on devices that expose USB gadget functionality, particularly camera or video streaming gadgets.

How to Mitigate CVE-2026-31726

Immediate Actions Required

• Apply the upstream Linux kernel patches referenced in the stable tree commits as soon as they are available for your distribution.
• Inventory all systems using the g_webcam or custom UVC gadget configurations, including embedded devices and SBCs.
• Restrict local access and USB gadget configfs permissions to trusted administrative users only.

Patch Information

The fix is distributed across multiple stable kernel commits, including Linux Kernel Commit 0587de7, Linux Kernel Commit 1aa9356, Linux Kernel Commit c038ba5, and Linux Kernel Commit eba2936. The patch introduces a func_unbound flag protected by a new uvc->lock mutex and uses a completion primitive to synchronize unbind with uvc_vdev_release(). Distribution maintainers should pull the corresponding stable backports for supported branches.

Workarounds

• Avoid unbinding the UVC gadget while the system is entering or exiting suspend; serialize gadget configuration changes outside PM transitions.
• Disable the UVC gadget function via configfs on systems that do not require USB video class functionality.
• Where feasible, disable system suspend on devices acting as USB video gadgets until patched kernels are deployed.

# Configuration example: disable the UVC gadget function via configfs
sudo sh -c 'echo "" > /sys/kernel/config/usb_gadget/g1/UDC'
sudo rm /sys/kernel/config/usb_gadget/g1/configs/c.1/uvc.0
sudo rmdir /sys/kernel/config/usb_gadget/g1/functions/uvc.0