CVE-2026-31726: usb: gadget: uvc: fix NULL pointer dereference during unbind race

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: uvc: fix NULL pointer dereference during unbind race

Commit b81ac4395bbe ("usb: gadget: uvc: allow for application to cleanly shutdown") introduced two stages of synchronization waits totaling 1500ms in uvcfunctionunbind() to prevent several types of kernel panics. However, this timing-based approach is insufficient during power management (PM) transitions.

When the PM subsystem starts freezing user space processes, the waiteventinterruptibletimeout() is aborted early, which allows the unbind thread to proceed and nullify the gadget pointer (cdev->gadget = NULL):

[ 814.123447][ T947] configfs-gadget.g1 gadget.0: uvc: uvcfunctionunbind() [ 814.178583][ T3173] PM: suspend entry (deep) [ 814.192487][ T3173] Freezing user space processes [ 814.197668][ T947] configfs-gadget.g1 gadget.0: uvc: uvcfunctionunbind no clean disconnect, wait for release

When the PM subsystem resumes or aborts the suspend and tasks are restarted, the V4L2 release path is executed and attempts to access the already nullified gadget pointer, triggering a kernel panic:

[ 814.292597][ C0] PM: pmsystemirqwakeup: 479 triggered dhdpciehostwake [ 814.386727][ T3173] Restarting tasks ... [ 814.403522][ T4558] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000030 [ 814.404021][ T4558] pc : usbgadgetdeactivate+0x14/0xf4 [ 814.404031][ T4558] lr : usbfunctiondeactivate+0x54/0x94 [ 814.404078][ T4558] Call trace: [ 814.404080][ T4558] usbgadgetdeactivate+0x14/0xf4 [ 814.404083][ T4558] usbfunctiondeactivate+0x54/0x94 [ 814.404087][ T4558] uvcfunctiondisconnect+0x1c/0x5c [ 814.404092][ T4558] uvcv4l2release+0x44/0xac [ 814.404095][ T4558] v4l2release+0xcc/0x130

Address the race condition and NULL pointer dereference by:

1. State Synchronization (flag + mutex) Introduce a 'funcunbound' flag in struct uvcdevice. This allows uvcfunctiondisconnect() to safely skip accessing the nullified cdev->gadget pointer. As suggested by Alan Stern, this flag is protected by a new mutex (uvc->lock) to ensure proper memory ordering and prevent instruction reordering or speculative loads. This mutex is also used to protect 'funcconnected' for consistent state management.

2. Explicit Synchronization (completion) Use a completion to synchronize uvcfunctionunbind() with the uvcvdevrelease() callback. This prevents Use-After-Free (UAF) by ensuring struct uvcdevice is freed after all video device resources are released.