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 uvc_function_unbind() 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

wait_event_interruptible_timeout() 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: uvc_function_unbind()

[ 814.178583][ T3173] PM: suspend entry (deep)

[ 814.192487][ T3173] Freezing user space processes

[ 814.197668][ T947] configfs-gadget.g1 gadget.0: uvc: uvc_function_unbind 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: pm_system_irq_wakeup: 479 triggered dhdpcie_host_wake

[ 814.386727][ T3173] Restarting tasks ...

[ 814.403522][ T4558] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000030

[ 814.404021][ T4558] pc : usb_gadget_deactivate+0x14/0xf4

[ 814.404031][ T4558] lr : usb_function_deactivate+0x54/0x94

[ 814.404078][ T4558] Call trace:

[ 814.404080][ T4558] usb_gadget_deactivate+0x14/0xf4

[ 814.404083][ T4558] usb_function_deactivate+0x54/0x94

[ 814.404087][ T4558] uvc_function_disconnect+0x1c/0x5c

[ 814.404092][ T4558] uvc_v4l2_release+0x44/0xac

[ 814.404095][ T4558] v4l2_release+0xcc/0x130

Address the race condition and NULL pointer dereference by:

1. State Synchronization (flag + mutex)

Introduce a 'func_unbound' flag in struct uvc_device. This allows

uvc_function_disconnect() 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 'func_connected' for consistent state management.

2. Explicit Synchronization (completion)

Use a completion to synchronize uvc_function_unbind() with the

uvc_vdev_release() callback. This prevents Use-After-Free (UAF) by

ensuring struct uvc_device is freed after all video device resources

are released.