CVE-2026-64233: usb: gadget: uvc: hold opts->lock across XU walks in uvc_function_bind

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

usb: gadget: uvc: hold opts->lock across XU walks in uvcfunctionbind

uvcfunctionbind() walks &opts->extensionunits twice without holding opts->lock:

- directly, for the iExtension string-descriptor fixup loop; - indirectly, four times via uvccopydescriptors() (once per speed), where the helper iterates uvc->desc.extensionunits (which aliases &opts->extensionunits) to size and emit XU descriptors.

The configfs side (uvcgextensionmake / uvcgextensiondrop, in drivers/usb/gadget/function/uvcconfigfs.c) takes opts->lock around its listaddtail / listdel operations. A privileged userspace process that holds the configfs subtree open and writes the gadget UDC name to bind the function while concurrently rmdir()'ing an extensions subdir can race uvcgextensiondrop() against the bind-time list walks and dereference a freed struct uvcgextension.

Hold opts->lock from the start of the XU string-descriptor fixup through the last uvccopydescriptors() call, releasing on the descriptor-error path via a new errorunlock label that drops the lock before falling through to the existing error label. This matches the locking discipline of the configfs callbacks and removes the only remaining unsynchronised reader of the XU list during bind.

Reachability: only privileged processes that can mount configfs and write to gadget UDC files can trigger the race, so this is a correctness fix rather than a security boundary.