Linux kernel: FunctionFS ioctl crashes on missing endpoint descriptor

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

usb: gadget: f_fs: Fix NULL pointer dereference in FUNCTIONFS_ENDPOINT_DESC

When user space sets up FunctionFS with only full-speed descriptors (e.g., flags = FUNCTIONFS_HAS_FS_DESC) and the gadget operates at a higher speed (such as USB_SPEED_HIGH or USB_SPEED_SUPER), config_ep_by_speed() detects that the function lacks descriptors for the current speed, logs a warning, and falls back to full-speed descriptors. The endpoint is then successfully enabled via usb_ep_enable(), and epfile->ep becomes valid.

However, when user space subsequently issues the FUNCTIONFS_ENDPOINT_DESC ioctl to query the endpoint descriptor, ffs_epfile_ioctl() selects the descriptor index solely based on gadget->speed:

switch (epfile->ffs->gadget->speed) {
case USB_SPEED_SUPER:
case USB_SPEED_SUPER_PLUS:
	desc_idx = 2;
	break;
case USB_SPEED_HIGH:
	desc_idx = 1;
	break;
default:
	desc_idx = 0;
}
desc = epfile->ep->descs[desc_idx];
memcpy(&desc1, desc, desc->bLength);

Because user space only supplied full-speed descriptors, epfile->ep->descs[1] is NULL. Dereferencing desc->bLength triggers an immediate kernel NULL pointer dereference panic.

Fix this by falling back through lower speed descriptors if the current speed descriptor was not supplied, matching the fallback logic in config_ep_by_speed() and historical f_fs behavior. If still missing, fall back to epfile->ep->ep->desc (the active descriptor), and return -EINVAL safely if no valid descriptor is found. In addition, mark desc as const to match struct usb_ep::desc and avoid discarding qualifiers.