In the Linux kernel, the following vulnerability has been resolved: usb: gadget: composite: fix...

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

usb: gadget: composite: fix dead empty check in the USB_DT_OTG handler

The OTG branch of composite_setup() falls back to the first configuration when none is selected:

if (cdev->config)
	config = cdev->config;
else
	config = list_first_entry(&cdev->configs,
				  struct usb_configuration, list);
if (!config)
	goto done;
...
memcpy(req->buf, config->descriptors[0], value);

list_first_entry() never returns NULL. On an empty list it returns container_of() of the list head. So the "if (!config)" check is dead.

When cdev->configs is empty, config points at the head inside struct usb_composite_dev. config->descriptors[0] reads whatever sits at that offset. The memcpy copies up to w_length bytes of it into the response buffer.

cdev->configs can be empty in two cases. One is a teardown race on gadget unbind with a control transfer in flight. The other is a driver that sets is_otg before it adds a config. A reproducer that holds cdev->configs empty triggers a KASAN fault in this branch.

Use list_first_entry_or_null() so the existing check does its job.