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

Overview

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.

Affected

• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < 2454264b2ab4cf0055c0bfd39e79f830452bd0db
• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < d3e72cfef2e38bd588055739a8100d14f9773b17
• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < 8ac463fe6c0f85bdb1ce8c30e8c9e060802e4483
• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < 56add2b9b2e89ec61c0761165d758f73004fdfdf
• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < 91b3ecd34b60f950c50c560974945b6596a6f207
• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < 01feaf024f29618d5ffa7ab0fd858e0579dcbf7b
• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < fcb21bf747640c9d6bd1eda9da85420f076d59c1
• Linux >= 53e6242db8d60da0587d36951cc9434d1a1c21dd < f8f680609c2b3ab795ffcd6f21585b6dfc46d395
• Linux 4.3

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.