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

[Maoyi Xie]

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.

Fixes: 53e6242db8d6 ("usb: gadget: composite: add USB_DT_OTG request handling")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Maoyi Xie <maoyixie.tju@xxxxxxxxx>
---
This is the fix for the problem I reported on linux-usb on 2026-05-20 [1].
The full KASAN report and the reproducer are in that message.

[1] https://lore.kernel.org/linux-usb/20260519184106.2356558-1-maoyixie.tju@xxxxxxxxx/

 drivers/usb/gadget/composite.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
index a902184bdf82..a5e7c6495949 100644
--- a/drivers/usb/gadget/composite.c
+++ b/drivers/usb/gadget/composite.c
@@ -1863,9 +1863,10 @@ composite_setup(struct usb_gadget *gadget, const struct usb_ctrlrequest *ctrl)
 				if (cdev->config)
 					config = cdev->config;
 				else
-					config = list_first_entry(
+					config = list_first_entry_or_null(
 							&cdev->configs,
-						struct usb_configuration, list);
+							struct usb_configuration,
+							list);
 				if (!config)
 					goto done;
 
-- 
2.34.1