USB: properly lock dynamic id list when showing an id

When walking the list of dynamic ids for a driver, no lock was being
held, which meant that an id could be removed or added while the list
was being iterated.  Fix this up by properly grabing the lock while we
walk the list.

Reported-by: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-usb@vger.kernel.org
Link: https://lore.kernel.org/r/2024111324-tubby-facecloth-d4a0@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: gregkh

drivers/usb/core/driver.c

@@ -116,6 +116,7 @@ ssize_t usb_show_dynids(struct usb_dynids *dynids, char *buf)
 	struct usb_dynid *dynid;
 	size_t count = 0;
 
+	guard(mutex)(&usb_dynids_lock);
 	list_for_each_entry(dynid, &dynids->list, node)
 		if (dynid->id.bInterfaceClass != 0)
 			count += scnprintf(&buf[count], PAGE_SIZE - count, "%04x %04x %02x\n",