usb: gadget: configfs: Prevent OOB read/write in usb_string_copy()

Userspace provided string 's' could trivially have the length zero. Left
unchecked this will firstly result in an OOB read in the form
`if (str[0 - 1] == '\n') followed closely by an OOB write in the form
`str[0 - 1] = '\0'`.

There is already a validating check to catch strings that are too long.
Let's supply an additional check for invalid strings that are too short.
Signed-off-by: Lee Jones <lee@kernel.org>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/20240705074339.633717-1-lee@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/configfs.c

@@ -115,9 +115,12 @@ static int usb_string_copy(const char *s, char **s_copy)
 	int ret;
 	char *str;
 	char *copy = *s_copy;
+
 	ret = strlen(s);
 	if (ret > USB_MAX_STRING_LEN)
 		return -EOVERFLOW;
+	if (ret < 1)
+		return -EINVAL;
 
 	if (copy) {
 		str = copy;