I update nginx to 1.4.7 and php to 5.5.12, After that I got the 502 error. Before I update everything works fine.
Alternative to broadening permissions in your php config, you could change the user specified in your nginx config.
On the first line of your nginx.conf excerpt above, the user and group are specified as www and www, respectively.
user www www;
Meanwhile, your php config probably specifies a user and group of www-data:
listen.owner = www-data
listen.group = www-data
You might change the line in your nginx.conf, to any of the following, then:
user www-data www;
user www-data www-data; # or any group, really, since you have the user matching
user www www-data; # requires that your php listen.mode gives rw access to the group