Nginx cannot access upstream .sock file and public folder

Given: fresh Archlinux VPS box on DigitalOcean. I created a user, 'app', and there's a file, /home/app/webapp.sock created by the binary started by systemd:

[Unit]
Description=Web application server
After=network.target

[Service]
Type=forking
User=app
PIDFile=/home/app/webapp.pid
ExecStart=/home/app/.gem/ruby/2.0.0/bin/thin -d --user app -e production --chdir /home/app/app --socket /home/app/webapp.sock --pid /home/app/webapp.pid --log /home/app/log/webapp.log start
ExecReload=/bin/kill -HUP $MAINPID
ExecStop=/bin/kill -QUIT $MAINPID

[Install]
WantedBy=multi-user.target

Don't want to run this app as http user, since at some point of time I may decide to run another web server on that machine under a different user, exposing only its .sock file to http user. Rails is known to have security flaws, so I would like to avoid the 'app' user from getting anywhere out of his home folder and its own data.

I have a sudo user, 'sudoer', and there's no way to read even a pid file:

[sudoer@host ~]$ cat /home/app/webapp.pid
cat: /home/app/webapp.pid: Permission denied
[sudoer@host ~]$ sudo su - app
[app@host ~]$ ls -l webapp.pid
-rw-r--r-- 1 app app 5 Dec  7 19:33 webapp.pid

It has 'r' rights for 'others', why is that 'sudoer' cannot 'cat' it?

I assume this is the reason for the following nginx errors too. Static file:

2013/12/07 18:58:05 [error] 18114#0: *2 open() "/home/app/app/public/favicon.ico" failed (13: Permission denied), client: 183.89.50.151, server: host.com, request: "GET /favicon.ico HTTP/1.1", host: "host.com"

Dynamic content:

2013/12/07 20:49:00 [crit] 21581#0: *1 connect() to unix:/home/app/webapp.sock failed (13: Permission denied) while connecting to upstream, client: 183.89.50.151, server: host.com, request: "GET /favicon.ico HTTP/1.1", upstream: "http://unix:/home/app/webapp.sock:/favicon.ico", host: "host.com"

Excerpt from nginx config:

upstream webapp {
  server unix:/home/app/webapp.sock fail_timeout=0;
}
server {
  listen 80;
  root /home/app/app/public;

Is that strengthened security? SELinux? CGroups? What am I doing wrong?

You must check the permission, not only of the socket (file) but of all the parent directories. If any of them deny access, your request will fail.

For instance:

# ls -ld /home/app
drwx------. 8 root root 4096 Dec 7 21:33 /home/app

Put your sock file in /var/run and when your sock file is created check for which user and group does it belong to. For your case user should be 'app'

Nginx cannot access upstream .sock file and public folder

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