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PHP-FPM and File Permissions

astorm

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First, let’s state the problem we’re going to examine

In a secure unix enviornment, the web server’s user unix account and the unix user account created for command line access (for deploying code and running command line tasks) should (for security reasons) be different. This, in turn, means PHP code that runs in one context (web) will creates files or directories that may not be readable or writable in other directories

This is a persistent problem for web developers of all stripes, but PHP developers feel it harder than most. Because of PHP’s success, there’s a massive enviornment of inexpensive web hosting available. As a result of its low-cost, these web environments are inflexible in the user account and permissions configurations they provide. While it’s easy to ignore this hosting ecosystem when you’re using PHP to build a service, things are a little different if you’re building PHP things for other PHP users to use. Not considering these sorts of enviornments means you’re either on the cusp of being negligent, or making a conscious decision to leave a majority (market share wise) of your PHP users behind.

What are FastCGI and PHP-FPM?

Running PHP in a FastCGI enviornment is often seen as a solution to these sorts of permissions problems. This is sort-of-true, but also sort-of-false. To understand why you need to understand what FastCGI and PHP-FPM are.

A web request in a traditional PHP enviornment goes something like this

Browser: Hey Apache, give me this URL

Apache: (to self) OK, this is a PHP url, let me run this PHP program to get the HTML page.

Apache: OK browser, here’s the HTML page

With FastCGI, the imaginary conversion goes something like this

Browser: Hey Apache, give me this URL

Apache: (to self) OK, this is a FastCGI URL. Hey FastCGI server, run this command with these parameters for me

FastCGI Server: OK, let me pass the command off to one of those processes I’ve already started and get an HTML page

Fast CGI Server: OK Apache, here’s the HTML page

Apache: Thanks FastCGI server!

Apache: OK browser, here’s the HTML

In a FastCGI setup, your web server is still exposed to the world, but there’s a second daemon process running. That second process is a FastCGI server that handles any URL that requires running a program. In other words, the web server still handles serving javascript files, CSS files, images, static HTML pages, etc., but the FastCGI server runs programs.

This has a few advantages. First, the FastCGI server is less exposed to direct world wide assault via network requests which means (or so says one line of security thinking) you can be more liberal as to which unix user account runs this server and its child processes.

The second advantage is you no longer need to embed your programming language in a web server module (MOD_PHP, MOD_PERL, etc.). So long as your web server can talk to a FastCGI server (both apache and nginx can), and your programming language has a FastCGI server, you’re good to go. Not tying your language to one web server is a Good Thing™

The third advantage, unrelated to permissions, is a FastCGI server doesn’t run into the expensive startup problem that plagues regular old CGI.

PHP-FPM stands for PHP FastCGI Process Management. PHP-FPM is PHP’s FastCGI server. Most PHP distributions will ship with a php-fpm binary and configuration files. For example, here’s where you can find the liip PHP-FPM binary and configuration files

/usr/local/php5/sbin/php-fpm
/usr/local/php5/etc/php-fpm.conf.default
/usr/local/php5/etc/php-fpm.d/www.conf.default

While most PHP distributions ship with PHP-FPM, many distributions will not be setup to automatically run your php-fpm server-daemon process. In other words, you may not have a launchctl, init.d, systemctl, or service script to run it. The default configuration files may not have smart defaults. All this, however, is another topic for another time.

PHP-FPM and Permissions

When folks try to get help with permissions problems online, you’ll often hear FastCGI or PHP-FPM brought up as a solution. There is some truth to this. As a developer, PHP-FPM can make your permissions problems go away, but only during development.

By running the FastCGI server (i.e. the PHP-FPM daemon) as your own user, or running it via the root user and configuring your www.conf file to run processes as your own user, most of your permissions problems will evaporate. With the two contexts (web, CLI) unified under a single user, files and directories created by the PHP application won’t have ownership by/from multiple user accounts. Developer bliss.

However — PHP-FPM doesn’t solve the permission problems for your deployment server. Tying the production PHP-FPM process to a specific user account is not a security best-practice. Also, with many PHP systems, there’s a second command line process (the cron user) to consider. In production we’re back to the problem of many user accounts reading and writing the same directories and we need to carefully orchestrate users, groups, umasks, and permissions in our environments.

What to Do

Whenever this topic comes up w/r/t an open source platform I’m — surprised? — how often it’s met by a shoulder shrug or the false-elitism of only losers use those environments. Sometimes this is couched in more politically sensitive language like

It wasn’t a priority but the team’s working hard on it and it’s a hard problem

or

Our user analysis shows those users aren’t adopting our platform and so we can’t allocate resources to it

or

Those users are not a market we’re targeting

What’s most frustrating about these attitudes is — this is a solved problem. PHP Frameworks solved it a long time ago with a few simple best practices

  1. Limit the file systems writes your PHP application needs to do when running in a web context
  2. Keep those writes under a limited number of directories (ideally one)
  3. Abstract any file system read/writes to a single system which allows you to
    • Control the PHP umask when making file system read/writes
    • Provide user-friendly error-messaging/experience when you run into a permission problem
    • Swap in a different system for reads and writes (everything from transfers/writes via SFTP, or replacing the whole thing with redis)

Off the top of my head — Joomla, WordPress, Drupal, Laravel — they all have (intentionally or not) created platforms that hew to the above. This means folks can use those platforms whether they’re running via a hand rolled FastCGI enviornment, or using good old mod_php from a WAMP/MAMP setup.

Wrap Up

There’s one last parting bit of advice for anyone running a dev team that’s shipping PHP code for other PHP developers to use: Give your team an enviornment (vagrant, docker, etc) where they feel the pain of these permissions problems. Maybe that’s MOD_PHP, maybe that’s PHP-FPM with proper user segregation.

When development teams just spin up a FastCGI enviornment with their own user accounts permissions problems will start to slip into your system, but no one will notice. By the time you ship to end users you’ll have hundreds of little paper cuts that you’ll probably never get around to fixing, but that your end-developer-users will nonetheless need to handle themselves.

When development teams feel this pain themselves they’ll seek out the best practices above on their own. You won’t need an endless round of post-launch sprints to clean these problems up — your team will have already solved the problem because it was in their interest to solve it.

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Copyright © Alan Storm 1975 – 2017 All Rights Reserved

Originally Posted: 6th July 2017