How to make a privileged call with oslo privsep

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Once you’ve added oslo privsep to your project, how do you make a privileged call? Its actually really easy to do. In this post I will assume you already have privsep running for your project, which at the time of writing limits you to OpenStack Nova in the OpenStack universe.

The first step is to write the code that will run with escalated permissions. In Nova, we have chosen to only have one set of escalated permissions, so its easy to decide which set to use. I’ll document how we reached that decision and alternative approaches in another post.

In Nova, all code that runs with escalated permissions is in the nova/privsep directory, which is a pattern I’d like to see repeated in other projects. This is partially because privsep maintains a whitelist of methods that are allowed to be run this way, but its also because it makes it very obvious to callers that the code being called is special in some way.

Let’s assume that we’re going to add a simple method which manipulates the filesystem of a hypervisor node as root. We’d write a method like this in a file inside nova/privsep:

import nova.privsep

...

@nova.privsep.sys_admin_pctxt.entrypoint
def update_motd(message):
    with open('/etc/motd', 'w') as f:
        f.write(message)

This method updates /etc/motd, which is the text which is displayed when a user interactively logs into the hypervisor node. “motd” stands for “message of the day” by the way. Here we just pass a new message of the day which clobbers the old value in the file.

The important thing is that entrypoint decorator at the start of the method. That’s how privsep decides to run this method with escalated permissions, and decides what permissions to use. In Nova at the moment we only have one set of escalated permissions, which we called sys_admin_pctxt because we’re artists. I’ll discuss in a later post how we came to that decision and what the other options were.

We can then call this method from anywhere else in Nova like this:

import nova.privsep.motd

...

nova.privsep.motd('This node is currently idle')

Note that we do imports for privsep code slightly differently. We always import the entire path, instead of creating a shortcut to just the module we’re using. In other words, we don’t do:

from nova.privsep import motd

...

motd('This node is a banana')

The above code would work, but is frowned on because it is less obvious here that the update_motd() method runs with escalated permissions — you’d have to go and read the imports to tell that.

That’s really all there is to it. The only other thing to mention is that there is a bit of a wart — code with escalated permissions can only use Nova code that is within the privsep directory. That’s been a problem when we’ve wanted to use a utility method from outside that path inside escalated code. The restriction happens for good reasons, so instead what we do in this case is move the utility into the privsep directory and fix up all the other callers to call the new location. Its not perfect, but its what we have for now.

There are some simple review criteria that should be used to assess a patch which implements new code that uses privsep in OpenStack Nova. They are:

  • Don’t use imports which create aliases. Use the “import nova.privsep.motd” form instead.
  • Keep methods with escalated permissions as simple as possible. Remember that these things are dangerous and should be as easy to understand as possible.
  • Calculate paths to manipulate¬†inside the escalated method — so, don’t let someone pass in a full path and the contents to write to that file as root, instead let them pass in the name of the network interface or whatever that you are manipulating and then calculate the path from there. That will make it harder for callers to use your code to clobber random files on the system.

Adding new code with escalated permissions is really easy in Nova now, and much more secure and faster than it was when we only had sudo and root command lines to do these sorts of things. Let me know if you have any questions.

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