A SELinux policy for incron: what does it do?


Sven Vermeulen Wed 22 May 2013

In this series of posts, we'll go through the creation of a SELinux policy for incron, a simple inotify based cron-like application. I will talk about the various steps that I would take in the creation of this policy, and give feedback when certain decisions are taken and why. At the end of the series, we'll have a hopefully well working policy.

The first step in developing a policy is to know what the application does and how/where it works. This allows us to check if its behavior matches an existing policy (and as such might be best just added to this policy) or if a new policy needs to be written. So, what does incron do?

From the documentation, we know that incron is a cron-like application that, unlike cron, works with file system notification events instead of time-related events. Other than that, it uses a similar way of working:

  • A daemon called incrond is the run-time application that reads in the incrontab files and creates the proper inotify watches. When a watch is triggered, it will execute the matching rule.
  • The daemon looks at two definitions (incrontabs): one system-wide (in /etc/incron.d) and one for users (in /var/spool/incron).
  • The user tabfiles are managed through incrontab (the command)
  • Logging is done through syslog
  • User commands are executed with the users' privileges (so the application calls setuid() and setgid())

With this, one can create a script to be executed when a file is uploaded (or deleted) to/from a file server, or when a process coredump occurred, or whatever automation you want to trigger when some file system event occurred. Events are plenty and can be found in /usr/include/sys/inotify.h.

So, with this information, it is safe to assume that we might be able to push incron in the existing cron policy. After all, it defines the contexts for all these and probably doesn't need any additional tweaking. And this seems to work at first, but a few tests reveal that the behavior is not that optimal.

# chcon -t crond_exec_t /usr/sbin/incrond
# chcon -t crontab_exec_t /usr/bin/incrontab
# chcon -R -t system_cron_spool_t /etc/incron.d
# chcon -t cron_log_t /var/log/cron.log
# chcon -R -t cron_spool_t /var/spool/incron

System tables work somewhat, but all commands are executed in the crond_t domain, not in a system_cronjob_t or related domain.
User tables fail when dealing with files in the users directories, since these too run in crond_t and thus have no read access to the user home directories.

The problems we notice come from the fact that the application is very simple in its code: it is not SELinux-aware (so it doesn't change the runtime context) as most cron daemons are, and when it changes the user id it does not call PAM, so we cannot trigger pam_selinux.so to handle context changes either. As a result, the entire daemon keeps running in crond_t.

This is one reason why a separate domain could be interesting: we might want to extend the rights of the daemon domain a bit, but don't want to extend these rights to the other cron daemons (who also run in crond_t). Another reason is that the cron policy has a few booleans that would not affect the behavior at all, making it less obvious for users to troubleshoot. As a result, we'll go for the separate policy instead - which will be for the next post.