zondag 30 september 2012

Determine whether Cron can execute jobs on behalf of the user with login user SELinux permissions

Cron would run jobs on behalf of users in a "cronjob" domain. This domain is reasonable restricted compared to the domain in which most users operate.

Fedora changed this behaviour to allow Cron to execute these jobs in the default login user domain of the user that owns the job.

Recently i added a boolean to Reference policy that enables one to tune the policy to allow Cron to run user cronjobs in the default login user domain conditionaly.

By default Cron will still execute the jobs in the Cron job domain but if you toggle the cron_userdomain_transition boolean to true then Cron will run the jobs in the default login user domain of the user owning the job.

This allows Cron to run jobs with the SE-Linux permissions that the owner of the job has.

Cron is SELinux aware. It queries the policy to see to which domain it should transition when running a job.

It does not do a automatic domain transition because it does not actually execute the Cron job. Instead if uses "setexec" to domain transition.

It looks to see to which domains the Cron job file is a entrypoint. Then it needs to be able to process transition to that domain and then it checks the default login user domain of a user and with this information it determines in which domain to execute the job

By default:

allow cronjob_t user_cron_spool_t:file entrypoint;
allow crond_t cronjob_t:process transition;

Then there is a default context of for staff this is for example:

system_r:crond_t:s0        staff_r:cronjob_t:s0

But when you tune the policy with cron_userdomain_transition then these rules will be replaced by:

allow $1 user_cron_spool_t:file entrypoint;
allow crond_t $1:process transition;

Where $1 is the calling login user domain, and then it will look up the default login user domain and role, for staff this is for example:

# semanage user -l | grep staff_u
staff_u         user       SystemLow  SystemLow-SystemHigh           staff_r sysadm_r system_r unconfined_r
That means:

"staff_r:staff_t"

Then Cron daemon has permission to run the job in the staff domain with the staff role.

The benefits of running jobs in the login user domain are that now your Cron job can interact with your processes and operate on your files as if it were you interacting with your processes and operating on your files.

Needless to say that other SE-Linux rules that apply to you now also apply to your Cron jobs.

The policy may still have some rough edges but it was tested on Gentoo and is said to work. I did make some changes after that but hopefully i have not introduced regression.

Currently Redhat distributions do not have this cron_userdomain_transition boolean and i am not sure if she will adopt it.

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