# SELinux

Starting with version 21.08, Slurm includes support for setting an SELinux context for jobs as a technology preview. The implementation may change in future releases, and support for it is not enabled by default.

## Architecture

When enabled, the Slurm job submission commands — salloc, sbatch, and srun — will automatically set a field with the current operating context. This field can be overwritten by the --context command line option.

It is important to note that this value can be directly manipulated by the end-user, and it is up to site-specific scripts to validate and control access to these contexts. At this time MUNGE, which Slurm users to security identify users and hosts on the cluster, does not provide an SELinux context field, and as such there is no secure mechanism to send the current context to the Slurm controller. Thus the context, as provided at job submission time, must be validated by a job_submit plugin running within the slurmctld.

Without such a script, no context is set or managed for a user's job.

## Installation

### Source:

SELinux support is disabled by default and must be enabled at configure time. It requires the libselinux1 library and development headers to build.

    configure --enable-selinux

## Setup

Once a version of Slurm that supports SELinux is installed, you will need to enable and create a job_submit plugin that will perform verification of the SELinux context, before passing it along to the slurmctld. At this time, there is not a reliable and secure way to get/verify contexts internally so you MUST create this script and perform verification in the job_submit plugin.

Example:

    function slurm_job_submit(job_desc, part_list, submit_uid)
      if job_desc.req_context then
        local element = 0
        for str in string.gmatch(job_desc.req_context, "([^:]+)") do
          if element == 0 and str ~= "unconfined_u" then
            slurm.log_user("Error: invalid SELinux context")
            return slurm.ERROR
          elseif element == 1 and str ~= "unconfined_r" then
            slurm.log_user("Error: %s is not a valid SELinux role")
            return slurm.ERROR
          end
          element = element + 1
        end
        job_desc.selinux_context = job_desc.req_context
      else
        -- Force a specific context if one wasn't requested
        job_desc.selinux_context = unconfined_u:unconfined_r:slurm_t:s0
      end
      return slurm.SUCCESS
    end

Note that job_desc.selinux_context is set based on the contents of job_desc.req_context if they are considered valid. job_desc.selinux_context is what set the context that will be used.

## Initial Testing

id is very useful for showing what context a user is currently in. As a test to make sure that we are switching contexts, you can run a quick test with srun.

    mcmult@master:~$ srun id
    uid=1000(mcmult) gid=1000(mcmult) groups=1000(mcmult),27(sudo) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
    mcmult@master:~$ srun --context=unconfined_u:unconfined_r:unconfined_t:s0 id
    uid=1000(mcmult) gid=1000(mcmult) groups=1000(mcmult),27(sudo) context=unconfined_u:unconfined_r:unconfined_t:s0

## Accounting

There is currently no support for tracking the SELinux context in Slurm's accounting. This may change as support evolves in future releases. If you need to keep track of the SELinux Context, it is possible to store it in the admin comment field as part of your job_submit plugin as is show in the example below.

Example:

    function slurm_job_submit(job_desc, part_list, submit_uid)
      if job_desc.req_context then
        local element = 0
        for str in string.gmatch(job_desc.req_context, "([^:]+)") do
          if element == 0 and str ~= "unconfined_u" then
            slurm.log_user("Error: invalid SELinux context")
            return slurm.ERROR
          elseif element == 1 and str ~= "unconfined_r" then
            slurm.log_user("Error: %s is not a valid SELinux role")
            return slurm.ERROR
          end
          element = element + 1
        end
        job_desc.selinux_context = job_desc.req_context
      else
        -- Force a specific context if one wasn't requested
        job_desc.selinux_context = unconfined_u:unconfined_r:slurm_t:s0
      end
      job_desc.admin_comment = "SELinuxContext=" .. job_desc.selinux_context
      return slurm.SUCCESS
    end

Note the addition of setting "job_desc.admin_comment" before returning. This will set the admin comment to show what context we will try to set for the job.

## Notes

If you wish to use pam_slurm_adopt with SELinux, see the pam_slurm_adopt documentation for hints on how to get this working. Note that that when using this feature and pam_slurm_adopt at the same time that the ssh session may not land in the same context as the job.

Last modified 22 September 2023