Reject SSH connections without SLURM allocation

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/fox/configuration.nix

@@ -56,4 +56,20 @@
     wantedBy = [ "multi-user.target" ];
     serviceConfig.ExecStart = script;
   };
+
+  # Only allow SSH connections from users who have a SLURM allocation
+  # See: https://slurm.schedmd.com/pam_slurm_adopt.html
+  security.pam.services.sshd.rules.account.slurm = {
+    control = "required";
+    enable = true;
+    modulePath = "${pkgs.slurm}/lib/security/pam_slurm_adopt.so";
+    args = [ "log_level=debug5" ];
+    order = 999999; # Make it last one
+  };
+
+  # Disable systemd session (pam_systemd.so) as it will conflict with the
+  # pam_slurm_adopt.so module. What happens is that the shell is first adopted
+  # into the slurmstepd task and then into the systemd session, which is not
+  # what we want, otherwise it will linger even if all jobs are gone.
+  security.pam.services.sshd.startSession = lib.mkForce false;
 }

m/module/slurm-client.nix

@@ -93,9 +93,29 @@ in {
       # Ignore memory constraints and only use unused cores to share a node with
       # other jobs.
       SelectTypeParameters=CR_Core
+
+      # Required for pam_slurm_adopt, see https://slurm.schedmd.com/pam_slurm_adopt.html
+      # This sets up the "extern" step into which ssh-launched processes will be
+      # adopted. Alloc runs the prolog at job allocation (salloc) rather than
+      # when a task runs (srun) so we can ssh early.
+      PrologFlags=Alloc,Contain,X11
+
+      # LaunchParameters=ulimit_pam_adopt will set RLIMIT_RSS in processes
+      # adopted by the external step, similar to tasks running in regular steps
+      # LaunchParameters=ulimit_pam_adopt
+      SlurmdDebug=debug5
+      #DebugFlags=Protocol,Cgroup
+    '';
+
+    extraCgroupConfig = ''
+      CgroupPlugin=cgroup/v2
+      #ConstrainCores=yes
     '';
   };
 
+  # Place the slurm config in /etc as this will be required by PAM
+  environment.etc.slurm.source = config.services.slurm.etcSlurm;
+
   age.secrets.mungeKey = {
     file = ../../secrets/munge-key.age;
     owner = "munge";

pkgs/overlay.nix

@@ -39,6 +39,18 @@ final: prev:
       # See https://bugs.schedmd.com/show_bug.cgi?id=19324
       ./slurm-rank-expansion.patch
     ];
+    # Install also the pam_slurm_adopt library to restrict users from accessing
+    # nodes with no job allocated.
+    postBuild = (old.postBuild or "") + ''
+      pushd contribs/pam_slurm_adopt
+        make "PAM_DIR=$out/lib/security"
+      popd
+    '';
+    postInstall = (old.postInstall or "") + ''
+      pushd contribs/pam_slurm_adopt
+        make "PAM_DIR=$out/lib/security" install
+      popd
+    '';
   });
 
   prometheus-slurm-exporter = prev.callPackage ./slurm-exporter.nix { };