hive-priv: lock personality, deny realtime, kernel logs and module loading

Adds LockPersonality, RestrictRealtime, ProtectKernelLogs and
ProtectKernelModules to the hive-priv unit. Nothing in hive-priv's
sources or its children uses personality(2), real-time scheduling,
/dev/kmsg, /proc/kmsg or explicit module loading.

A module-eval case pins the four directives on the rendered unit.

docs/trust-boundary/security.md

@@ -322,7 +322,13 @@ above for how the hard-coded unit name scopes its access instead.
 **Socket-activated** — systemd starts `hive-priv` on the first
 incoming connection (`LISTEN_FDS=1`); it's not running between calls.
 The `ProtectSystem=strict` + `ReadWritePaths` sandbox limits filesystem
-writes to only the paths `hive-priv` legitimately needs.
+writes to only the paths `hive-priv` legitimately needs. The unit also sets
+`LockPersonality`, `RestrictRealtime`, `ProtectKernelLogs` and
+`ProtectKernelModules`: it changes no execution domain, schedules nothing in
+real time, reads no kernel log, and loads no module. It sets none of
+`RestrictNamespaces` (`nsenter` into a container needs `setns`),
+`RestrictAddressFamilies`, `PrivateDevices`, `CapabilityBoundingSet` or
+`SystemCallFilter`; those need a watched deploy to judge.
 
 ### Privilege boundary summary
 

nix/host-modules/hive-priv.nix

@@ -148,6 +148,15 @@ in
         # meta/applied repos; libgit2 refuses them without safe.directory.
         # See safeDirGitconfig above.
         ExecStartPre = "+-${pkgs.coreutils}/bin/cp ${safeDirGitconfig} /var/lib/hive-priv/.gitconfig";
+        # Kernel surfaces neither hive-priv nor its children use: no
+        # personality(2), no real-time scheduling, no /dev/kmsg or
+        # /proc/kmsg, no explicit module loading (kernel-initiated autoload
+        # still works). Container start/stop run in `container@.service`,
+        # not under this unit.
+        LockPersonality = true;
+        RestrictRealtime = true;
+        ProtectKernelLogs = true;
+        ProtectKernelModules = true;
       };
     };
   };

nix/module-eval/core-toggle.nix

@@ -1083,6 +1083,20 @@ let
         { unit, ... }: !(bare.systemd.services ? ${unit}) && !(bare.systemd.timers ? ${unit})
       ) storeRefreshed;
     }
+    {
+      # `or false` so a dropped key fails this case by name instead of
+      # aborting eval. Only the rendered unit is pinned; eval cannot show
+      # that hive-priv still works under the directives.
+      name = "hive-priv locks personality and denies real-time scheduling, kernel logs and module loading";
+      ok =
+        let
+          sc = allLocal.systemd.services.hive-priv.serviceConfig;
+        in
+        (sc.LockPersonality or false)
+        && (sc.RestrictRealtime or false)
+        && (sc.ProtectKernelLogs or false)
+        && (sc.ProtectKernelModules or false);
+    }
   ]
   # Starting a unit that stays `active (exited)` is a no-op, so a timer aimed
   # at one fires and fetches nothing.