From 525cbe0355cd40690fc8140d485a8a7a129dbabd Mon Sep 17 00:00:00 2001
From: atlas <atlas@hyperhive.local>
Date: Thu, 29 Oct 2026 00:09:12 +0100
Subject: [PATCH] count readers, not writers, of the pull token in the cache's
 roster pin

---
 nix/module-eval/bao-grants.nix | 37 +++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/nix/module-eval/bao-grants.nix b/nix/module-eval/bao-grants.nix
index 85057868..c1b43d95 100644
--- a/nix/module-eval/bao-grants.nix
+++ b/nix/module-eval/bao-grants.nix
@@ -316,6 +316,15 @@ let
         best: g: if best == null || lib.stringLength g.path > lib.stringLength best.path then g else best
       ) null globs;
 
+  # Whether the policy text in `text` lets its holder read `secret/data/swarm/<leaf>`,
+  # by `grantFor`'s rule, so a glob that reaches the leaf counts and a write does not.
+  grantReads =
+    leaf: text:
+    let
+      g = grantFor (grantsIn text) "secret/data/swarm/${leaf}";
+    in
+    g != null && lib.elem "read" g.caps;
+
   ungranted =
     grants: units:
     lib.concatLists (
@@ -2139,20 +2148,42 @@ let
     {
       # The signing key is the cache's alone; the push token is also the
       # builder's, and is read there. The pull token is also the controller's,
-      # which only reads it (pinned above).
-      name = "no granting unit but the cache's names its signing key, and only it and the controller name its pull token";
+      # which only reads it (pinned above). The pull token's value is what
+      # the roster guards, so it counts the units that may read it: the
+      # operator's write-only grant on it, which replaces the token and
+      # exposes nothing, stays outside.
+      name = "no granting unit but the cache's names its signing key, and only it and the controller read its pull token";
       ok =
         lib.filter (
           u: lib.hasInfix "services/attic/jwt-secret" baoGrantHere.systemd.services.${u}.script
         ) grantingUnitNames == [ "swarm-bao-attic-policy" ]
         &&
           lib.filter (
-            u: lib.hasInfix "services/attic/pull-token" baoGrantHere.systemd.services.${u}.script
+            u: grantReads "services/attic/pull-token" baoGrantHere.systemd.services.${u}.script
           ) grantingUnitNames == [
             "swarm-bao-controller-policy"
             "swarm-bao-attic-policy"
           ];
     }
+    {
+      # Control for the property above: `grantReads` tells a read from a write
+      # on the same path, and the operator's policy does name the pull token.
+      name = "a grant that only writes the pull token is not a read of it, and one that reads it is";
+      ok =
+        let
+          stanza = caps: ''
+            path "secret/data/swarm/services/attic/pull-token" {
+              capabilities = [${caps}]
+            }
+          '';
+          viewer = baoGrantHere.systemd.services.swarm-bao-operator-viewer-policy.script;
+        in
+        !(grantReads "services/attic/pull-token" (stanza ''"create", "update"''))
+        && grantReads "services/attic/pull-token" (stanza ''"read"'')
+        && grantReads "services/attic/pull-token" (stanza ''"create", "read"'')
+        && lib.hasInfix "services/attic/pull-token" viewer
+        && !(grantReads "services/attic/pull-token" viewer);
+    }
     {
       name = "the PKI unit signs the cache's leaf under its own subject";
       ok =