fleetd #184: report member trust model

The correction removed a false claim (blocking the socket breaks git over
SSH) but took a true one with it: the socket is a live handle to the agent,
so a member holding it can sign with every key the agent holds. Without that,
the entry reads as if the setting does not matter, and an operator has no
reason left not to set it to allow. Fixing an overclaim must not leave an
underclaim.

Also record the measurement and the mistake behind the old claim, so the next
person does not re-argue it from scratch.

fleetd/fleetd.example.yaml

@@ -686,6 +686,19 @@ guard:
 #            confidentiality boundary. A real boundary needs a different OS user or OS-level
 #            confinement, such as a container or VM. That is the open question in fleetd #184.
 #
+#            Still do not set this to "allow" casually. SSH_AUTH_SOCK is a live handle to YOUR
+#            ssh-agent, so a member holding it can sign with EVERY key the agent holds. It sits in
+#            no secret file and looks like no credential, which is why it slipped past three
+#            earlier tickets (gitea #110). Blocking it does not contain a member, but allowing it
+#            hands one a signing capability for no gain — the block costs nothing, so keep it.
+#
+#            Both halves of this are measured, not argued. 2026-08-28: a member with
+#            SSH_AUTH_SOCK blanked pushed to the forge over SSH successfully, because `ssh -G`
+#            resolves an IdentityFile outside ~/.ssh that is readable and has no passphrase. An
+#            earlier version of this comment claimed blocking the socket BREAKS git over SSH. It
+#            does not. That claim came from looking only in ~/.ssh, which holds nothing but four
+#            `Include` lines — looking in one place and concluding about the whole host.
+#
 # HOT-RELOADABLE the same way `fleet:` is (CB-559): read fresh on every spawn, so editing this list
 # and reloading config (or restarting) changes what the NEXT spawn inherits; already-running members
 # are unaffected either way.

fleetd/src/main/java/dev/ltms/fleet/Fleetd.java

@@ -121,6 +121,7 @@ public final class Fleetd {
         // else can fail on a silently-empty one. A daemon started without a login shell (launchd)
         // boots fine either way — this is the only thing that says so out loud.
         reportRequiredSecrets(cfg);
+        reportMemberTrustModel(cfg);
         // CB-596: an absent (or empty) memberCredentials: block blocks NOTHING — no credential
         // name is hardcoded any more to fall back on. Say so loudly, the same way a missing
         // secret is reported above, so upgrading past this commit never silently drops CB-592's
@@ -1136,6 +1137,24 @@ public final class Fleetd {
         });
     }
 
+    /**
+     * fleetd #184: state the member trust model at startup. Environment controls and worktrees do
+     * not make a sandbox when fleetd and its members use the same OS user. A separate herdr may
+     * provide that boundary, but fleetd cannot inspect the uid at the other end of its socket.
+     */
+    static void reportMemberTrustModel(FleetConfig cfg) {
+        if (cfg.memberHerdrSocket() != null && !cfg.memberHerdrSocket().isBlank()) {
+            log.info("member trust model: members are routed to a separate herdr through "
+                    + "memberHerdrSocket. fleetd cannot see that herdr's uid, so confirm it runs "
+                    + "as a different OS user before treating it as a boundary.");
+            return;
+        }
+        log.info("member trust model: members run as the same OS user as fleetd, not in a sandbox. "
+                + "A member can read any file this user can read, including SSH keys and credential "
+                + "stores, whatever memberCredentials says. To add a real boundary, route members to "
+                + "a second herdr under a different OS user with memberHerdrSocket.");
+    }
+
     /**
      * CB-596: {@code known:} empty (block absent entirely, or present but empty) means {@link
      * FleetConfig.MemberCredentials#blockedSet()} is empty too — every member pane inherits the

fleetd/src/test/java/dev/ltms/fleet/MemberTrustModelReportTest.java

@@ -0,0 +1,73 @@
+package dev.ltms.fleet;
+
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
+import dev.ltms.fleet.config.FleetConfig;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.slf4j.LoggerFactory;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * fleetd #184: startup must state whether members share fleetd's OS user or use a separate herdr.
+ */
+class MemberTrustModelReportTest {
+
+    private static FleetConfig load(Path dir, String yaml) throws Exception {
+        Path f = dir.resolve("fleetd.yaml");
+        Files.writeString(f, yaml);
+        return FleetConfig.load(f);
+    }
+
+    private static ListAppender<ILoggingEvent> attach() {
+        Logger logger = (Logger) LoggerFactory.getLogger(Fleetd.class);
+        ListAppender<ILoggingEvent> appender = new ListAppender<>();
+        appender.start();
+        logger.addAppender(appender);
+        return appender;
+    }
+
+    private static void detach(ListAppender<ILoggingEvent> appender) {
+        ((Logger) LoggerFactory.getLogger(Fleetd.class)).detachAppender(appender);
+    }
+
+    private static String report(FleetConfig cfg) {
+        Logger logger = (Logger) LoggerFactory.getLogger(Fleetd.class);
+        ch.qos.logback.classic.Level original = logger.getLevel();
+        logger.setLevel(ch.qos.logback.classic.Level.INFO);
+        ListAppender<ILoggingEvent> appender = attach();
+        try {
+            Fleetd.reportMemberTrustModel(cfg);
+        } finally {
+            detach(appender);
+            logger.setLevel(original);
+        }
+        return appender.list.getFirst().getFormattedMessage();
+    }
+
+    @Test
+    void unsetMemberHerdrSocketStatesThatMembersAreNotSandboxed(@TempDir Path dir) throws Exception {
+        FleetConfig cfg = load(dir, "bind:\n  host: 127.0.0.1\n  port: 8765\n");
+
+        assertEquals("member trust model: members run as the same OS user as fleetd, not in a sandbox. "
+                        + "A member can read any file this user can read, including SSH keys and credential "
+                        + "stores, whatever memberCredentials says. To add a real boundary, route members to "
+                        + "a second herdr under a different OS user with memberHerdrSocket.",
+                report(cfg));
+    }
+
+    @Test
+    void configuredMemberHerdrSocketStatesThatFleetdCannotConfirmTheBoundary(@TempDir Path dir) throws Exception {
+        FleetConfig cfg = load(dir, "memberHerdrSocket: /tmp/member-herdr.sock\n");
+
+        assertEquals("member trust model: members are routed to a separate herdr through "
+                        + "memberHerdrSocket. fleetd cannot see that herdr's uid, so confirm it runs "
+                        + "as a different OS user before treating it as a boundary.",
+                report(cfg));
+    }
+}