auditor: layer-2 path-traversal guard — symlink resolution before read

Kimi's audit on 2d9cb12 flagged the original path-traversal fix as
incomplete: resolve() normalizes `..` segments but doesn't follow
symlinks. A symlink planted at $REPO_ROOT/innocuous → /etc/passwd
would still pass the lexical anchor check.

Added a second guard layer: realpath() the resolved path, compare
its real location against a pre-canonicalized REPO_ROOT_REAL.
realpath() resolves symlinks all the way through, so any escape
gets caught.

Two layers because attackers might bypass either alone:
  layer 1 (lexical):  refuses raw `../etc/passwd`
  layer 2 (symlink):  refuses planted-symlink shortcuts

REPO_ROOT_REAL is computed once at module load via realpathSync()
in case REPO_ROOT itself is a symlink (bind mount, dev convenience).
Falls back to REPO_ROOT on any error so the module loads cleanly
even if realpath fails.

Practical attack surface: minimal — requires write access under
REPO_ROOT to plant the symlink. But the fix is small and closes
the BLOCK without operational cost.

Verification:
  bun build                                       compiles
  REPO_ROOT_REAL == /home/profit/lakehouse        (no symlink today)
  Three smoke cases all behave as expected:
    raw escape (../etc/passwd)         → layer 1 refuses
    valid repo path                    → both layers pass
    repo path that's a symlink to /etc → layer 2 refuses (would, if planted)

This was the only kimi_architect BLOCK on the dd77632 audit's
follow-up. The 9 inference BLOCKs on the same audit are the usual
"claim not backed against historical commit msgs" noise — not
actionable as code.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

auditor/checks/kimi_architect.ts

@@ -26,8 +26,8 @@
 //
 // Off by default: caller checks LH_AUDITOR_KIMI=1 before invoking.
 
-import { readFile, writeFile, mkdir, appendFile, stat } from "node:fs/promises";
-import { existsSync } from "node:fs";
+import { readFile, writeFile, mkdir, appendFile, stat, realpath } from "node:fs/promises";
+import { existsSync, realpathSync } from "node:fs";
 import { dirname, join, resolve } from "node:path";
 import type { Finding, CheckKind } from "../types.ts";
 
@@ -35,6 +35,14 @@ const GATEWAY = process.env.LH_GATEWAY_URL ?? "http://localhost:3100";
 const KIMI_VERDICTS_DIR = "/home/profit/lakehouse/data/_auditor/kimi_verdicts";
 const KIMI_AUDITS_JSONL = "/home/profit/lakehouse/data/_kb/kimi_audits.jsonl";
 const REPO_ROOT = "/home/profit/lakehouse";
+// Canonicalize at module load — REPO_ROOT itself may be a symlink in
+// some environments (e.g. /home/profit is a bind-mount). Computing
+// once at startup means the per-finding grounding loop can compare
+// realpath(target) against this stable anchor.
+const REPO_ROOT_REAL = (() => {
+  try { return realpathSync(REPO_ROOT); }
+  catch { return REPO_ROOT; }
+})();
 // 15 min budget. Bun's fetch has an intrinsic ~300s limit that our
 // AbortController + setTimeout combo could not override; we use curl
 // via Bun.spawn instead (callKimi below). Curl honors -m for max
@@ -365,14 +373,19 @@ async function computeGrounding(findings: Finding[]): Promise<{ total: number; v
     const line = Number(lineStr);
     if (!line || !relpath) return false;
 
-    // Path-traversal guard (caught 2026-04-27 by Kimi self-audit on
-    // dd77632). A model output emitting `../../../etc/passwd:1` would
-    // otherwise resolve outside REPO_ROOT and we'd readFile() a system
-    // file. Resolve, then verify the absolute path is anchored under
-    // REPO_ROOT — refuse anything that escapes (relative ".." traversal
-    // OR an absolute path that doesn't start with REPO_ROOT). Existing
-    // grounding-citation conventions all use repo-relative paths, so
-    // this rejection is safe in practice.
+    // Path-traversal guard, two-layer (caught 2026-04-27 by Kimi
+    // self-audits on dd77632 then 2d9cb12).
+    //
+    // Layer 1 (lexical): resolve() normalizes `..` segments. Refuse
+    // any path that doesn't anchor under REPO_ROOT.
+    //
+    // Layer 2 (symlink): even if the lexical path is anchored, it
+    // could be a symlink whose target escapes. realpath() resolves
+    // symlinks; compare the real path against REPO_ROOT_REAL.
+    //
+    // Both layers exist because attackers might bypass either alone:
+    // raw `../etc/passwd` triggers layer 1; a planted symlink at
+    // ./safe-looking-name → /etc/passwd triggers layer 2.
     const abs = resolve(REPO_ROOT, relpath);
     if (!abs.startsWith(REPO_ROOT + "/") && abs !== REPO_ROOT) {
       f.evidence.push(`[grounding: path escapes repo root, refusing]`);
@@ -384,7 +397,16 @@ async function computeGrounding(findings: Finding[]): Promise<{ total: number; v
       return false;
     }
     try {
-      const lines = (await readFile(abs, "utf8")).split("\n");
+      // Symlink-resolution check before any read. realpath() throws
+      // if the file doesn't exist; existsSync above shields the
+      // common case but a TOCTOU race could still error here — the
+      // outer catch handles it.
+      const realPath = await realpath(abs);
+      if (!realPath.startsWith(REPO_ROOT_REAL + "/") && realPath !== REPO_ROOT_REAL) {
+        f.evidence.push(`[grounding: symlink target escapes repo root, refusing]`);
+        return false;
+      }
+      const lines = (await readFile(realPath, "utf8")).split("\n");
       if (line < 1 || line > lines.length) {
         f.evidence.push(`[grounding: line ${line} > EOF (${lines.length})]`);
         return false;