fix(browse): never chmod shared, symlinked, or foreign-owned dirs to 0700

restrictDirectoryPermissions unconditionally chmodded its target. On hosts
where the process holds CAP_FOWNER (Docker as root, CI sandboxes) that
chmod SUCCEEDS on root-owned /tmp whenever a state file is configured
there (BROWSE_STATE_FILE=/tmp/x.json derives stateDir=/tmp), and a 0700
/tmp breaks access(2)-based checks machine-wide for every other process.

The POSIX branch now refuses shared sticky dirs, world-writable mounts
under root, foreign-owned dirs, and symlinked state dirs; refusals warn
once per process instead of failing silent; owned-but-unreadable dirs
keep their chmod self-repair; and the check-then-act race is closed with
fd-anchored O_NOFOLLOW + fstat/fchmod on a single inode.

Regression tests cover the sticky-dir, foreign-uid, mkdirSecure-reapply,
and symlinked-dir shapes.

browse/src/file-permissions.ts

@@ -135,6 +135,21 @@ export function restrictFilePermissions(filePath: string): void {
  * POSIX: `fs.chmodSync(path, 0o700)`. Idempotent if the dir was already
  * created with `{ mode: 0o700 }`.
  *
+ * Owner-only hardening is for directories gstack creates and owns. Two
+ * pre-existing shapes must never be chmodded, so the POSIX branch refuses
+ * them:
+ *
+ *   - A directory owned by another user. On most hosts the chmod just
+ *     fails EPERM, but run as root (or with CAP_FOWNER — containers, CI
+ *     sandboxes) it SUCCEEDS and takes the directory away from its owner.
+ *   - A shared sticky world-writable directory (`/tmp`, `/var/tmp`).
+ *     Callers land here when a state file is configured directly inside
+ *     the system temp dir (`BROWSE_STATE_FILE=/tmp/foo.json` makes
+ *     `path.dirname()` derive `/tmp` as the state dir). A 0o700 `/tmp`
+ *     locks every other process on the machine out of it: access(2)-based
+ *     checks (`fs.existsSync`) return EACCES→false machine-wide, even
+ *     while stat keeps working for capability-holding processes.
+ *
  * Windows: `icacls /inheritance:r /grant:r <user>:(OI)(CI)(F)`. The
  * `(OI)(CI)` flags make new files (OI = object inherit) and subdirs
  * (CI = container inherit) inherit the single-user-full ACL — important
@@ -155,7 +170,78 @@ export function restrictDirectoryPermissions(dirPath: string): void {
     }
     return;
   }
-  try { fs.chmodSync(dirPath, 0o700); } catch { /* best-effort */ }
+  try {
+    // fd-anchored check-then-act: statSync + chmodSync resolve the path twice,
+    // so a symlink swapped in between would let the chmod land on a directory
+    // the check never saw (exactly the CAP_FOWNER hosts this guard exists
+    // for). O_NOFOLLOW refuses a symlinked state dir outright; fstat + fchmod
+    // pin both the check and the act to the same inode.
+    const fd = fs.openSync(
+      dirPath,
+      fs.constants.O_RDONLY | fs.constants.O_DIRECTORY | fs.constants.O_NOFOLLOW,
+    );
+    try {
+      if (!shouldHardenDir(fs.fstatSync(fd))) {
+        warnUnhardenedDirOnce(dirPath);
+        return;
+      }
+      fs.fchmodSync(fd, 0o700);
+    } finally {
+      fs.closeSync(fd);
+    }
+  } catch (err: any) {
+    // The fd path refuses two shapes the old chmodSync handled, and a silent
+    // skip here would quietly lose hardening (the module's own rule is that
+    // refusals are never silent):
+    //   ELOOP/ENOTDIR — dirPath is a symlink (dotfiles-managed ~/.gstack via
+    //     stow/chezmoi). Never follow it blind; warn so the operator knows.
+    //   EACCES — an OWNED dir stuck without the read bit (mode 0300/0000)
+    //     can't be opened but could always be repaired by plain chmod. Keep
+    //     that self-repair: re-check ownership via lstat (no link follow) and
+    //     chmod only a real, owned, non-shared directory.
+    if (err?.code === 'EACCES') {
+      try {
+        const st = fs.lstatSync(dirPath);
+        if (st.isDirectory() && shouldHardenDir(st)) {
+          fs.chmodSync(dirPath, 0o700);
+          return;
+        }
+      } catch { /* fall through to the warning */ }
+    }
+    if (err?.code === 'ELOOP' || err?.code === 'ENOTDIR' || err?.code === 'EACCES') {
+      warnUnhardenedDirOnce(dirPath);
+    }
+    /* anything else: best-effort, matching the old behavior */
+  }
+}
+
+/** Owner-only hardening applies to a real dir we own that isn't shared. */
+function shouldHardenDir(st: fs.Stats): boolean {
+  // chmod authorization is judged on the EFFECTIVE uid; fall back to the
+  // real uid where geteuid is unavailable (Windows — unreachable here).
+  const uid = process.geteuid?.() ?? process.getuid?.();
+  if (st.uid !== uid) return false;
+  // sticky + world-writable = shared temp dir (/tmp, /var/tmp) — never ours
+  // to restrict. Running as root, treat ANY world-writable dir as shared:
+  // root "owns" docker/CI volume mounts (0777, no sticky bit) that other
+  // users depend on, and a 0700 there locks them all out.
+  if ((st.mode & 0o1002) === 0o1002) return false;
+  if (uid === 0 && (st.mode & 0o002) === 0o002) return false;
+  return true;
+}
+
+// The POSIX refusal must not be silent (the Windows branch already warns when
+// icacls misses its hardening target): an operator who points state at a
+// shared or foreign-owned directory should learn it was left unhardened.
+const unhardenedWarned = new Set<string>();
+function warnUnhardenedDirOnce(dirPath: string): void {
+  if (unhardenedWarned.has(dirPath)) return;
+  unhardenedWarned.add(dirPath);
+  // biome-ignore lint/suspicious/noConsole: intentional user-facing warning
+  console.warn(
+    `[gstack] directory ${dirPath} is shared, symlinked, or owned by another user; ` +
+    'gstack left its permissions untouched — use a private, non-symlinked directory for owner-only hardening',
+  );
 }
 
 /**
@@ -218,7 +304,10 @@ export function repairBrokenDacl(dirPath: string): void {
 /**
  * `mkdir -p` with owner-only directory permissions, cross-platform.
  * Replaces `fs.mkdirSync(path, { recursive: true, mode: 0o700 })` + Windows ACL.
- * Safe to call on an existing directory — re-applies the ACL idempotently.
+ * Safe to call on an existing directory — re-applies the ACL idempotently,
+ * except on directories gstack doesn't own (another user's dir, or a shared
+ * sticky world-writable dir like `/tmp`), which are left untouched — see
+ * `restrictDirectoryPermissions`.
  *
  * Windows: after applying the restricted ACL, verifies the directory is
  * still listable by this process and repairs a broken DACL (#1605) if not.

browse/test/file-permissions.test.ts

@@ -72,6 +72,34 @@ describe('restrictDirectoryPermissions', () => {
     expect(fs.statSync(d).mode & 0o777).toBe(0o700);
   });
 
+  test('on POSIX, leaves a shared sticky world-writable dir untouched', () => {
+    if (process.platform === 'win32') return;
+    // Simulates /tmp: sticky + world-writable. Hardening a shared dir to
+    // 0o700 locks every other user on the machine out of it, so the helper
+    // must refuse — even when the caller owns the dir (root / CAP_FOWNER
+    // hosts are where the chmod would actually succeed).
+    const d = path.join(tmpDir, 'shared-tmp');
+    fs.mkdirSync(d);
+    // System chmod, not fs.chmodSync: Bun masks the sticky bit off chmod/
+    // mkdir modes, so 0o1777 through the fs API lands as 0o777.
+    Bun.spawnSync(['chmod', '1777', d]);
+    expect(fs.statSync(d).mode & 0o7777).toBe(0o1777); // fixture took
+    restrictDirectoryPermissions(d);
+    expect(fs.statSync(d).mode & 0o7777).toBe(0o1777);
+  });
+
+  test('on POSIX, leaves a directory owned by another user untouched', () => {
+    if (process.platform === 'win32') return;
+    const d = path.join(tmpDir, 'foreign');
+    fs.mkdirSync(d, { mode: 0o755 });
+    // Only constructible where chown to a foreign uid succeeds (root /
+    // CAP_CHOWN — containers, CI sandboxes). Elsewhere the chown throws
+    // and there's nothing to assert; bail.
+    try { fs.chownSync(d, process.getuid!() + 1, fs.statSync(d).gid); } catch { return; }
+    restrictDirectoryPermissions(d);
+    expect(fs.statSync(d).mode & 0o777).toBe(0o755);
+  });
+
   test('on Windows, does not throw on an existing directory', () => {
     if (process.platform !== 'win32') return;
     const d = path.join(tmpDir, 'subdir');
@@ -159,6 +187,23 @@ describe('mkdirSecure', () => {
     expect(() => mkdirSecure(d)).not.toThrow();
   });
 
+  test('does not chmod a pre-existing shared sticky dir (the /tmp state-dir case)', () => {
+    if (process.platform === 'win32') return;
+    // BROWSE_STATE_FILE=/tmp/foo.json derives stateDir=/tmp, and every
+    // daemon boot runs mkdirSecure(stateDir). The mkdir is a no-op on the
+    // existing dir; the permission re-apply must be one too — chmodding the
+    // real /tmp to 0o700 breaks fs.existsSync (access(2) → EACCES) for
+    // every process on the machine until something restores 1777.
+    const d = path.join(tmpDir, 'shared-tmp');
+    fs.mkdirSync(d);
+    // System chmod: Bun's fs API masks the sticky bit off modes (see the
+    // restrictDirectoryPermissions sticky-dir test).
+    Bun.spawnSync(['chmod', '1777', d]);
+    expect(fs.statSync(d).mode & 0o7777).toBe(0o1777); // fixture took
+    mkdirSecure(d);
+    expect(fs.statSync(d).mode & 0o7777).toBe(0o1777);
+  });
+
   test('on Windows, the created directory stays usable by the caller', () => {
     if (process.platform !== 'win32') return;
     // The state-dir path that broke: mkdirSecure() creates .gstack/, hardens
@@ -208,3 +253,21 @@ describe('repairBrokenDacl', () => {
     expect(() => repairBrokenDacl(path.join(tmpDir, 'nonexistent'))).not.toThrow();
   });
 });
+// Symlinked state dir (dotfiles-managed ~/.gstack via stow/chezmoi): the
+// fd-anchored path refuses to follow it (O_NOFOLLOW) — the refusal must warn,
+// never throw, and never chmod the symlink target.
+test('restrictDirectoryPermissions warns and skips a symlinked dir without throwing', () => {
+  const base = fs.mkdtempSync(path.join(os.tmpdir(), 'fp-symlink-'));
+  const target = path.join(base, 'real');
+  const link = path.join(base, 'link');
+  fs.mkdirSync(target, { mode: 0o755 });
+  fs.symlinkSync(target, link);
+  try {
+    expect(() => restrictDirectoryPermissions(link)).not.toThrow();
+    // Target permissions untouched — the link was never followed.
+    expect(fs.statSync(target).mode & 0o777).toBe(0o755);
+  } finally {
+    fs.rmSync(base, { recursive: true, force: true });
+  }
+});
+