fix(firecracker): harden committed-snapshot resume against guest-controlled data

Address the codex review on #398:

- P1: inject_guest_boot no longer follows a symlink at bb-init/bb-dropbear.
  A committed snapshot is guest-controlled and could plant those paths as
  symlinks aimed at a host file (e.g. bb-init -> ~/.bashrc); write_text /
  copy2 would then overwrite the target as the host user during resume.
  Replace any pre-existing entry and create the files with
  O_EXCL|O_NOFOLLOW so the write stays inside the staging tree.

- P2: write the snapshot tar owner-only (0600). It can contain the bottle's
  private workspace; it was being created world-readable (0644).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

tests/unit/test_firecracker_helpers.py

@@ -7,6 +7,7 @@ from __future__ import annotations
 
 import json
 import os
+import stat
 import subprocess
 import tempfile
 import unittest
@@ -218,5 +219,59 @@ class TestBuildCommittedRootfsDir(unittest.TestCase):
                 self.assertEqual(2, calls["n"])
 
 
+class TestInjectGuestBootSymlinkSafe(unittest.TestCase):
+    """A committed snapshot is guest-controlled: inject_guest_boot must not
+    follow a planted symlink and overwrite a host file during resume."""
+
+    def test_planted_symlink_does_not_escape_staging_tree(self):
+        with tempfile.TemporaryDirectory(prefix="fc-inject.") as d:
+            tmp = Path(d)
+            dropbear = tmp / "dropbear"
+            dropbear.write_bytes(b"DROPBEAR")
+            # A host file the malicious snapshot tries to clobber.
+            victim = tmp / "victim"
+            victim.write_text("original")
+
+            rootfs = tmp / "rootfs"
+            rootfs.mkdir()
+            # The snapshot planted bb-init/bb-dropbear as symlinks to it.
+            (rootfs / "bb-init").symlink_to(victim)
+            (rootfs / "bb-dropbear").symlink_to(victim)
+
+            with patch.object(util, "dropbear_path", return_value=dropbear):
+                util.inject_guest_boot(rootfs, init_script="#!/bin/sh\nreal\n")
+
+            # Host file untouched; the staged paths are fresh regular files.
+            self.assertEqual("original", victim.read_text())
+            self.assertFalse((rootfs / "bb-init").is_symlink())
+            self.assertFalse((rootfs / "bb-dropbear").is_symlink())
+            self.assertEqual("#!/bin/sh\nreal\n", (rootfs / "bb-init").read_text())
+            self.assertEqual(b"DROPBEAR", (rootfs / "bb-dropbear").read_bytes())
+
+
+class TestCommitRootfsPermissions(unittest.TestCase):
+    """The snapshot tar can hold the bottle's private workspace, so the
+    freezer must write it owner-only (0600)."""
+
+    def test_snapshot_created_owner_only(self):
+        with tempfile.TemporaryDirectory(prefix="fc-freeze.") as d:
+            tmp = Path(d)
+            key = tmp / "key"
+            key.write_text("K")
+            tar_path = tmp / "state" / "committed-rootfs.tar"
+
+            def fake_run(argv: list[str], *a: Any, **k: Any) -> Any:
+                # Emulate the ssh|tar pipe streaming the snapshot to stdout.
+                k["stdout"].write(b"TARDATA")
+                return subprocess.CompletedProcess(argv, 0, b"", b"")
+
+            with patch.object(freezer.util, "ssh_base_argv", return_value=["ssh"]), \
+                 patch.object(freezer.subprocess, "run", side_effect=fake_run):
+                freezer._commit_rootfs_via_ssh(key, "10.0.0.1", tar_path)
+
+            self.assertEqual(b"TARDATA", tar_path.read_bytes())
+            self.assertEqual(0o600, stat.S_IMODE(tar_path.stat().st_mode))
+
+
 if __name__ == "__main__":
     unittest.main()