Enhance security by ensuring unique SSH host keys for VMs and protecting registry credentials. Update documentation to reflect changes in SSH key management and improve state file permissions.

uvm/images.py

@@ -24,6 +24,11 @@ _FIRECRACKER_DEMO_PUBLIC_KEY = (
 )
 _MODE_PATTERN = re.compile(r"Mode:\s+0*([0-7]+)")
 _OWNER_PATTERN = re.compile(r"User:\s+(\d+)\s+Group:\s+(\d+)")
+_HOST_KEY_TYPES = (
+    ("rsa", "3072"),
+    ("ecdsa", "256"),
+    ("ed25519", None),
+)
 
 
 @dataclass(frozen=True, slots=True)
@@ -144,6 +149,7 @@ class ImageStore:
 
                 self._write_guest_file(disk, "/etc/shadow", shadow)
                 self._write_guest_file(disk, "/etc/ssh/sshd_config", sshd_config)
+                self._replace_guest_host_keys(disk, temporary)
                 if authorized_keys is not None:
                     self._remove_firecracker_demo_key(disk, authorized_keys)
         except (OSError, UnicodeError) as error:
@@ -179,9 +185,12 @@ class ImageStore:
         mode_match = _MODE_PATTERN.search(stat_result.stdout)
         owner_match = _OWNER_PATTERN.search(stat_result.stdout)
         if mode_match is None or owner_match is None:
-            if required:
+            output = f"{stat_result.stdout}\n{stat_result.stderr}".lower()
+            if not required and "file not found" in output:
+                return None
+            if required and "file not found" in output:
                 raise UvmError(f"guest rootfs is missing required file: {guest_path}")
-            return None
+            raise UvmError(f"could not inspect guest rootfs file: {guest_path}")
 
         self._runner.run(
             ("debugfs", "-R", f"dump {guest_path} {destination}", disk),
@@ -229,6 +238,43 @@ class ImageStore:
         ):
             raise UvmError(f"could not verify updated guest rootfs file: {guest_path}")
 
+    def _replace_guest_host_keys(self, disk: Path, temporary: Path) -> None:
+        for key_type, bits in _HOST_KEY_TYPES:
+            name = f"ssh_host_{key_type}_key"
+            private_path = temporary / name
+            command: list[str | Path] = [
+                "ssh-keygen",
+                "-q",
+                "-t",
+                key_type,
+                "-N",
+                "",
+                "-C",
+                "",
+                "-f",
+                private_path,
+            ]
+            if bits is not None:
+                command[4:4] = ["-b", bits]
+            self._runner.run(command, capture=True)
+
+            for suffix, default_mode in (("", 0o600), (".pub", 0o644)):
+                guest_path = f"/etc/ssh/{name}{suffix}"
+                existing = self._read_guest_file(
+                    disk,
+                    guest_path,
+                    temporary / f"existing-{name}{suffix}",
+                    required=False,
+                )
+                generated = private_path.with_name(f"{name}{suffix}")
+                source = _GuestFile(
+                    path=generated,
+                    mode=existing.mode if existing is not None else default_mode,
+                    uid=existing.uid if existing is not None else 0,
+                    gid=existing.gid if existing is not None else 0,
+                )
+                self._write_guest_file(disk, guest_path, source)
+
     def _remove_firecracker_demo_key(self, disk: Path, authorized_keys: _GuestFile) -> None:
         contents = authorized_keys.path.read_text(encoding="utf-8")
         updated = _without_firecracker_demo_key(contents)
@@ -269,10 +315,29 @@ def _set_shadow_password(contents: str, username: str, password_hash: str) -> st
 
 
 def _enable_ssh_password_authentication(contents: str, username: str) -> str:
-    directives = ["PasswordAuthentication yes"]
+    managed_directives = {"hostkey", "passwordauthentication", "permitrootlogin"}
+    unmanaged_lines = []
+    for line in contents.splitlines():
+        stripped = line.lstrip()
+        directive = stripped.split(None, 1)[0].lower() if stripped else ""
+        if stripped.startswith("#") or directive not in managed_directives:
+            unmanaged_lines.append(line)
+
+    directives = [
+        "HostKey /etc/ssh/ssh_host_rsa_key",
+        "HostKey /etc/ssh/ssh_host_ecdsa_key",
+        "HostKey /etc/ssh/ssh_host_ed25519_key",
+        "PasswordAuthentication no",
+    ]
     if username == "root":
         directives.append("PermitRootLogin yes")
-    return "# Managed by uvm\n" + "\n".join(directives) + "\n" + contents
+    return (
+        "# Managed by uvm\n"
+        + "\n".join(directives)
+        + f"\nMatch User {username}\n    PasswordAuthentication yes\nMatch all\n"
+        + "\n".join(unmanaged_lines).rstrip()
+        + "\n"
+    )
 
 
 def _without_firecracker_demo_key(contents: str) -> str | None: