fix: never let root read the SSH configuration from stdin

A driver gains root with sudo -S and puts the password in front of stdin.
A sudo that does not prompt -- NOPASSWD, a cached timestamp -- leaves it
there, and the cat under sudo wrote it into the drop-in as its first line.
Seen on the first real run (NetOrk/netork#1027): sshd -t rejected the file
and the rollback removed it, but the password was in a 0644 file for a
moment and in the error the caller showed.

The content now goes without privilege to a mktemp file only the login user
can read; root installs that file (mode 644, root-owned) and removes it.
The fake host in the tests prepends the password to every privileged stdin,
as a non-prompting sudo does.

Bumps to 4.5.1. NetOrk/netork#1027, NetOrk/netork#1029

tests/test_ssh_server.py

@@ -188,10 +188,29 @@ class FakeHost(OpenSshServerMixin):
                     eff[kind] = parts[1].split(",")
         return eff
 
+    #: What a real ``sudo -S`` leaves on stdin when it does not prompt (NOPASSWD,
+    #: or a cached timestamp): the password the driver put in front of the data.
+    SUDO_PASSWORD = "hunter2"
+
     def run_command(self, command: str, *, privileged: bool = False, timeout: float = 60,
                     stdin: Optional[bytes] = None) -> CommandResult:
         self.ran.append(command)
+        if privileged and stdin is not None:
+            stdin = f"{self.SUDO_PASSWORD}\n".encode() + stdin
+        m = re.search(r"mktemp", command)
+        if m:
+            assert not privileged, "the content must reach a file without sudo in the way"
+            path = f"/tmp/netork-ssh.{len(self.files)}"
+            self.files[path] = (stdin or b"").decode()
+            return CommandResult(path, "", 0)
         assert privileged, command
+        m = re.search(r"install -m 644 -- '?([^' ]+)'? '?([^' ]+)'?", command)
+        if m:
+            src, tmp = m.group(1), m.group(2)
+            target = re.search(r"mv -f -- '?[^' ]+'? '?([^' ;]+)'?", command).group(1)
+            self.files[target] = self.files.pop(src)
+            assert tmp.endswith(".netork-tmp")
+            return CommandResult("", "", 0)
         if command.startswith("sshd -T") or "sshd -T" in command:
             eff = self._effective()
             out = "".join(f"{d} {','.join(eff[k])}\n" for d, k in _DIRECTIVE.items())
@@ -204,10 +223,6 @@ class FakeHost(OpenSshServerMixin):
             if path in self.files:
                 return CommandResult(self.files[path], "", 0)
             return CommandResult("", "No such file", 1)
-        m = re.search(r"mv -f -- \"\$tmp\" '?([^' ]+)'?", command)
-        if m:
-            self.files[m.group(1)] = (stdin or b"").decode()
-            return CommandResult("", "", 0)
         m = re.match(r"rm -f -- '?([^' ]+)'?$", command)
         if m:
             self.files.pop(m.group(1), None)
@@ -301,3 +316,22 @@ def test_main_config_second_fix_keeps_what_the_first_set():
     eff = host._effective()
     assert "ecdsa-sha2-nistp256" not in eff["hostkey"]
     assert "hmac-sha1" not in eff["mac"]
+
+
+def test_the_sudo_password_never_reaches_a_file():
+    """napalm-linux puts the sudo password in front of stdin for ``sudo -S``. A
+    sudo that does not prompt leaves it there, and a ``cat`` under sudo wrote it
+    into the drop-in (NetOrk/netork#1027, first run on a real host). The content
+    now reaches a file of the login user first; root only installs that file."""
+    host = FakeHost()
+    host.harden_ssh_server({"mac": ["hmac-sha1"]})
+    assert host.SUDO_PASSWORD not in host.files[DROPIN_PATH]
+    assert host.files[DROPIN_PATH].startswith("# Written by netOrk")
+    assert not [p for p in host.files if p.startswith("/tmp/")]
+
+
+def test_restoring_an_earlier_dropin_leaks_nothing_either():
+    host = FakeHost()
+    host.files[DROPIN_PATH] = "# netOrk\nHostKeyAlgorithms ssh-ed25519\n"
+    host.harden_ssh_server({"mac": ["hmac-sha1"]}, verify=lambda: False)
+    assert host.files[DROPIN_PATH] == "# netOrk\nHostKeyAlgorithms ssh-ed25519\n"