beiboot: support for temporary UserKnownHostsFile

In the bastion host case, create a temporary UserKnownHostsFile for ssh
and populate it with data from the authorization (if provided).  On
successful login, send the updated file back to the user via the
long-disused `x-login-data` mechanism.  That'll get it added to the JSON
blob that is sent as the reply to the `GET /login` request.

src/cockpit/beiboot.py

@@ -237,6 +237,7 @@ def flatpak_spawn(cmd: Sequence[str], env: Sequence[str]) -> tuple[Sequence[str]
 
 
 class SshPeer(Peer):
+    user_known_hosts_file: 'Path | None' = None
     always: bool
 
     def __init__(self, router: Router, destination: str, args: argparse.Namespace):
@@ -266,15 +267,17 @@ async def connect_from_flatpak(self) -> None:
 
     async def connect_from_bastion_host(self) -> None:
         basic_password = None
+        known_hosts = ''
         username_opts = []
 
         # do we have user/password (Basic auth) from the login page?
         auth = await self.router.request_authorization_object("*")
         response = get_str(auth, 'response')
-        user_known_hosts = get_str(auth, 'user-known-hosts', '')
 
         if response.startswith('Basic '):
-            user, basic_password = base64.b64decode(response.split(' ', 1)[1]).decode().split(':', 1)
+            decoded = base64.b64decode(response[6:]).decode()
+            user_password, _, known_hosts = decoded.partition('\0')
+            user, _, basic_password = user_password.partition(':')
             if user:  # this can be empty, i.e. auth is just ":"
                 logger.debug("got username %s and password from Basic auth", user)
                 username_opts = ['-l', user]
@@ -290,20 +293,14 @@ async def connect_from_bastion_host(self) -> None:
             logger.error("Could not find cockpit-askpass helper at %r", askpass)
 
         with tempfile.TemporaryDirectory() as tmpdir:
-            user_known_hosts_file = Path(tmpdir) / 'user-known-hosts'
-            user_known_hosts_file.write_text(user_known_hosts)
+            self.user_known_hosts_file = Path(tmpdir) / 'user-known-hosts'
+            self.user_known_hosts_file.write_text(known_hosts)
 
             cmd, env = via_ssh(cmd, self.destination, ssh_askpass, *username_opts,
-                               '-o', f'UserKnownHostsfile={user_known_hosts_file!s}')
+                               '-o', f'UserKnownHostsfile={self.user_known_hosts_file!s}')
 
             await self.boot(cmd, env, basic_password)
 
-            self.router.write_control(
-                command='authorize', challenge='x-login-data', cookie='-', login_data={
-                    'user-known-hosts': user_known_hosts_file.read_text()
-                }
-            )
-
     async def boot(self, cmd: Sequence[str], env: Sequence[str], basic_password: str | None = None) -> None:
         beiboot_helper = BridgeBeibootHelper(self)
         agent = ferny.InteractionAgent([AuthorizeResponder(self.router, basic_password), beiboot_helper])
@@ -375,6 +372,13 @@ async def run(args) -> None:
     try:
         message = dict(await bridge.ssh_peer.start())
 
+        if bridge.ssh_peer.user_known_hosts_file:
+            bridge.write_control(
+                command='authorize', challenge='x-login-data', cookie='-', login_data={
+                    'known-hosts': bridge.ssh_peer.user_known_hosts_file.read_text()
+                }
+            )
+
         # See comment in do_init() above: we tell cockpit-ws that we support
         # this and then handle it ourselves when we get the init message.
         capabilities = message.setdefault('capabilities', {})