Repository: matrix-org/synapse

Issue:
Need a way to allow non-SSO users to use Key Backup on SSO-using hosts
**Description:**

On a host with both SSO users and non-SSO (e.g. bots) coexisting, it's impossible to use Key Backup as the latter. If we disable password auth (which makes sense when using SSO), even if we use our non-SSO user with its access token, an auth check will still be performed when uploading keys to key backup – and that will fail due to no available auth flow (and a 401) since there is no auth flow that the bot user can use.

Code-wise we end up [here](https://github.com/matrix-org/synapse/blob/67aae05ece9b6e07fedc73f737c0d6db6351d6c7/synapse/rest/client/keys.py#L75-L116) when setting up Key Backup in Element, and then [this](https://github.com/matrix-org/synapse/blob/67aae05ece9b6e07fedc73f737c0d6db6351d6c7/synapse/handlers/auth.py#L389) gives us no available options.


Discussion:
erikjohnston: Do we want to retarget this at `release-v1.60`?

Patch:
diff --git a/changelog.d/12883.feature b/changelog.d/12883.feature
new file mode 100644
index 000000000..83626771c
--- /dev/null
+++ b/changelog.d/12883.feature
@@ -0,0 +1,1 @@
+Add an option allowing users to use their password to reauthenticate for privileged actions even though password login is disabled.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index ee98d193c..e0abcd3b0 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -2216,7 +2216,9 @@ sso:
 
 
 password_config:
-   # Uncomment to disable password login
+   # Uncomment to disable password login.
+   # Set to `only_for_reauth` to permit reauthentication for users that
+   # have passwords and are already logged in.
    #
    #enabled: false
 
diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index 0f5bda32b..295cece7e 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -2930,6 +2930,9 @@ Use this setting to enable password-based logins.
 
 This setting has the following sub-options:
 * `enabled`: Defaults to true.
+   Set to false to disable password authentication.
+   Set to `only_for_reauth` to allow users with existing passwords to use them
+   to log in and reauthenticate, whilst preventing new users from setting passwords.
 * `localdb_enabled`: Set to false to disable authentication against the local password
    database. This is ignored if `enabled` is false, and is only useful
    if you have other `password_providers`. Defaults to true. 
diff --git a/synapse/config/auth.py b/synapse/config/auth.py
index bb417a235..265a554a5 100644
--- a/synapse/config/auth.py
+++ b/synapse/config/auth.py
@@ -29,7 +29,18 @@ class AuthConfig(Config):
         if password_config is None:
             password_config = {}
 
-        self.password_enabled = password_config.get("enabled", True)
+        passwords_enabled = password_config.get("enabled", True)
+        # 'only_for_reauth' allows users who have previously set a password to use it,
+        # even though passwords would otherwise be disabled.
+        passwords_for_reauth_only = passwords_enabled == "only_for_reauth"
+
+        self.password_enabled_for_login = (
+            passwords_enabled and not passwords_for_reauth_only
+        )
+        self.password_enabled_for_reauth = (
+            passwords_for_reauth_only or passwords_enabled
+        )
+
         self.password_localdb_enabled = password_config.get("localdb_enabled", True)
         self.password_pepper = password_config.get("pepper", "")
 
@@ -46,7 +57,9 @@ class AuthConfig(Config):
     def generate_config_section(self, **kwargs: Any) -> str:
         return """\
         password_config:
-           # Uncomment to disable password login
+           # Uncomment to disable password login.
+           # Set to `only_for_reauth` to permit reauthentication for users that
+           # have passwords and are already logged in.
            #
            #enabled: false
 
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 1b9050ea9..fbafbbee6 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -210,7 +210,8 @@ class AuthHandler:
 
         self.hs = hs  # FIXME better possibility to access registrationHandler later?
         self.macaroon_gen = hs.get_macaroon_generator()
-        self._password_enabled = hs.config.auth.password_enabled
+        self._password_enabled_for_login = hs.config.auth.password_enabled_for_login
+        self._password_enabled_for_reauth = hs.config.auth.password_enabled_for_reauth
         self._password_localdb_enabled = hs.config.auth.password_localdb_enabled
         self._third_party_rules = hs.get_third_party_event_rules()
 
@@ -387,13 +388,13 @@ class AuthHandler:
         return params, session_id
 
     async def _get_available_ui_auth_types(self, user: UserID) -> Iterable[str]:
-        """Get a list of the authentication types this user can use"""
+        """Get a list of the user-interactive authentication types this user can use."""
 
         ui_auth_types = set()
 
         # if the HS supports password auth, and the user has a non-null password, we
         # support password auth
-        if self._password_localdb_enabled and self._password_enabled:
+        if self._password_localdb_enabled and self._password_enabled_for_reauth:
             lookupres = await self._find_user_id_and_pwd_hash(user.to_string())
             if lookupres:
                 _, password_hash = lookupres
@@ -402,7 +403,7 @@ class AuthHandler:
 
         # also allow auth from password providers
         for t in self.password_auth_provider.get_supported_login_types().keys():
-            if t == LoginType.PASSWORD and not self._password_enabled:
+            if t == LoginType.PASSWORD and not self._password_enabled_for_reauth:
                 continue
             ui_auth_types.add(t)
 
@@ -710,7 +711,7 @@ class AuthHandler:
             return res
 
         # fall back to the v1 login flow
-        canonical_id, _ = await self.validate_login(authdict)
+        canonical_id, _ = await self.validate_login(authdict, is_reauth=True)
         return canonical_id
 
     def _get_params_recaptcha(self) -> dict:
@@ -1064,7 +1065,7 @@ class AuthHandler:
         Returns:
             Whether users on this server are allowed to change or set a password
         """
-        return self._password_enabled and self._password_localdb_enabled
+        return self._password_enabled_for_login and self._password_localdb_enabled
 
     def get_supported_login_types(self) -> Iterable[str]:
         """Get a the login types supported for the /login API
@@ -1089,9 +1090,9 @@ class AuthHandler:
         # that comes first, where it's present.
         if LoginType.PASSWORD in types:
             types.remove(LoginType.PASSWORD)
-            if self._password_enabled:
+            if self._password_enabled_for_login:
                 types.insert(0, LoginType.PASSWORD)
-        elif self._password_localdb_enabled and self._password_enabled:
+        elif self._password_localdb_enabled and self._password_enabled_for_login:
             types.insert(0, LoginType.PASSWORD)
 
         return types
@@ -1100,6 +1101,7 @@ class AuthHandler:
         self,
         login_submission: Dict[str, Any],
         ratelimit: bool = False,
+        is_reauth: bool = False,
     ) -> Tuple[str, Optional[Callable[["LoginResponse"], Awaitable[None]]]]:
         """Authenticates the user for the /login API
 
@@ -1110,6 +1112,9 @@ class AuthHandler:
             login_submission: the whole of the login submission
                 (including 'type' and other relevant fields)
             ratelimit: whether to apply the failed_login_attempt ratelimiter
+            is_reauth: whether this is part of a User-Interactive Authorisation
+                flow to reauthenticate for a privileged action (rather than a
+                new login)
         Returns:
             A tuple of the canonical user id, and optional callback
                 to be called once the access token and device id are issued
@@ -1132,8 +1137,14 @@ class AuthHandler:
         # special case to check for "password" for the check_password interface
         # for the auth providers
         password = login_submission.get("password")
+
         if login_type == LoginType.PASSWORD:
-            if not self._password_enabled:
+            if is_reauth:
+                passwords_allowed_here = self._password_enabled_for_reauth
+            else:
+                passwords_allowed_here = self._password_enabled_for_login
+
+            if not passwords_allowed_here:
                 raise SynapseError(400, "Password login has been disabled.")
             if not isinstance(password, str):
                 raise SynapseError(400, "Bad parameter: password", Codes.INVALID_PARAM)


Test patch:
diff --git a/tests/rest/client/test_auth.py b/tests/rest/client/test_auth.py
index 9653f4583..05355c7fb 100644
--- a/tests/rest/client/test_auth.py
+++ b/tests/rest/client/test_auth.py
@@ -195,8 +195,17 @@ class UIAuthTests(unittest.HomeserverTestCase):
         self.user_pass = "pass"
         self.user = self.register_user("test", self.user_pass)
         self.device_id = "dev1"
+
+        # Force-enable password login for just long enough to log in.
+        auth_handler = self.hs.get_auth_handler()
+        allow_auth_for_login = auth_handler._password_enabled_for_login
+        auth_handler._password_enabled_for_login = True
+
         self.user_tok = self.login("test", self.user_pass, self.device_id)
 
+        # Restore password login to however it was.
+        auth_handler._password_enabled_for_login = allow_auth_for_login
+
     def delete_device(
         self,
         access_token: str,
@@ -263,6 +272,38 @@ class UIAuthTests(unittest.HomeserverTestCase):
             },
         )
 
+    @override_config({"password_config": {"enabled": "only_for_reauth"}})
+    def test_ui_auth_with_passwords_for_reauth_only(self) -> None:
+        """
+        Test user interactive authentication outside of registration.
+        """
+
+        # Attempt to delete this device.
+        # Returns a 401 as per the spec
+        channel = self.delete_device(
+            self.user_tok, self.device_id, HTTPStatus.UNAUTHORIZED
+        )
+
+        # Grab the session
+        session = channel.json_body["session"]
+        # Ensure that flows are what is expected.
+        self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
+
+        # Make another request providing the UI auth flow.
+        self.delete_device(
+            self.user_tok,
+            self.device_id,
+            HTTPStatus.OK,
+            {
+                "auth": {
+                    "type": "m.login.password",
+                    "identifier": {"type": "m.id.user", "user": self.user},
+                    "password": self.user_pass,
+                    "session": session,
+                },
+            },
+        )
+
     def test_grandfathered_identifier(self) -> None:
         """Check behaviour without "identifier" dict