Add initial support for a "pick your IdP" page (#9017)

During login, if there are multiple IdPs enabled, offer the user a choice of
IdPs.

9 files changed, 168 insertions, 3 deletions

diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index 8d9b53be53..b1d9817a6a 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -63,6 +63,7 @@ from synapse.rest import ClientRestResource
 from synapse.rest.admin import AdminRestResource
 from synapse.rest.health import HealthResource
 from synapse.rest.key.v2 import KeyApiV2Resource
+from synapse.rest.synapse.client.pick_idp import PickIdpResource
 from synapse.rest.synapse.client.pick_username import pick_username_resource
 from synapse.rest.well_known import WellKnownResource
 from synapse.server import HomeServer
@@ -194,6 +195,7 @@ class SynapseHomeServer(HomeServer):
                     "/.well-known/matrix/client": WellKnownResource(self),
                     "/_synapse/admin": AdminRestResource(self),
                     "/_synapse/client/pick_username": pick_username_resource(self),
+                    "/_synapse/client/pick_idp": PickIdpResource(self),
                 }
             )
 
diff --git a/synapse/config/sso.py b/synapse/config/sso.py
index 93bbd40937..1aeb1c5c92 100644
--- a/synapse/config/sso.py
+++ b/synapse/config/sso.py
@@ -31,6 +31,7 @@ class SSOConfig(Config):
 
         # Read templates from disk
         (
+            self.sso_login_idp_picker_template,
             self.sso_redirect_confirm_template,
             self.sso_auth_confirm_template,
             self.sso_error_template,
@@ -38,6 +39,7 @@ class SSOConfig(Config):
             sso_auth_success_template,
         ) = self.read_templates(
             [
+                "sso_login_idp_picker.html",
                 "sso_redirect_confirm.html",
                 "sso_auth_confirm.html",
                 "sso_error.html",
@@ -98,6 +100,31 @@ class SSOConfig(Config):
             #
             # Synapse will look for the following templates in this directory:
             #
+            # * HTML page to prompt the user to choose an Identity Provider during
+            #   login: 'sso_login_idp_picker.html'.
+            #
+            #   This is only used if multiple SSO Identity Providers are configured.
+            #
+            #   When rendering, this template is given the following variables:
+            #     * redirect_url: the URL that the user will be redirected to after
+            #       login. Needs manual escaping (see
+            #       https://jinja.palletsprojects.com/en/2.11.x/templates/#html-escaping).
+            #
+            #     * server_name: the homeserver's name.
+            #
+            #     * providers: a list of available Identity Providers. Each element is
+            #       an object with the following attributes:
+            #         * idp_id: unique identifier for the IdP
+            #         * idp_name: user-facing name for the IdP
+            #
+            #   The rendered HTML page should contain a form which submits its results
+            #   back as a GET request, with the following query parameters:
+            #
+            #     * redirectUrl: the client redirect URI (ie, the `redirect_url` passed
+            #       to the template)
+            #
+            #     * idp: the 'idp_id' of the chosen IDP.
+            #
             # * HTML page for a confirmation step before redirecting back to the client
             #   with the login token: 'sso_redirect_confirm.html'.
             #
diff --git a/synapse/handlers/cas_handler.py b/synapse/handlers/cas_handler.py
index 295974c521..f3430c6713 100644
--- a/synapse/handlers/cas_handler.py
+++ b/synapse/handlers/cas_handler.py
@@ -77,6 +77,9 @@ class CasHandler:
         # identifier for the external_ids table
         self.idp_id = "cas"
 
+        # user-facing name of this auth provider
+        self.idp_name = "CAS"
+
         self._sso_handler = hs.get_sso_handler()
 
         self._sso_handler.register_identity_provider(self)
diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
index 3e2b60eb7b..6835c6c462 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -121,6 +121,9 @@ class OidcHandler(BaseHandler):
         # identifier for the external_ids table
         self.idp_id = "oidc"
 
+        # user-facing name of this auth provider
+        self.idp_name = "OIDC"
+
         self._sso_handler = hs.get_sso_handler()
 
         self._sso_handler.register_identity_provider(self)
diff --git a/synapse/handlers/saml_handler.py b/synapse/handlers/saml_handler.py
index 6106237f1f..a8376543c9 100644
--- a/synapse/handlers/saml_handler.py
+++ b/synapse/handlers/saml_handler.py
@@ -75,6 +75,9 @@ class SamlHandler(BaseHandler):
         # identifier for the external_ids table
         self.idp_id = "saml"
 
+        # user-facing name of this auth provider
+        self.idp_name = "SAML"
+
         # a map from saml session id to Saml2SessionData object
         self._outstanding_requests_dict = {}  # type: Dict[str, Saml2SessionData]
 
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index d8fb8cdd05..2da1ea2223 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -14,7 +14,8 @@
 # limitations under the License.
 import abc
 import logging
-from typing import TYPE_CHECKING, Awaitable, Callable, Dict, List, Optional
+from typing import TYPE_CHECKING, Awaitable, Callable, Dict, List, Mapping, Optional
+from urllib.parse import urlencode
 
 import attr
 from typing_extensions import NoReturn, Protocol
@@ -66,6 +67,11 @@ class SsoIdentityProvider(Protocol):
         Eg, "saml", "cas", "github"
         """
 
+    @property
+    @abc.abstractmethod
+    def idp_name(self) -> str:
+        """User-facing name for this provider"""
+
     @abc.abstractmethod
     async def handle_redirect_request(
         self,
@@ -156,6 +162,10 @@ class SsoHandler:
         assert p_id not in self._identity_providers
         self._identity_providers[p_id] = p
 
+    def get_identity_providers(self) -> Mapping[str, SsoIdentityProvider]:
+        """Get the configured identity providers"""
+        return self._identity_providers
+
     def render_error(
         self,
         request: Request,
@@ -203,8 +213,10 @@ class SsoHandler:
             ap = next(iter(self._identity_providers.values()))
             return await ap.handle_redirect_request(request, client_redirect_url)
 
-        # otherwise, we have a configuration error
-        raise Exception("Multiple SSO identity providers have been configured!")
+        # otherwise, redirect to the IDP picker
+        return "/_synapse/client/pick_idp?" + urlencode(
+            (("redirectUrl", client_redirect_url),)
+        )
 
     async def get_sso_user_by_remote_user_id(
         self, auth_provider_id: str, remote_user_id: str
diff --git a/synapse/res/templates/sso_login_idp_picker.html b/synapse/res/templates/sso_login_idp_picker.html
new file mode 100644
index 0000000000..f53c9cd679
--- /dev/null
+++ b/synapse/res/templates/sso_login_idp_picker.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<html lang="en">
+    <head>
+        <meta charset="UTF-8">
+        <link rel="stylesheet" href="/_matrix/static/client/login/style.css">
+        <title>{{server_name | e}} Login</title>
+    </head>
+    <body>
+        <div id="container">
+            <h1 id="title">{{server_name | e}} Login</h1>
+            <div class="login_flow">
+                <p>Choose one of the following identity providers:</p>
+            <form>
+                <input type="hidden" name="redirectUrl" value="{{redirect_url | e}}">
+                <ul class="radiobuttons">
+{% for p in providers %}
+                    <li>
+                        <input type="radio" name="idp" id="prov{{loop.index}}" value="{{p.idp_id}}">
+                        <label for="prov{{loop.index}}">{{p.idp_name | e}}</label>
+                    </li>
+{% endfor %}
+                </ul>
+                <input type="submit" class="button button--full-width" id="button-submit" value="Submit">
+            </form>
+            </div>
+        </div>
+    </body>
+</html>
diff --git a/synapse/rest/synapse/client/pick_idp.py b/synapse/rest/synapse/client/pick_idp.py
new file mode 100644
index 0000000000..e5b720bbca
--- /dev/null
+++ b/synapse/rest/synapse/client/pick_idp.py
@@ -0,0 +1,82 @@
+# -*- coding: utf-8 -*-
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+from typing import TYPE_CHECKING
+
+from synapse.http.server import (
+    DirectServeHtmlResource,
+    finish_request,
+    respond_with_html,
+)
+from synapse.http.servlet import parse_string
+from synapse.http.site import SynapseRequest
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class PickIdpResource(DirectServeHtmlResource):
+    """IdP picker resource.
+
+    This resource gets mounted under /_synapse/client/pick_idp. It serves an HTML page
+    which prompts the user to choose an Identity Provider from the list.
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self._sso_handler = hs.get_sso_handler()
+        self._sso_login_idp_picker_template = (
+            hs.config.sso.sso_login_idp_picker_template
+        )
+        self._server_name = hs.hostname
+
+    async def _async_render_GET(self, request: SynapseRequest) -> None:
+        client_redirect_url = parse_string(request, "redirectUrl", required=True)
+        idp = parse_string(request, "idp", required=False)
+
+        # if we need to pick an IdP, do so
+        if not idp:
+            return await self._serve_id_picker(request, client_redirect_url)
+
+        # otherwise, redirect to the IdP's redirect URI
+        providers = self._sso_handler.get_identity_providers()
+        auth_provider = providers.get(idp)
+        if not auth_provider:
+            logger.info("Unknown idp %r", idp)
+            self._sso_handler.render_error(
+                request, "unknown_idp", "Unknown identity provider ID"
+            )
+            return
+
+        sso_url = await auth_provider.handle_redirect_request(
+            request, client_redirect_url.encode("utf8")
+        )
+        logger.info("Redirecting to %s", sso_url)
+        request.redirect(sso_url)
+        finish_request(request)
+
+    async def _serve_id_picker(
+        self, request: SynapseRequest, client_redirect_url: str
+    ) -> None:
+        # otherwise, serve up the IdP picker
+        providers = self._sso_handler.get_identity_providers()
+        html = self._sso_login_idp_picker_template.render(
+            redirect_url=client_redirect_url,
+            server_name=self._server_name,
+            providers=providers.values(),
+        )
+        respond_with_html(request, 200, html)
diff --git a/synapse/static/client/login/style.css b/synapse/static/client/login/style.css
index 83e4f6abc8..dd76714a92 100644
--- a/synapse/static/client/login/style.css
+++ b/synapse/static/client/login/style.css
@@ -31,6 +31,11 @@ form {
     margin: 10px 0 0 0;
 }
 
+ul.radiobuttons {
+    text-align: left;
+    list-style: none;
+}
+
 /*
  * Add some padding to the viewport.
  */