SAML2: render a comprehensible error page if something goes wrong

If an error happened while processing a SAML AuthN response, or a client
ends up doing a `GET` request to `/authn_response`, then render a
customisable error page rather than a confusing error.

synapse/handlers/saml_handler.py

@@ -23,6 +23,7 @@ from saml2.client import Saml2Client
 
 from synapse.api.errors import SynapseError
 from synapse.config import ConfigError
+from synapse.http.server import finish_request
 from synapse.http.servlet import parse_string
 from synapse.module_api import ModuleApi
 from synapse.types import (
@@ -73,6 +74,8 @@ class SamlHandler:
         # a lock on the mappings
         self._mapping_lock = Linearizer(name="saml_mapping", clock=self._clock)
 
+        self._error_html_content = hs.config.saml2_error_html_content
+
     def handle_redirect_request(self, client_redirect_url):
         """Handle an incoming request to /login/sso/redirect
 
@@ -114,7 +117,22 @@ class SamlHandler:
         # the dict.
         self.expire_sessions()
 
-        user_id = await self._map_saml_response_to_user(resp_bytes, relay_state)
+        try:
+            user_id = await self._map_saml_response_to_user(resp_bytes, relay_state)
+        except Exception as e:
+            # If decoding the response or mapping it to a user failed, then log the
+            # error and tell the user that something went wrong.
+            logger.error(e)
+
+            request.setResponseCode(400)
+            request.setHeader(b"Content-Type", b"text/html; charset=utf-8")
+            request.setHeader(
+                b"Content-Length", b"%d" % (len(self._error_html_content),)
+            )
+            request.write(self._error_html_content.encode("utf8"))
+            finish_request(request)
+            return
+
         self._auth_handler.complete_sso_login(user_id, request, relay_state)
 
     async def _map_saml_response_to_user(self, resp_bytes, client_redirect_url):