Clean up exception handling in SAML2ResponseResource (#7614)

* Expose `return_html_error`, and allow it to take a Jinja2 template instead of a raw string

* Clean up exception handling in SAML2ResponseResource

  * use the existing code in `return_html_error` instead of re-implementing it
    (giving it a jinja2 template rather than inventing a new form of template)

  * do the exception-catching in the REST layer rather than in the handler
    layer, to make sure we catch all exceptions.

synapse/handlers/saml_handler.py

@@ -23,11 +23,9 @@ 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.http.site import SynapseRequest
 from synapse.module_api import ModuleApi
-from synapse.module_api.errors import RedirectException
 from synapse.types import (
     UserID,
     map_username_to_mxid_localpart,
@@ -80,8 +78,6 @@ 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: bytes, ui_auth_session_id: Optional[str] = None
     ) -> bytes:
@@ -129,26 +125,9 @@ class SamlHandler:
         # the dict.
         self.expire_sessions()
 
-        try:
-            user_id, current_session = await self._map_saml_response_to_user(
-                resp_bytes, relay_state
-            )
-        except RedirectException:
-            # Raise the exception as per the wishes of the SAML module response
-            raise
-        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
+        user_id, current_session = await self._map_saml_response_to_user(
+            resp_bytes, relay_state
+        )
 
         # Complete the interactive auth session or the login.
         if current_session and current_session.ui_auth_session_id:
@@ -171,6 +150,11 @@ class SamlHandler:
 
         Returns:
              Tuple of the user ID and SAML session associated with this response.
+
+        Raises:
+            SynapseError if there was a problem with the response.
+            RedirectException: some mapping providers may raise this if they need
+                to redirect to an interstitial page.
         """
         try:
             saml2_auth = self._saml_client.parse_authn_request_response(
@@ -179,11 +163,9 @@ class SamlHandler:
                 outstanding=self._outstanding_requests_dict,
             )
         except Exception as e:
-            logger.warning("Exception parsing SAML2 response: %s", e)
             raise SynapseError(400, "Unable to parse SAML2 response: %s" % (e,))
 
         if saml2_auth.not_signed:
-            logger.warning("SAML2 response was not signed")
             raise SynapseError(400, "SAML2 response was not signed")
 
         logger.debug("SAML2 response: %s", saml2_auth.origxml)
@@ -264,11 +246,10 @@ class SamlHandler:
 
                 localpart = attribute_dict.get("mxid_localpart")
                 if not localpart:
-                    logger.error(
-                        "SAML mapping provider plugin did not return a "
-                        "mxid_localpart object"
+                    raise Exception(
+                        "Error parsing SAML2 response: SAML mapping provider plugin "
+                        "did not return a mxid_localpart value"
                     )
-                    raise SynapseError(500, "Error parsing SAML2 response")
 
                 displayname = attribute_dict.get("displayname")
                 emails = attribute_dict.get("emails", [])