UI Auth via SSO: redirect the user to an appropriate SSO. (#9081)

If we have integrations with multiple identity providers, when the user does a UI Auth, we need to redirect them to the right one.

There are a few steps to this. First of all we actually need to store the userid of the user we are trying to validate in the UIA session, since the /auth/sso/fallback/web request is unauthenticated.

Then, once we get the /auth/sso/fallback/web request, we can fish the user id out of the session, and use it to look up the external id mappings, and hence pick an SSO provider for them.

3 files changed, 110 insertions, 18 deletions

diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 2f5b2b61aa..4f881a439a 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -50,7 +50,10 @@ from synapse.api.errors import (
 )
 from synapse.api.ratelimiting import Ratelimiter
 from synapse.handlers._base import BaseHandler
-from synapse.handlers.ui_auth import INTERACTIVE_AUTH_CHECKERS
+from synapse.handlers.ui_auth import (
+    INTERACTIVE_AUTH_CHECKERS,
+    UIAuthSessionDataConstants,
+)
 from synapse.handlers.ui_auth.checkers import UserInteractiveAuthChecker
 from synapse.http import get_request_user_agent
 from synapse.http.server import finish_request, respond_with_html
@@ -335,10 +338,10 @@ class AuthHandler(BaseHandler):
                 request_body.pop("auth", None)
                 return request_body, None
 
-        user_id = requester.user.to_string()
+        requester_user_id = requester.user.to_string()
 
         # Check if we should be ratelimited due to too many previous failed attempts
-        self._failed_uia_attempts_ratelimiter.ratelimit(user_id, update=False)
+        self._failed_uia_attempts_ratelimiter.ratelimit(requester_user_id, update=False)
 
         # build a list of supported flows
         supported_ui_auth_types = await self._get_available_ui_auth_types(
@@ -346,13 +349,16 @@ class AuthHandler(BaseHandler):
         )
         flows = [[login_type] for login_type in supported_ui_auth_types]
 
+        def get_new_session_data() -> JsonDict:
+            return {UIAuthSessionDataConstants.REQUEST_USER_ID: requester_user_id}
+
         try:
             result, params, session_id = await self.check_ui_auth(
-                flows, request, request_body, description
+                flows, request, request_body, description, get_new_session_data,
             )
         except LoginError:
             # Update the ratelimiter to say we failed (`can_do_action` doesn't raise).
-            self._failed_uia_attempts_ratelimiter.can_do_action(user_id)
+            self._failed_uia_attempts_ratelimiter.can_do_action(requester_user_id)
             raise
 
         # find the completed login type
@@ -360,14 +366,14 @@ class AuthHandler(BaseHandler):
             if login_type not in result:
                 continue
 
-            user_id = result[login_type]
+            validated_user_id = result[login_type]
             break
         else:
             # this can't happen
             raise Exception("check_auth returned True but no successful login type")
 
         # check that the UI auth matched the access token
-        if user_id != requester.user.to_string():
+        if validated_user_id != requester_user_id:
             raise AuthError(403, "Invalid auth")
 
         # Note that the access token has been validated.
@@ -399,13 +405,9 @@ class AuthHandler(BaseHandler):
 
         # if sso is enabled, allow the user to log in via SSO iff they have a mapping
         # from sso to mxid.
-        if self.hs.config.saml2.saml2_enabled or self.hs.config.oidc.oidc_enabled:
-            if await self.store.get_external_ids_by_user(user.to_string()):
-                ui_auth_types.add(LoginType.SSO)
-
-        # Our CAS impl does not (yet) correctly register users in user_external_ids,
-        # so always offer that if it's available.
-        if self.hs.config.cas.cas_enabled:
+        if await self.hs.get_sso_handler().get_identity_providers_for_user(
+            user.to_string()
+        ):
             ui_auth_types.add(LoginType.SSO)
 
         return ui_auth_types
@@ -424,6 +426,7 @@ class AuthHandler(BaseHandler):
         request: SynapseRequest,
         clientdict: Dict[str, Any],
         description: str,
+        get_new_session_data: Optional[Callable[[], JsonDict]] = None,
     ) -> Tuple[dict, dict, str]:
         """
         Takes a dictionary sent by the client in the login / registration
@@ -447,6 +450,13 @@ class AuthHandler(BaseHandler):
             description: A human readable string to be displayed to the user that
                          describes the operation happening on their account.
 
+            get_new_session_data:
+                an optional callback which will be called when starting a new session.
+                it should return data to be stored as part of the session.
+
+                The keys of the returned data should be entries in
+                UIAuthSessionDataConstants.
+
         Returns:
             A tuple of (creds, params, session_id).
 
@@ -474,10 +484,15 @@ class AuthHandler(BaseHandler):
 
         # If there's no session ID, create a new session.
         if not sid:
+            new_session_data = get_new_session_data() if get_new_session_data else {}
+
             session = await self.store.create_ui_auth_session(
                 clientdict, uri, method, description
             )
 
+            for k, v in new_session_data.items():
+                await self.set_session_data(session.session_id, k, v)
+
         else:
             try:
                 session = await self.store.get_ui_auth_session(sid)
@@ -639,7 +654,8 @@ class AuthHandler(BaseHandler):
 
         Args:
             session_id: The ID of this session as returned from check_auth
-            key: The key to store the data under
+            key: The key to store the data under. An entry from
+                UIAuthSessionDataConstants.
             value: The data to store
         """
         try:
@@ -655,7 +671,8 @@ class AuthHandler(BaseHandler):
 
         Args:
             session_id: The ID of this session as returned from check_auth
-            key: The key to store the data under
+            key: The key the data was stored under. An entry from
+                UIAuthSessionDataConstants.
             default: Value to return if the key has not been set
         """
         try:
@@ -1329,12 +1346,12 @@ class AuthHandler(BaseHandler):
         else:
             return False
 
-    async def start_sso_ui_auth(self, redirect_url: str, session_id: str) -> str:
+    async def start_sso_ui_auth(self, request: SynapseRequest, session_id: str) -> str:
         """
         Get the HTML for the SSO redirect confirmation page.
 
         Args:
-            redirect_url: The URL to redirect to the SSO provider.
+            request: The incoming HTTP request
             session_id: The user interactive authentication session ID.
 
         Returns:
@@ -1344,6 +1361,35 @@ class AuthHandler(BaseHandler):
             session = await self.store.get_ui_auth_session(session_id)
         except StoreError:
             raise SynapseError(400, "Unknown session ID: %s" % (session_id,))
+
+        user_id_to_verify = await self.get_session_data(
+            session_id, UIAuthSessionDataConstants.REQUEST_USER_ID
+        )  # type: str
+
+        idps = await self.hs.get_sso_handler().get_identity_providers_for_user(
+            user_id_to_verify
+        )
+
+        if not idps:
+            # we checked that the user had some remote identities before offering an SSO
+            # flow, so either it's been deleted or the client has requested SSO despite
+            # it not being offered.
+            raise SynapseError(400, "User has no SSO identities")
+
+        # for now, just pick one
+        idp_id, sso_auth_provider = next(iter(idps.items()))
+        if len(idps) > 0:
+            logger.warning(
+                "User %r has previously logged in with multiple SSO IdPs; arbitrarily "
+                "picking %r",
+                user_id_to_verify,
+                idp_id,
+            )
+
+        redirect_url = await sso_auth_provider.handle_redirect_request(
+            request, None, session_id
+        )
+
         return self._sso_auth_confirm_template.render(
             description=session.description, redirect_url=redirect_url,
         )
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index 740df7e4a0..d096e0b091 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -167,6 +167,37 @@ class SsoHandler:
         """Get the configured identity providers"""
         return self._identity_providers
 
+    async def get_identity_providers_for_user(
+        self, user_id: str
+    ) -> Mapping[str, SsoIdentityProvider]:
+        """Get the SsoIdentityProviders which a user has used
+
+        Given a user id, get the identity providers that that user has used to log in
+        with in the past (and thus could use to re-identify themselves for UI Auth).
+
+        Args:
+            user_id: MXID of user to look up
+
+        Raises:
+            a map of idp_id to SsoIdentityProvider
+        """
+        external_ids = await self._store.get_external_ids_by_user(user_id)
+
+        valid_idps = {}
+        for idp_id, _ in external_ids:
+            idp = self._identity_providers.get(idp_id)
+            if not idp:
+                logger.warning(
+                    "User %r has an SSO mapping for IdP %r, but this is no longer "
+                    "configured.",
+                    user_id,
+                    idp_id,
+                )
+            else:
+                valid_idps[idp_id] = idp
+
+        return valid_idps
+
     def render_error(
         self,
         request: Request,
diff --git a/synapse/handlers/ui_auth/__init__.py b/synapse/handlers/ui_auth/__init__.py
index 824f37f8f8..a68d5e790e 100644
--- a/synapse/handlers/ui_auth/__init__.py
+++ b/synapse/handlers/ui_auth/__init__.py
@@ -20,3 +20,18 @@ TODO: move more stuff out of AuthHandler in here.
 """
 
 from synapse.handlers.ui_auth.checkers import INTERACTIVE_AUTH_CHECKERS  # noqa: F401
+
+
+class UIAuthSessionDataConstants:
+    """Constants for use with AuthHandler.set_session_data"""
+
+    # used during registration and password reset to store a hashed copy of the
+    # password, so that the client does not need to submit it each time.
+    PASSWORD_HASH = "password_hash"
+
+    # used during registration to store the mxid of the registered user
+    REGISTERED_USER_ID = "registered_user_id"
+
+    # used by validate_user_via_ui_auth to store the mxid of the user we are validating
+    # for.
+    REQUEST_USER_ID = "request_user_id"