Allow appservice users to /login (#8320)

Add ability for ASes to /login using the `uk.half-shot.msc2778.login.application_service` login `type`.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

synapse/rest/client/v1/login.py

@@ -18,6 +18,7 @@ from typing import Awaitable, Callable, Dict, Optional
 
 from synapse.api.errors import Codes, LoginError, SynapseError
 from synapse.api.ratelimiting import Ratelimiter
+from synapse.appservice import ApplicationService
 from synapse.handlers.auth import (
     convert_client_dict_legacy_fields_to_identifier,
     login_id_phone_to_thirdparty,
@@ -44,6 +45,7 @@ class LoginRestServlet(RestServlet):
     TOKEN_TYPE = "m.login.token"
     JWT_TYPE = "org.matrix.login.jwt"
     JWT_TYPE_DEPRECATED = "m.login.jwt"
+    APPSERVICE_TYPE = "uk.half-shot.msc2778.login.application_service"
 
     def __init__(self, hs):
         super(LoginRestServlet, self).__init__()
@@ -61,6 +63,8 @@ class LoginRestServlet(RestServlet):
         self.cas_enabled = hs.config.cas_enabled
         self.oidc_enabled = hs.config.oidc_enabled
 
+        self.auth = hs.get_auth()
+
         self.auth_handler = self.hs.get_auth_handler()
         self.registration_handler = hs.get_registration_handler()
         self.handlers = hs.get_handlers()
@@ -107,6 +111,8 @@ class LoginRestServlet(RestServlet):
             ({"type": t} for t in self.auth_handler.get_supported_login_types())
         )
 
+        flows.append({"type": LoginRestServlet.APPSERVICE_TYPE})
+
         return 200, {"flows": flows}
 
     def on_OPTIONS(self, request: SynapseRequest):
@@ -116,8 +122,12 @@ class LoginRestServlet(RestServlet):
         self._address_ratelimiter.ratelimit(request.getClientIP())
 
         login_submission = parse_json_object_from_request(request)
+
         try:
-            if self.jwt_enabled and (
+            if login_submission["type"] == LoginRestServlet.APPSERVICE_TYPE:
+                appservice = self.auth.get_appservice_by_req(request)
+                result = await self._do_appservice_login(login_submission, appservice)
+            elif self.jwt_enabled and (
                 login_submission["type"] == LoginRestServlet.JWT_TYPE
                 or login_submission["type"] == LoginRestServlet.JWT_TYPE_DEPRECATED
             ):
@@ -134,6 +144,33 @@ class LoginRestServlet(RestServlet):
             result["well_known"] = well_known_data
         return 200, result
 
+    def _get_qualified_user_id(self, identifier):
+        if identifier["type"] != "m.id.user":
+            raise SynapseError(400, "Unknown login identifier type")
+        if "user" not in identifier:
+            raise SynapseError(400, "User identifier is missing 'user' key")
+
+        if identifier["user"].startswith("@"):
+            return identifier["user"]
+        else:
+            return UserID(identifier["user"], self.hs.hostname).to_string()
+
+    async def _do_appservice_login(
+        self, login_submission: JsonDict, appservice: ApplicationService
+    ):
+        logger.info(
+            "Got appservice login request with identifier: %r",
+            login_submission.get("identifier"),
+        )
+
+        identifier = convert_client_dict_legacy_fields_to_identifier(login_submission)
+        qualified_user_id = self._get_qualified_user_id(identifier)
+
+        if not appservice.is_interested_in_user(qualified_user_id):
+            raise LoginError(403, "Invalid access_token", errcode=Codes.FORBIDDEN)
+
+        return await self._complete_login(qualified_user_id, login_submission)
+
     async def _do_other_login(self, login_submission: JsonDict) -> Dict[str, str]:
         """Handle non-token/saml/jwt logins
 
@@ -219,15 +256,7 @@ class LoginRestServlet(RestServlet):
 
         # by this point, the identifier should be an m.id.user: if it's anything
         # else, we haven't understood it.
-        if identifier["type"] != "m.id.user":
-            raise SynapseError(400, "Unknown login identifier type")
-        if "user" not in identifier:
-            raise SynapseError(400, "User identifier is missing 'user' key")
-
-        if identifier["user"].startswith("@"):
-            qualified_user_id = identifier["user"]
-        else:
-            qualified_user_id = UserID(identifier["user"], self.hs.hostname).to_string()
+        qualified_user_id = self._get_qualified_user_id(identifier)
 
         # Check if we've hit the failed ratelimit (but don't update it)
         self._failed_attempts_ratelimiter.ratelimit(