Factor out a validate_user_via_ui_auth method

Collect together all the places that validate a logged-in user via UI auth.

synapse/handlers/auth.py

@@ -88,6 +88,49 @@ class AuthHandler(BaseHandler):
                 )
         self._supported_login_types = frozenset(login_types)
 
+    @defer.inlineCallbacks
+    def validate_user_via_ui_auth(self, requester, request_body, clientip):
+        """
+        Checks that the user is who they claim to be, via a UI auth.
+
+        This is used for things like device deletion and password reset where
+        the user already has a valid access token, but we want to double-check
+        that it isn't stolen by re-authenticating them.
+
+        Args:
+            requester (Requester): The user, as given by the access token
+
+            request_body (dict): The body of the request sent by the client
+
+            clientip (str): The IP address of the client.
+
+        Returns:
+            defer.Deferred[dict]: the parameters for this request (which may
+                have been given only in a previous call).
+
+        Raises:
+            InteractiveAuthIncompleteError if the client has not yet completed
+                any of the permitted login flows
+
+            AuthError if the client has completed a login flow, and it gives
+                a different user to `requester`
+        """
+
+        # we only support password login here
+        flows = [[LoginType.PASSWORD]]
+
+        result, params, _ = yield self.check_auth(
+            flows, request_body, clientip,
+        )
+
+        user_id = result[LoginType.PASSWORD]
+
+        # check that the UI auth matched the access token
+        if user_id != requester.user.to_string():
+            raise AuthError(403, "Invalid auth")
+
+        defer.returnValue(params)
+
     @defer.inlineCallbacks
     def check_auth(self, flows, clientdict, clientip):
         """