Interactive Auth: Return 401 from for incorrect password

This requires a bit of fettling, because I want to return a helpful error
message too but we don't want to distinguish between unknown user and invalid
password. To avoid hardcoding the error message into 15 places in the code,
I've had to refactor a few methods to return None instead of throwing.

Fixes https://matrix.org/jira/browse/SYN-744

synapse/handlers/auth.py

@@ -58,7 +58,6 @@ def __init__(self, hs):
         }
         self.bcrypt_rounds = hs.config.bcrypt_rounds
         self.sessions = {}
-        self.INVALID_TOKEN_HTTP_STATUS = 401
 
         self.ldap_enabled = hs.config.ldap_enabled
         if self.ldap_enabled:
@@ -148,13 +147,19 @@ def check_auth(self, flows, clientdict, clientip):
         creds = session['creds']
 
         # check auth type currently being presented
+        errordict = {}
         if 'type' in authdict:
             if authdict['type'] not in self.checkers:
                 raise LoginError(400, "", Codes.UNRECOGNIZED)
-            result = yield self.checkers[authdict['type']](authdict, clientip)
-            if result:
-                creds[authdict['type']] = result
-                self._save_session(session)
+            try:
+                result = yield self.checkers[authdict['type']](authdict, clientip)
+                if result:
+                    creds[authdict['type']] = result
+                    self._save_session(session)
+            except LoginError, e:
+                # this step failed. Merge the error dict into the response
+                # so that the client can have another go.
+                errordict = e.error_dict()
 
         for f in flows:
             if len(set(f) - set(creds.keys())) == 0:
@@ -163,6 +168,7 @@ def check_auth(self, flows, clientdict, clientip):
 
         ret = self._auth_dict_for_flows(flows, session)
         ret['completed'] = creds.keys()
+        ret.update(errordict)
         defer.returnValue((False, ret, clientdict, session['id']))
 
     @defer.inlineCallbacks
@@ -430,37 +436,40 @@ def check_user_exists(self, user_id):
             defer.Deferred: (str) canonical_user_id, or None if zero or
             multiple matches
         """
-        try:
-            res = yield self._find_user_id_and_pwd_hash(user_id)
+        res = yield self._find_user_id_and_pwd_hash(user_id)
+        if res is not None:
             defer.returnValue(res[0])
-        except LoginError:
-            defer.returnValue(None)
+        defer.returnValue(None)
 
     @defer.inlineCallbacks
     def _find_user_id_and_pwd_hash(self, user_id):
         """Checks to see if a user with the given id exists. Will check case
-        insensitively, but will throw if there are multiple inexact matches.
+        insensitively, but will return None if there are multiple inexact
+        matches.
 
         Returns:
             tuple: A 2-tuple of `(canonical_user_id, password_hash)`
+            None: if there is not exactly one match
         """
         user_infos = yield self.store.get_users_by_id_case_insensitive(user_id)
+
+        result = None
         if not user_infos:
             logger.warn("Attempted to login as %s but they do not exist", user_id)
-            raise LoginError(403, "", errcode=Codes.FORBIDDEN)
-
-        if len(user_infos) > 1:
-            if user_id not in user_infos:
-                logger.warn(
-                    "Attempted to login as %s but it matches more than one user "
-                    "inexactly: %r",
-                    user_id, user_infos.keys()
-                )
-                raise LoginError(403, "", errcode=Codes.FORBIDDEN)
-
-            defer.returnValue((user_id, user_infos[user_id]))
+        elif len(user_infos) == 1:
+            # a single match (possibly not exact)
+            result = user_infos.popitem()
+        elif user_id in user_infos:
+            # multiple matches, but one is exact
+            result = (user_id, user_infos[user_id])
         else:
-            defer.returnValue(user_infos.popitem())
+            # multiple matches, none of them exact
+            logger.warn(
+                "Attempted to login as %s but it matches more than one user "
+                "inexactly: %r",
+                user_id, user_infos.keys()
+            )
+        defer.returnValue(result)
 
     @defer.inlineCallbacks
     def _check_password(self, user_id, password):
@@ -474,34 +483,45 @@ def _check_password(self, user_id, password):
         Returns:
             (str) the canonical_user_id
         Raises:
-            LoginError if the password was incorrect
+            LoginError if login fails
         """
         valid_ldap = yield self._check_ldap_password(user_id, password)
         if valid_ldap:
             defer.returnValue(user_id)
 
-        result = yield self._check_local_password(user_id, password)
-        defer.returnValue(result)
+        canonical_user_id = yield self._check_local_password(user_id, password)
+
+        if canonical_user_id:
+            defer.returnValue(canonical_user_id)
+
+        # unknown username or invalid password. We raise a 403 here, but note
+        # that if we're doing user-interactive login, it turns all LoginErrors
+        # into a 401 anyway.
+        raise LoginError(
+            403, "Invalid password",
+            errcode=Codes.FORBIDDEN
+        )
 
     @defer.inlineCallbacks
     def _check_local_password(self, user_id, password):
         """Authenticate a user against the local password database.
 
-        user_id is checked case insensitively, but will throw if there are
+        user_id is checked case insensitively, but will return None if there are
         multiple inexact matches.
 
         Args:
             user_id (str): complete @user:id
         Returns:
-            (str) the canonical_user_id
-        Raises:
-            LoginError if the password was incorrect
+            (str) the canonical_user_id, or None if unknown user / bad password
         """
-        user_id, password_hash = yield self._find_user_id_and_pwd_hash(user_id)
+        lookupres = yield self._find_user_id_and_pwd_hash(user_id)
+        if not lookupres:
+            defer.returnValue(None)
+        (user_id, password_hash) = lookupres
         result = self.validate_hash(password, password_hash)
         if not result:
             logger.warn("Failed password login for user %s", user_id)
-            raise LoginError(403, "", errcode=Codes.FORBIDDEN)
+            defer.returnValue(None)
         defer.returnValue(user_id)
 
     @defer.inlineCallbacks