Allow SAML username provider plugins (#6411)

2 files changed, 296 insertions, 88 deletions

diff --git a/synapse/config/saml2_config.py b/synapse/config/saml2_config.py
index c5ea2d43a1..b91414aa35 100644
--- a/synapse/config/saml2_config.py
+++ b/synapse/config/saml2_config.py
@@ -14,17 +14,19 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import re
+import logging
 
 from synapse.python_dependencies import DependencyException, check_requirements
-from synapse.types import (
-    map_username_to_mxid_localpart,
-    mxid_localpart_allowed_characters,
-)
-from synapse.util.module_loader import load_python_module
+from synapse.util.module_loader import load_module, load_python_module
 
 from ._base import Config, ConfigError
 
+logger = logging.getLogger(__name__)
+
+DEFAULT_USER_MAPPING_PROVIDER = (
+    "synapse.handlers.saml_handler.DefaultSamlMappingProvider"
+)
+
 
 def _dict_merge(merge_dict, into_dict):
     """Do a deep merge of two dicts
@@ -75,15 +77,69 @@ class SAML2Config(Config):
 
         self.saml2_enabled = True
 
-        self.saml2_mxid_source_attribute = saml2_config.get(
-            "mxid_source_attribute", "uid"
-        )
-
         self.saml2_grandfathered_mxid_source_attribute = saml2_config.get(
             "grandfathered_mxid_source_attribute", "uid"
         )
 
-        saml2_config_dict = self._default_saml_config_dict()
+        # user_mapping_provider may be None if the key is present but has no value
+        ump_dict = saml2_config.get("user_mapping_provider") or {}
+
+        # Use the default user mapping provider if not set
+        ump_dict.setdefault("module", DEFAULT_USER_MAPPING_PROVIDER)
+
+        # Ensure a config is present
+        ump_dict["config"] = ump_dict.get("config") or {}
+
+        if ump_dict["module"] == DEFAULT_USER_MAPPING_PROVIDER:
+            # Load deprecated options for use by the default module
+            old_mxid_source_attribute = saml2_config.get("mxid_source_attribute")
+            if old_mxid_source_attribute:
+                logger.warning(
+                    "The config option saml2_config.mxid_source_attribute is deprecated. "
+                    "Please use saml2_config.user_mapping_provider.config"
+                    ".mxid_source_attribute instead."
+                )
+                ump_dict["config"]["mxid_source_attribute"] = old_mxid_source_attribute
+
+            old_mxid_mapping = saml2_config.get("mxid_mapping")
+            if old_mxid_mapping:
+                logger.warning(
+                    "The config option saml2_config.mxid_mapping is deprecated. Please "
+                    "use saml2_config.user_mapping_provider.config.mxid_mapping instead."
+                )
+                ump_dict["config"]["mxid_mapping"] = old_mxid_mapping
+
+        # Retrieve an instance of the module's class
+        # Pass the config dictionary to the module for processing
+        (
+            self.saml2_user_mapping_provider_class,
+            self.saml2_user_mapping_provider_config,
+        ) = load_module(ump_dict)
+
+        # Ensure loaded user mapping module has defined all necessary methods
+        # Note parse_config() is already checked during the call to load_module
+        required_methods = [
+            "get_saml_attributes",
+            "saml_response_to_user_attributes",
+        ]
+        missing_methods = [
+            method
+            for method in required_methods
+            if not hasattr(self.saml2_user_mapping_provider_class, method)
+        ]
+        if missing_methods:
+            raise ConfigError(
+                "Class specified by saml2_config."
+                "user_mapping_provider.module is missing required "
+                "methods: %s" % (", ".join(missing_methods),)
+            )
+
+        # Get the desired saml auth response attributes from the module
+        saml2_config_dict = self._default_saml_config_dict(
+            *self.saml2_user_mapping_provider_class.get_saml_attributes(
+                self.saml2_user_mapping_provider_config
+            )
+        )
         _dict_merge(
             merge_dict=saml2_config.get("sp_config", {}), into_dict=saml2_config_dict
         )
@@ -103,22 +159,27 @@ class SAML2Config(Config):
             saml2_config.get("saml_session_lifetime", "5m")
         )
 
-        mapping = saml2_config.get("mxid_mapping", "hexencode")
-        try:
-            self.saml2_mxid_mapper = MXID_MAPPER_MAP[mapping]
-        except KeyError:
-            raise ConfigError("%s is not a known mxid_mapping" % (mapping,))
-
-    def _default_saml_config_dict(self):
+    def _default_saml_config_dict(
+        self, required_attributes: set, optional_attributes: set
+    ):
+        """Generate a configuration dictionary with required and optional attributes that
+        will be needed to process new user registration
+
+        Args:
+            required_attributes: SAML auth response attributes that are
+                necessary to function
+            optional_attributes: SAML auth response attributes that can be used to add
+                additional information to Synapse user accounts, but are not required
+
+        Returns:
+            dict: A SAML configuration dictionary
+        """
         import saml2
 
         public_baseurl = self.public_baseurl
         if public_baseurl is None:
             raise ConfigError("saml2_config requires a public_baseurl to be set")
 
-        required_attributes = {"uid", self.saml2_mxid_source_attribute}
-
-        optional_attributes = {"displayName"}
         if self.saml2_grandfathered_mxid_source_attribute:
             optional_attributes.add(self.saml2_grandfathered_mxid_source_attribute)
         optional_attributes -= required_attributes
@@ -207,33 +268,58 @@ class SAML2Config(Config):
           #
           #config_path: "%(config_dir_path)s/sp_conf.py"
 
-          # the lifetime of a SAML session. This defines how long a user has to
+          # The lifetime of a SAML session. This defines how long a user has to
           # complete the authentication process, if allow_unsolicited is unset.
           # The default is 5 minutes.
           #
           #saml_session_lifetime: 5m
 
-          # The SAML attribute (after mapping via the attribute maps) to use to derive
-          # the Matrix ID from. 'uid' by default.
+          # An external module can be provided here as a custom solution to
+          # mapping attributes returned from a saml provider onto a matrix user.
           #
-          #mxid_source_attribute: displayName
-
-          # The mapping system to use for mapping the saml attribute onto a matrix ID.
-          # Options include:
-          #  * 'hexencode' (which maps unpermitted characters to '=xx')
-          #  * 'dotreplace' (which replaces unpermitted characters with '.').
-          # The default is 'hexencode'.
-          #
-          #mxid_mapping: dotreplace
-
-          # In previous versions of synapse, the mapping from SAML attribute to MXID was
-          # always calculated dynamically rather than stored in a table. For backwards-
-          # compatibility, we will look for user_ids matching such a pattern before
-          # creating a new account.
+          user_mapping_provider:
+            # The custom module's class. Uncomment to use a custom module.
+            #
+            #module: mapping_provider.SamlMappingProvider
+
+            # Custom configuration values for the module. Below options are
+            # intended for the built-in provider, they should be changed if
+            # using a custom module. This section will be passed as a Python
+            # dictionary to the module's `parse_config` method.
+            #
+            config:
+              # The SAML attribute (after mapping via the attribute maps) to use
+              # to derive the Matrix ID from. 'uid' by default.
+              #
+              # Note: This used to be configured by the
+              # saml2_config.mxid_source_attribute option. If that is still
+              # defined, its value will be used instead.
+              #
+              #mxid_source_attribute: displayName
+
+              # The mapping system to use for mapping the saml attribute onto a
+              # matrix ID.
+              #
+              # Options include:
+              #  * 'hexencode' (which maps unpermitted characters to '=xx')
+              #  * 'dotreplace' (which replaces unpermitted characters with
+              #     '.').
+              # The default is 'hexencode'.
+              #
+              # Note: This used to be configured by the
+              # saml2_config.mxid_mapping option. If that is still defined, its
+              # value will be used instead.
+              #
+              #mxid_mapping: dotreplace
+
+          # In previous versions of synapse, the mapping from SAML attribute to
+          # MXID was always calculated dynamically rather than stored in a
+          # table. For backwards- compatibility, we will look for user_ids
+          # matching such a pattern before creating a new account.
           #
           # This setting controls the SAML attribute which will be used for this
-          # backwards-compatibility lookup. Typically it should be 'uid', but if the
-          # attribute maps are changed, it may be necessary to change it.
+          # backwards-compatibility lookup. Typically it should be 'uid', but if
+          # the attribute maps are changed, it may be necessary to change it.
           #
           # The default is 'uid'.
           #
@@ -241,23 +327,3 @@ class SAML2Config(Config):
         """ % {
             "config_dir_path": config_dir_path
         }
-
-
-DOT_REPLACE_PATTERN = re.compile(
-    ("[^%s]" % (re.escape("".join(mxid_localpart_allowed_characters)),))
-)
-
-
-def dot_replace_for_mxid(username: str) -> str:
-    username = username.lower()
-    username = DOT_REPLACE_PATTERN.sub(".", username)
-
-    # regular mxids aren't allowed to start with an underscore either
-    username = re.sub("^_", "", username)
-    return username
-
-
-MXID_MAPPER_MAP = {
-    "hexencode": map_username_to_mxid_localpart,
-    "dotreplace": dot_replace_for_mxid,
-}
diff --git a/synapse/handlers/saml_handler.py b/synapse/handlers/saml_handler.py
index cc9e6b9bd0..0082f85c26 100644
--- a/synapse/handlers/saml_handler.py
+++ b/synapse/handlers/saml_handler.py
@@ -13,20 +13,36 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import re
+from typing import Tuple
 
 import attr
 import saml2
+import saml2.response
 from saml2.client import Saml2Client
 
 from synapse.api.errors import SynapseError
+from synapse.config import ConfigError
 from synapse.http.servlet import parse_string
 from synapse.rest.client.v1.login import SSOAuthHandler
-from synapse.types import UserID, map_username_to_mxid_localpart
+from synapse.types import (
+    UserID,
+    map_username_to_mxid_localpart,
+    mxid_localpart_allowed_characters,
+)
 from synapse.util.async_helpers import Linearizer
 
 logger = logging.getLogger(__name__)
 
 
+@attr.s
+class Saml2SessionData:
+    """Data we track about SAML2 sessions"""
+
+    # time the session was created, in milliseconds
+    creation_time = attr.ib()
+
+
 class SamlHandler:
     def __init__(self, hs):
         self._saml_client = Saml2Client(hs.config.saml2_sp_config)
@@ -37,11 +53,14 @@ class SamlHandler:
         self._datastore = hs.get_datastore()
         self._hostname = hs.hostname
         self._saml2_session_lifetime = hs.config.saml2_session_lifetime
-        self._mxid_source_attribute = hs.config.saml2_mxid_source_attribute
         self._grandfathered_mxid_source_attribute = (
             hs.config.saml2_grandfathered_mxid_source_attribute
         )
-        self._mxid_mapper = hs.config.saml2_mxid_mapper
+
+        # plugin to do custom mapping from saml response to mxid
+        self._user_mapping_provider = hs.config.saml2_user_mapping_provider_class(
+            hs.config.saml2_user_mapping_provider_config
+        )
 
         # identifier for the external_ids table
         self._auth_provider_id = "saml"
@@ -118,22 +137,10 @@ class SamlHandler:
             remote_user_id = saml2_auth.ava["uid"][0]
         except KeyError:
             logger.warning("SAML2 response lacks a 'uid' attestation")
-            raise SynapseError(400, "uid not in SAML2 response")
-
-        try:
-            mxid_source = saml2_auth.ava[self._mxid_source_attribute][0]
-        except KeyError:
-            logger.warning(
-                "SAML2 response lacks a '%s' attestation", self._mxid_source_attribute
-            )
-            raise SynapseError(
-                400, "%s not in SAML2 response" % (self._mxid_source_attribute,)
-            )
+            raise SynapseError(400, "'uid' not in SAML2 response")
 
         self._outstanding_requests_dict.pop(saml2_auth.in_response_to, None)
 
-        displayName = saml2_auth.ava.get("displayName", [None])[0]
-
         with (await self._mapping_lock.queue(self._auth_provider_id)):
             # first of all, check if we already have a mapping for this user
             logger.info(
@@ -173,22 +180,46 @@ class SamlHandler:
                     )
                     return registered_user_id
 
-            # figure out a new mxid for this user
-            base_mxid_localpart = self._mxid_mapper(mxid_source)
+            # Map saml response to user attributes using the configured mapping provider
+            for i in range(1000):
+                attribute_dict = self._user_mapping_provider.saml_response_to_user_attributes(
+                    saml2_auth, i
+                )
+
+                logger.debug(
+                    "Retrieved SAML attributes from user mapping provider: %s "
+                    "(attempt %d)",
+                    attribute_dict,
+                    i,
+                )
+
+                localpart = attribute_dict.get("mxid_localpart")
+                if not localpart:
+                    logger.error(
+                        "SAML mapping provider plugin did not return a "
+                        "mxid_localpart object"
+                    )
+                    raise SynapseError(500, "Error parsing SAML2 response")
 
-            suffix = 0
-            while True:
-                localpart = base_mxid_localpart + (str(suffix) if suffix else "")
+                displayname = attribute_dict.get("displayname")
+
+                # Check if this mxid already exists
                 if not await self._datastore.get_users_by_id_case_insensitive(
                     UserID(localpart, self._hostname).to_string()
                 ):
+                    # This mxid is free
                     break
-                suffix += 1
-            logger.info("Allocating mxid for new user with localpart %s", localpart)
+            else:
+                # Unable to generate a username in 1000 iterations
+                # Break and return error to the user
+                raise SynapseError(
+                    500, "Unable to generate a Matrix ID from the SAML response"
+                )
 
             registered_user_id = await self._registration_handler.register_user(
-                localpart=localpart, default_display_name=displayName
+                localpart=localpart, default_display_name=displayname
             )
+
             await self._datastore.record_user_external_id(
                 self._auth_provider_id, remote_user_id, registered_user_id
             )
@@ -205,9 +236,120 @@ class SamlHandler:
             del self._outstanding_requests_dict[reqid]
 
 
+DOT_REPLACE_PATTERN = re.compile(
+    ("[^%s]" % (re.escape("".join(mxid_localpart_allowed_characters)),))
+)
+
+
+def dot_replace_for_mxid(username: str) -> str:
+    username = username.lower()
+    username = DOT_REPLACE_PATTERN.sub(".", username)
+
+    # regular mxids aren't allowed to start with an underscore either
+    username = re.sub("^_", "", username)
+    return username
+
+
+MXID_MAPPER_MAP = {
+    "hexencode": map_username_to_mxid_localpart,
+    "dotreplace": dot_replace_for_mxid,
+}
+
+
 @attr.s
-class Saml2SessionData:
-    """Data we track about SAML2 sessions"""
+class SamlConfig(object):
+    mxid_source_attribute = attr.ib()
+    mxid_mapper = attr.ib()
 
-    # time the session was created, in milliseconds
-    creation_time = attr.ib()
+
+class DefaultSamlMappingProvider(object):
+    __version__ = "0.0.1"
+
+    def __init__(self, parsed_config: SamlConfig):
+        """The default SAML user mapping provider
+
+        Args:
+            parsed_config: Module configuration
+        """
+        self._mxid_source_attribute = parsed_config.mxid_source_attribute
+        self._mxid_mapper = parsed_config.mxid_mapper
+
+    def saml_response_to_user_attributes(
+        self, saml_response: saml2.response.AuthnResponse, failures: int = 0,
+    ) -> dict:
+        """Maps some text from a SAML response to attributes of a new user
+
+        Args:
+            saml_response: A SAML auth response object
+
+            failures: How many times a call to this function with this
+                saml_response has resulted in a failure
+
+        Returns:
+            dict: A dict containing new user attributes. Possible keys:
+                * mxid_localpart (str): Required. The localpart of the user's mxid
+                * displayname (str): The displayname of the user
+        """
+        try:
+            mxid_source = saml_response.ava[self._mxid_source_attribute][0]
+        except KeyError:
+            logger.warning(
+                "SAML2 response lacks a '%s' attestation", self._mxid_source_attribute,
+            )
+            raise SynapseError(
+                400, "%s not in SAML2 response" % (self._mxid_source_attribute,)
+            )
+
+        # Use the configured mapper for this mxid_source
+        base_mxid_localpart = self._mxid_mapper(mxid_source)
+
+        # Append suffix integer if last call to this function failed to produce
+        # a usable mxid
+        localpart = base_mxid_localpart + (str(failures) if failures else "")
+
+        # Retrieve the display name from the saml response
+        # If displayname is None, the mxid_localpart will be used instead
+        displayname = saml_response.ava.get("displayName", [None])[0]
+
+        return {
+            "mxid_localpart": localpart,
+            "displayname": displayname,
+        }
+
+    @staticmethod
+    def parse_config(config: dict) -> SamlConfig:
+        """Parse the dict provided by the homeserver's config
+        Args:
+            config: A dictionary containing configuration options for this provider
+        Returns:
+            SamlConfig: A custom config object for this module
+        """
+        # Parse config options and use defaults where necessary
+        mxid_source_attribute = config.get("mxid_source_attribute", "uid")
+        mapping_type = config.get("mxid_mapping", "hexencode")
+
+        # Retrieve the associating mapping function
+        try:
+            mxid_mapper = MXID_MAPPER_MAP[mapping_type]
+        except KeyError:
+            raise ConfigError(
+                "saml2_config.user_mapping_provider.config: '%s' is not a valid "
+                "mxid_mapping value" % (mapping_type,)
+            )
+
+        return SamlConfig(mxid_source_attribute, mxid_mapper)
+
+    @staticmethod
+    def get_saml_attributes(config: SamlConfig) -> Tuple[set, set]:
+        """Returns the required attributes of a SAML
+
+        Args:
+            config: A SamlConfig object containing configuration params for this provider
+
+        Returns:
+            tuple[set,set]: The first set equates to the saml auth response
+                attributes that are required for the module to function, whereas the
+                second set consists of those attributes which can be used if
+                available, but are not necessary
+        """
+        return {"uid", config.mxid_source_attribute}, {"displayName"}