Implement stable support for MSC3882 to allow an existing device/session to generate a login token for use on a new device/session (#15388)

Implements stable support for MSC3882; this involves updating Synapse's support to
match the MSC / the spec says.

Continue to support the unstable version to allow clients to transition.

6 files changed, 75 insertions, 33 deletions

diff --git a/synapse/config/auth.py b/synapse/config/auth.py
index 12e853980e..c7ab428f28 100644
--- a/synapse/config/auth.py
+++ b/synapse/config/auth.py
@@ -60,3 +60,13 @@ class AuthConfig(Config):
         self.ui_auth_session_timeout = self.parse_duration(
             ui_auth.get("session_timeout", 0)
         )
+
+        # Logging in with an existing session.
+        login_via_existing = config.get("login_via_existing_session", {})
+        self.login_via_existing_enabled = login_via_existing.get("enabled", False)
+        self.login_via_existing_require_ui_auth = login_via_existing.get(
+            "require_ui_auth", True
+        )
+        self.login_via_existing_token_timeout = self.parse_duration(
+            login_via_existing.get("token_timeout", "5m")
+        )
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 1d189b2e26..a9e002cf08 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -192,10 +192,10 @@ class MSC3861:
                 ("captcha", "enable_registration_captcha"),
             )
 
-        if root.experimental.msc3882_enabled:
+        if root.auth.login_via_existing_enabled:
             raise ConfigError(
-                "MSC3882 cannot be enabled when OAuth delegation is enabled",
-                ("experimental_features", "msc3882_enabled"),
+                "Login via existing session cannot be enabled when OAuth delegation is enabled",
+                ("login_via_existing_session", "enabled"),
             )
 
         if root.registration.refresh_token_lifetime:
@@ -319,13 +319,6 @@ class ExperimentalConfig(Config):
         # MSC3881: Remotely toggle push notifications for another client
         self.msc3881_enabled: bool = experimental.get("msc3881_enabled", False)
 
-        # MSC3882: Allow an existing session to sign in a new session
-        self.msc3882_enabled: bool = experimental.get("msc3882_enabled", False)
-        self.msc3882_ui_auth: bool = experimental.get("msc3882_ui_auth", True)
-        self.msc3882_token_timeout = self.parse_duration(
-            experimental.get("msc3882_token_timeout", "5m")
-        )
-
         # MSC3874: Filtering /messages with rel_types / not_rel_types.
         self.msc3874_enabled: bool = experimental.get("msc3874_enabled", False)
 
diff --git a/synapse/rest/client/capabilities.py b/synapse/rest/client/capabilities.py
index 0dbf8f6818..3154b9f77e 100644
--- a/synapse/rest/client/capabilities.py
+++ b/synapse/rest/client/capabilities.py
@@ -65,6 +65,9 @@ class CapabilitiesRestServlet(RestServlet):
                 "m.3pid_changes": {
                     "enabled": self.config.registration.enable_3pid_changes
                 },
+                "m.get_login_token": {
+                    "enabled": self.config.auth.login_via_existing_enabled,
+                },
             }
         }
 
diff --git a/synapse/rest/client/login.py b/synapse/rest/client/login.py
index d4dc2462b9..6493b00bb8 100644
--- a/synapse/rest/client/login.py
+++ b/synapse/rest/client/login.py
@@ -104,6 +104,9 @@ class LoginRestServlet(RestServlet):
             and hs.config.experimental.msc3866.require_approval_for_new_accounts
         )
 
+        # Whether get login token is enabled.
+        self._get_login_token_enabled = hs.config.auth.login_via_existing_enabled
+
         self.auth = hs.get_auth()
 
         self.clock = hs.get_clock()
@@ -142,6 +145,9 @@ class LoginRestServlet(RestServlet):
             # to SSO.
             flows.append({"type": LoginRestServlet.CAS_TYPE})
 
+        # The login token flow requires m.login.token to be advertised.
+        support_login_token_flow = self._get_login_token_enabled
+
         if self.cas_enabled or self.saml2_enabled or self.oidc_enabled:
             flows.append(
                 {
@@ -153,14 +159,23 @@ class LoginRestServlet(RestServlet):
                 }
             )
 
-            # While it's valid for us to advertise this login type generally,
-            # synapse currently only gives out these tokens as part of the
-            # SSO login flow.
-            # Generally we don't want to advertise login flows that clients
-            # don't know how to implement, since they (currently) will always
-            # fall back to the fallback API if they don't understand one of the
-            # login flow types returned.
-            flows.append({"type": LoginRestServlet.TOKEN_TYPE})
+            # SSO requires a login token to be generated, so we need to advertise that flow
+            support_login_token_flow = True
+
+        # While it's valid for us to advertise this login type generally,
+        # synapse currently only gives out these tokens as part of the
+        # SSO login flow or as part of login via an existing session.
+        #
+        # Generally we don't want to advertise login flows that clients
+        # don't know how to implement, since they (currently) will always
+        # fall back to the fallback API if they don't understand one of the
+        # login flow types returned.
+        if support_login_token_flow:
+            tokenTypeFlow: Dict[str, Any] = {"type": LoginRestServlet.TOKEN_TYPE}
+            # If the login token flow is enabled advertise the get_login_token flag.
+            if self._get_login_token_enabled:
+                tokenTypeFlow["get_login_token"] = True
+            flows.append(tokenTypeFlow)
 
         flows.extend({"type": t} for t in self.auth_handler.get_supported_login_types())
 
diff --git a/synapse/rest/client/login_token_request.py b/synapse/rest/client/login_token_request.py
index 43ea21d5e6..b1629f94a5 100644
--- a/synapse/rest/client/login_token_request.py
+++ b/synapse/rest/client/login_token_request.py
@@ -15,6 +15,7 @@
 import logging
 from typing import TYPE_CHECKING, Tuple
 
+from synapse.api.ratelimiting import Ratelimiter
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 from synapse.http.site import SynapseRequest
@@ -33,7 +34,7 @@ class LoginTokenRequestServlet(RestServlet):
 
     Request:
 
-    POST /login/token HTTP/1.1
+    POST /login/get_token HTTP/1.1
     Content-Type: application/json
 
     {}
@@ -43,30 +44,45 @@ class LoginTokenRequestServlet(RestServlet):
     HTTP/1.1 200 OK
     {
         "login_token": "ABDEFGH",
-        "expires_in": 3600,
+        "expires_in_ms": 3600000,
     }
     """
 
-    PATTERNS = client_patterns(
-        "/org.matrix.msc3882/login/token$", releases=[], v1=False, unstable=True
-    )
+    PATTERNS = [
+        *client_patterns(
+            "/login/get_token$", releases=["v1"], v1=False, unstable=False
+        ),
+        # TODO: this is no longer needed once unstable MSC3882 does not need to be supported:
+        *client_patterns(
+            "/org.matrix.msc3882/login/token$", releases=[], v1=False, unstable=True
+        ),
+    ]
 
     def __init__(self, hs: "HomeServer"):
         super().__init__()
         self.auth = hs.get_auth()
-        self.store = hs.get_datastores().main
-        self.clock = hs.get_clock()
-        self.server_name = hs.config.server.server_name
+        self._main_store = hs.get_datastores().main
         self.auth_handler = hs.get_auth_handler()
-        self.token_timeout = hs.config.experimental.msc3882_token_timeout
-        self.ui_auth = hs.config.experimental.msc3882_ui_auth
+        self.token_timeout = hs.config.auth.login_via_existing_token_timeout
+        self._require_ui_auth = hs.config.auth.login_via_existing_require_ui_auth
+
+        # Ratelimit aggressively to a maxmimum of 1 request per minute.
+        #
+        # This endpoint can be used to spawn additional sessions and could be
+        # abused by a malicious client to create many sessions.
+        self._ratelimiter = Ratelimiter(
+            store=self._main_store,
+            clock=hs.get_clock(),
+            rate_hz=1 / 60,
+            burst_count=1,
+        )
 
     @interactive_auth_handler
     async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         requester = await self.auth.get_user_by_req(request)
         body = parse_json_object_from_request(request)
 
-        if self.ui_auth:
+        if self._require_ui_auth:
             await self.auth_handler.validate_user_via_ui_auth(
                 requester,
                 request,
@@ -75,9 +91,12 @@ class LoginTokenRequestServlet(RestServlet):
                 can_skip_ui_auth=False,  # Don't allow skipping of UI auth
             )
 
+        # Ensure that this endpoint isn't being used too often. (Ensure this is
+        # done *after* UI auth.)
+        await self._ratelimiter.ratelimit(None, requester.user.to_string().lower())
+
         login_token = await self.auth_handler.create_login_token_for_user_id(
             user_id=requester.user.to_string(),
-            auth_provider_id="org.matrix.msc3882.login_token_request",
             duration_ms=self.token_timeout,
         )
 
@@ -85,11 +104,13 @@ class LoginTokenRequestServlet(RestServlet):
             200,
             {
                 "login_token": login_token,
+                # TODO: this is no longer needed once unstable MSC3882 does not need to be supported:
                 "expires_in": self.token_timeout // 1000,
+                "expires_in_ms": self.token_timeout,
             },
         )
 
 
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
-    if hs.config.experimental.msc3882_enabled:
+    if hs.config.auth.login_via_existing_enabled:
         LoginTokenRequestServlet(hs).register(http_server)
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index 32df054f56..547bf34df1 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -113,8 +113,8 @@ class VersionsRestServlet(RestServlet):
                     "fi.mau.msc2815": self.config.experimental.msc2815_enabled,
                     # Adds a ping endpoint for appservices to check HS->AS connection
                     "fi.mau.msc2659.stable": True,  # TODO: remove when "v1.7" is added above
-                    # Adds support for login token requests as per MSC3882
-                    "org.matrix.msc3882": self.config.experimental.msc3882_enabled,
+                    # TODO: this is no longer needed once unstable MSC3882 does not need to be supported:
+                    "org.matrix.msc3882": self.config.auth.login_via_existing_enabled,
                     # Adds support for remotely enabling/disabling pushers, as per MSC3881
                     "org.matrix.msc3881": self.config.experimental.msc3881_enabled,
                     # Adds support for filtering /messages by event relation.