Add an Admin API to temporarily grant the ability to update an existing cross-signing key without UIA (#16634)

6 files changed, 165 insertions, 12 deletions

diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index d06524495f..70fa931d17 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -1450,19 +1450,25 @@ class E2eKeysHandler:
 
         return desired_key_data
 
-    async def is_cross_signing_set_up_for_user(self, user_id: str) -> bool:
+    async def check_cross_signing_setup(self, user_id: str) -> Tuple[bool, bool]:
         """Checks if the user has cross-signing set up
 
         Args:
             user_id: The user to check
 
-        Returns:
-            True if the user has cross-signing set up, False otherwise
+        Returns: a 2-tuple of booleans
+            - whether the user has cross-signing set up, and
+            - whether the user's master cross-signing key may be replaced without UIA.
         """
-        existing_master_key = await self.store.get_e2e_cross_signing_key(
-            user_id, "master"
-        )
-        return existing_master_key is not None
+        (
+            exists,
+            ts_replacable_without_uia_before,
+        ) = await self.store.get_master_cross_signing_key_updatable_before(user_id)
+
+        if ts_replacable_without_uia_before is None:
+            return exists, False
+        else:
+            return exists, self.clock.time_msec() < ts_replacable_without_uia_before
 
 
 def _check_cross_signing_key(
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index 9bd0d764f8..91edfd45d7 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -88,6 +88,7 @@ from synapse.rest.admin.users import (
     UserByThreePid,
     UserMembershipRestServlet,
     UserRegisterServlet,
+    UserReplaceMasterCrossSigningKeyRestServlet,
     UserRestServletV2,
     UsersRestServletV2,
     UserTokenRestServlet,
@@ -292,6 +293,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListDestinationsRestServlet(hs).register(http_server)
     RoomMessagesRestServlet(hs).register(http_server)
     RoomTimestampToEventRestServlet(hs).register(http_server)
+    UserReplaceMasterCrossSigningKeyRestServlet(hs).register(http_server)
     UserByExternalId(hs).register(http_server)
     UserByThreePid(hs).register(http_server)
 
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 73878dd99d..9900498fbe 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -1270,6 +1270,46 @@ class AccountDataRestServlet(RestServlet):
         }
 
 
+class UserReplaceMasterCrossSigningKeyRestServlet(RestServlet):
+    """Allow a given user to replace their master cross-signing key without UIA.
+
+    This replacement is permitted for a limited period (currently 10 minutes).
+
+    While this is exposed via the admin API, this is intended for use by the
+    Matrix Authentication Service rather than server admins.
+    """
+
+    PATTERNS = admin_patterns(
+        "/users/(?P<user_id>[^/]*)/_allow_cross_signing_replacement_without_uia"
+    )
+    REPLACEMENT_PERIOD_MS = 10 * 60 * 1000  # 10 minutes
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastores().main
+
+    async def on_POST(
+        self,
+        request: SynapseRequest,
+        user_id: str,
+    ) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self._auth, request)
+
+        if user_id is None:
+            raise NotFoundError("User not found")
+
+        timestamp = (
+            await self._store.allow_master_cross_signing_key_replacement_without_uia(
+                user_id, self.REPLACEMENT_PERIOD_MS
+            )
+        )
+
+        if timestamp is None:
+            raise NotFoundError("User has no master cross-signing key")
+
+        return HTTPStatus.OK, {"updatable_without_uia_before_ms": timestamp}
+
+
 class UserByExternalId(RestServlet):
     """Find a user based on an external ID from an auth provider"""
 
diff --git a/synapse/rest/client/keys.py b/synapse/rest/client/keys.py
index 70b8be1aa2..add8045439 100644
--- a/synapse/rest/client/keys.py
+++ b/synapse/rest/client/keys.py
@@ -376,9 +376,10 @@ class SigningKeyUploadServlet(RestServlet):
         user_id = requester.user.to_string()
         body = parse_json_object_from_request(request)
 
-        is_cross_signing_setup = (
-            await self.e2e_keys_handler.is_cross_signing_set_up_for_user(user_id)
-        )
+        (
+            is_cross_signing_setup,
+            master_key_updatable_without_uia,
+        ) = await self.e2e_keys_handler.check_cross_signing_setup(user_id)
 
         # Before MSC3967 we required UIA both when setting up cross signing for the
         # first time and when resetting the device signing key. With MSC3967 we only
@@ -386,9 +387,14 @@ class SigningKeyUploadServlet(RestServlet):
         # time. Because there is no UIA in MSC3861, for now we throw an error if the
         # user tries to reset the device signing key when MSC3861 is enabled, but allow
         # first-time setup.
+        #
+        # XXX: We now have a get-out clause by which MAS can temporarily mark the master
+        # key as replaceable. It should do its own equivalent of user interactive auth
+        # before doing so.
         if self.hs.config.experimental.msc3861.enabled:
-            # There is no way to reset the device signing key with MSC3861
-            if is_cross_signing_setup:
+            # The auth service has to explicitly mark the master key as replaceable
+            # without UIA to reset the device signing key with MSC3861.
+            if is_cross_signing_setup and not master_key_updatable_without_uia:
                 raise SynapseError(
                     HTTPStatus.NOT_IMPLEMENTED,
                     "Resetting cross signing keys is not yet supported with MSC3861",
diff --git a/synapse/storage/databases/main/end_to_end_keys.py b/synapse/storage/databases/main/end_to_end_keys.py
index 8cb61eaee3..9e98729330 100644
--- a/synapse/storage/databases/main/end_to_end_keys.py
+++ b/synapse/storage/databases/main/end_to_end_keys.py
@@ -1383,6 +1383,51 @@ class EndToEndKeyWorkerStore(EndToEndKeyBackgroundStore, CacheInvalidationWorker
 
         return otk_rows
 
+    async def get_master_cross_signing_key_updatable_before(
+        self, user_id: str
+    ) -> Tuple[bool, Optional[int]]:
+        """Get time before which a master cross-signing key may be replaced without UIA.
+
+        (UIA means "User-Interactive Auth".)
+
+        There are three cases to distinguish:
+         (1) No master cross-signing key.
+         (2) The key exists, but there is no replace-without-UI timestamp in the DB.
+         (3) The key exists, and has such a timestamp recorded.
+
+        Returns: a 2-tuple of:
+          - a boolean: is there a master cross-signing key already?
+          - an optional timestamp, directly taken from the DB.
+
+        In terms of the cases above, these are:
+         (1) (False, None).
+         (2) (True, None).
+         (3) (True, <timestamp in ms>).
+
+        """
+
+        def impl(txn: LoggingTransaction) -> Tuple[bool, Optional[int]]:
+            # We want to distinguish between three cases:
+            txn.execute(
+                """
+                SELECT updatable_without_uia_before_ms
+                FROM e2e_cross_signing_keys
+                WHERE user_id = ? AND keytype = 'master'
+                ORDER BY stream_id DESC
+                LIMIT 1
+            """,
+                (user_id,),
+            )
+            row = cast(Optional[Tuple[Optional[int]]], txn.fetchone())
+            if row is None:
+                return False, None
+            return True, row[0]
+
+        return await self.db_pool.runInteraction(
+            "e2e_cross_signing_keys",
+            impl,
+        )
+
 
 class EndToEndKeyStore(EndToEndKeyWorkerStore, SQLBaseStore):
     def __init__(
@@ -1630,3 +1675,42 @@ class EndToEndKeyStore(EndToEndKeyWorkerStore, SQLBaseStore):
             ],
             desc="add_e2e_signing_key",
         )
+
+    async def allow_master_cross_signing_key_replacement_without_uia(
+        self, user_id: str, duration_ms: int
+    ) -> Optional[int]:
+        """Mark this user's latest master key as being replaceable without UIA.
+
+        Said replacement will only be permitted for a short time after calling this
+        function. That time period is controlled by the duration argument.
+
+        Returns:
+            None, if there is no such key.
+            Otherwise, the timestamp before which replacement is allowed without UIA.
+        """
+        timestamp = self._clock.time_msec() + duration_ms
+
+        def impl(txn: LoggingTransaction) -> Optional[int]:
+            txn.execute(
+                """
+                UPDATE e2e_cross_signing_keys
+                SET updatable_without_uia_before_ms = ?
+                WHERE stream_id = (
+                    SELECT stream_id
+                    FROM e2e_cross_signing_keys
+                    WHERE user_id = ? AND keytype = 'master'
+                    ORDER BY stream_id DESC
+                    LIMIT 1
+                )
+            """,
+                (timestamp, user_id),
+            )
+            if txn.rowcount == 0:
+                return None
+
+            return timestamp
+
+        return await self.db_pool.runInteraction(
+            "allow_master_cross_signing_key_replacement_without_uia",
+            impl,
+        )
diff --git a/synapse/storage/schema/main/delta/83/05_cross_signing_key_update_grant.sql b/synapse/storage/schema/main/delta/83/05_cross_signing_key_update_grant.sql
new file mode 100644
index 0000000000..b74bdd71fa
--- /dev/null
+++ b/synapse/storage/schema/main/delta/83/05_cross_signing_key_update_grant.sql
@@ -0,0 +1,15 @@
+/* Copyright 2023 The Matrix.org Foundation C.I.C
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+ALTER TABLE e2e_cross_signing_keys ADD COLUMN updatable_without_uia_before_ms bigint DEFAULT NULL;
\ No newline at end of file