bugfix: make msc3967 idempotent (#16943)

MSC3967 was updated recently to make it more robust to network failures:

> there is an existing cross-signing master key and it exactly matches
the cross-signing master key provided in the request body. If there are
any additional keys provided in the request (self signing key, user
signing key) they MUST also match the existing keys stored on the
server. In other words, the request contains no new keys. If there are
new keys, UIA MUST be performed.


https://github.com/matrix-org/matrix-spec-proposals/blob/hughns/device-signing-upload-uia/proposals/3967-device-signing-upload-uia.md#proposal

This covers the case where the 200 OK is lost in transit so the client
retries the upload, only to then get UIA'd.

Complement tests: https://github.com/matrix-org/complement/pull/713 -
passing example
https://github.com/element-hq/synapse/actions/runs/7976948122/job/21778795094?pr=16943#step:7:8820

### Pull Request Checklist

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https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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- Feel free to credit yourself, by adding a sentence "Contributed by
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* [x] [Code
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correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: reivilibre <oliverw@matrix.org>

6 files changed, 102 insertions, 3 deletions

diff --git a/changelog.d/16943.bugfix b/changelog.d/16943.bugfix
new file mode 100644
index 0000000000..4360741132
--- /dev/null
+++ b/changelog.d/16943.bugfix
@@ -0,0 +1 @@
+Make the CSAPI endpoint `/keys/device_signing/upload` idempotent.
\ No newline at end of file
diff --git a/docker/complement/conf/workers-shared-extra.yaml.j2 b/docker/complement/conf/workers-shared-extra.yaml.j2
index 2b11b487f6..32eada4419 100644
--- a/docker/complement/conf/workers-shared-extra.yaml.j2
+++ b/docker/complement/conf/workers-shared-extra.yaml.j2
@@ -102,6 +102,8 @@ experimental_features:
   msc3391_enabled: true
   # Filtering /messages by relation type.
   msc3874_enabled: true
+  # no UIA for x-signing upload for the first time
+  msc3967_enabled: true
 
 server_notices:
   system_mxid_localpart: _server
diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index b1a8724b7e..2a779f8255 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -214,7 +214,7 @@ fi
 
 extra_test_args=()
 
-test_packages="./tests/csapi ./tests ./tests/msc3874 ./tests/msc3890 ./tests/msc3391 ./tests/msc3930 ./tests/msc3902"
+test_packages="./tests/csapi ./tests ./tests/msc3874 ./tests/msc3890 ./tests/msc3391 ./tests/msc3930 ./tests/msc3902 ./tests/msc3967"
 
 # Enable dirty runs, so tests will reuse the same container where possible.
 # This significantly speeds up tests, but increases the possibility of test pollution.
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 63e00f102e..1ece54ccfc 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -1476,6 +1476,42 @@ class E2eKeysHandler:
         else:
             return exists, self.clock.time_msec() < ts_replacable_without_uia_before
 
+    async def has_different_keys(self, user_id: str, body: JsonDict) -> bool:
+        """
+        Check if a key provided in `body` differs from the same key stored in the DB. Returns
+        true on the first difference. If a key exists in `body` but does not exist in the DB,
+        returns True. If `body` has no keys, this always returns False.
+        Note by 'key' we mean Matrix key rather than JSON key.
+
+        The purpose of this function is to detect whether or not we need to apply UIA checks.
+        We must apply UIA checks if any key in the database is being overwritten. If a key is
+        being inserted for the first time, or if the key exactly matches what is in the database,
+        then no UIA check needs to be performed.
+
+        Args:
+            user_id: The user who sent the `body`.
+            body: The JSON request body from POST /keys/device_signing/upload
+        Returns:
+            True if any key in `body` has a different value in the database.
+        """
+        # Ensure that each key provided in the request body exactly matches the one we have stored.
+        # The first time we see the DB having a different key to the matching request key, bail.
+        # Note: we do not care if the DB has a key which the request does not specify, as we only
+        # care about *replacements* or *insertions* (i.e UPSERT)
+        req_body_key_to_db_key = {
+            "master_key": "master",
+            "self_signing_key": "self_signing",
+            "user_signing_key": "user_signing",
+        }
+        for req_body_key, db_key in req_body_key_to_db_key.items():
+            if req_body_key in body:
+                existing_key = await self.store.get_e2e_cross_signing_key(
+                    user_id, db_key
+                )
+                if existing_key != body[req_body_key]:
+                    return True
+        return False
+
 
 def _check_cross_signing_key(
     key: JsonDict, user_id: str, key_type: str, signing_key: Optional[VerifyKey] = None
diff --git a/synapse/rest/client/keys.py b/synapse/rest/client/keys.py
index b6d9ee074a..86c9515854 100644
--- a/synapse/rest/client/keys.py
+++ b/synapse/rest/client/keys.py
@@ -409,7 +409,18 @@ class SigningKeyUploadServlet(RestServlet):
             # But first-time setup is fine
 
         elif self.hs.config.experimental.msc3967_enabled:
-            # If we already have a master key then cross signing is set up and we require UIA to reset
+            # MSC3967 allows this endpoint to 200 OK for idempotency. Resending exactly the same
+            # keys should just 200 OK without doing a UIA prompt.
+            keys_are_different = await self.e2e_keys_handler.has_different_keys(
+                user_id, body
+            )
+            if not keys_are_different:
+                # FIXME: we do not fallthrough to upload_signing_keys_for_user because confusingly
+                # if we do, we 500 as it looks like it tries to INSERT the same key twice, causing a
+                # unique key constraint violation. This sounds like a bug?
+                return 200, {}
+            # the keys are different, is x-signing set up? If no, then the keys don't exist which is
+            # why they are different. If yes, then we need to UIA to change them.
             if is_cross_signing_setup:
                 await self.auth_handler.validate_user_via_ui_auth(
                     requester,
@@ -420,7 +431,6 @@ class SigningKeyUploadServlet(RestServlet):
                     can_skip_ui_auth=False,
                 )
             # Otherwise we don't require UIA since we are setting up cross signing for first time
-
         else:
             # Previous behaviour is to always require UIA but allow it to be skipped
             await self.auth_handler.validate_user_via_ui_auth(
diff --git a/tests/handlers/test_e2e_keys.py b/tests/handlers/test_e2e_keys.py
index 3d931abb06..0e6352ff4b 100644
--- a/tests/handlers/test_e2e_keys.py
+++ b/tests/handlers/test_e2e_keys.py
@@ -1101,6 +1101,56 @@ class E2eKeysHandlerTestCase(unittest.HomeserverTestCase):
             },
         )
 
+    def test_has_different_keys(self) -> None:
+        """check that has_different_keys returns True when the keys provided are different to what
+        is in the database."""
+        local_user = "@boris:" + self.hs.hostname
+        keys1 = {
+            "master_key": {
+                # private key: 2lonYOM6xYKdEsO+6KrC766xBcHnYnim1x/4LFGF8B0
+                "user_id": local_user,
+                "usage": ["master"],
+                "keys": {
+                    "ed25519:nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unI9kDYcHwk": "nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unI9kDYcHwk"
+                },
+            }
+        }
+        self.get_success(self.handler.upload_signing_keys_for_user(local_user, keys1))
+        is_different = self.get_success(
+            self.handler.has_different_keys(
+                local_user,
+                {
+                    "master_key": keys1["master_key"],
+                },
+            )
+        )
+        self.assertEqual(is_different, False)
+        # change the usage => different keys
+        keys1["master_key"]["usage"] = ["develop"]
+        is_different = self.get_success(
+            self.handler.has_different_keys(
+                local_user,
+                {
+                    "master_key": keys1["master_key"],
+                },
+            )
+        )
+        self.assertEqual(is_different, True)
+        keys1["master_key"]["usage"] = ["master"]  # reset
+        # change the key => different keys
+        keys1["master_key"]["keys"] = {
+            "ed25519:nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unIc0rncs": "nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unIc0rncs"
+        }
+        is_different = self.get_success(
+            self.handler.has_different_keys(
+                local_user,
+                {
+                    "master_key": keys1["master_key"],
+                },
+            )
+        )
+        self.assertEqual(is_different, True)
+
     def test_query_devices_remote_sync(self) -> None:
         """Tests that querying keys for a remote user that we share a room with,
         but haven't yet fetched the keys for, returns the cross signing keys