MatrixSynapse/synapse/federation/federation_base.py

# Copyright 2015, 2016 OpenMarket Ltd
# Copyright 2020 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.
import logging
from typing import TYPE_CHECKING

from synapse.api.constants import MAX_DEPTH, EventContentFields, EventTypes, Membership
from synapse.api.errors import Codes, SynapseError
from synapse.api.room_versions import EventFormatVersions, RoomVersion
from synapse.crypto.event_signing import check_event_content_hash
from synapse.crypto.keyring import Keyring
from synapse.events import EventBase, make_event_from_dict
from synapse.events.utils import prune_event, validate_canonicaljson
from synapse.http.servlet import assert_params_in_dict
from synapse.logging.opentracing import log_kv, trace
from synapse.types import JsonDict, get_domain_from_id

if TYPE_CHECKING:
    from synapse.server import HomeServer


logger = logging.getLogger(__name__)


class InvalidEventSignatureError(RuntimeError):
    """Raised when the signature on an event is invalid.

    The stringification of this exception is just the error message without reference
    to the event id. The event id is available as a property.
    """

    def __init__(self, message: str, event_id: str):
        super().__init__(message)
        self.event_id = event_id


class FederationBase:
    def __init__(self, hs: "HomeServer"):
        self.hs = hs

        self.server_name = hs.hostname
        self.keyring = hs.get_keyring()
        self.spam_checker = hs.get_spam_checker()
        self.store = hs.get_datastores().main
        self._clock = hs.get_clock()
        self._storage_controllers = hs.get_storage_controllers()

    @trace
    async def _check_sigs_and_hash(
        self, room_version: RoomVersion, pdu: EventBase
    ) -> EventBase:
        """Checks that event is correctly signed by the sending server.

        Also checks the content hash, and redacts the event if there is a mismatch.

        Also runs the event through the spam checker; if it fails, redacts the event
        and flags it as soft-failed.

        Args:
            room_version: The room version of the PDU
            pdu: the event to be checked

        Returns:
              * the original event if the checks pass
              * a redacted version of the event (if the signature
                matched but the hash did not). In this case a warning will be logged.

        Raises:
          InvalidEventSignatureError if the signature check failed. Nothing
             will be logged in this case.
        """
        await _check_sigs_on_pdu(self.keyring, room_version, pdu)

        if not check_event_content_hash(pdu):
            # let's try to distinguish between failures because the event was
            # redacted (which are somewhat expected) vs actual ball-tampering
            # incidents.
            #
            # This is just a heuristic, so we just assume that if the keys are
            # about the same between the redacted and received events, then the
            # received event was probably a redacted copy (but we then use our
            # *actual* redacted copy to be on the safe side.)
            redacted_event = prune_event(pdu)
            if set(redacted_event.keys()) == set(pdu.keys()) and set(
                redacted_event.content.keys()
            ) == set(pdu.content.keys()):
                logger.debug(
                    "Event %s seems to have been redacted; using our redacted copy",
                    pdu.event_id,
                )
                log_kv(
                    {
                        "message": "Event seems to have been redacted; using our redacted copy",
                        "event_id": pdu.event_id,
                    }
                )
            else:
                logger.warning(
                    "Event %s content has been tampered, redacting",
                    pdu.event_id,
                )
                log_kv(
                    {
                        "message": "Event content has been tampered, redacting",
                        "event_id": pdu.event_id,
                    }
                )
            return redacted_event

        spam_check = await self.spam_checker.check_event_for_spam(pdu)

        if spam_check != self.spam_checker.NOT_SPAM:
            logger.warning("Event contains spam, soft-failing %s", pdu.event_id)
            log_kv(
                {
                    "message": "Event contains spam, redacting (to save disk space) "
                    "as well as soft-failing (to stop using the event in prev_events)",
                    "event_id": pdu.event_id,
                }
            )
            # we redact (to save disk space) as well as soft-failing (to stop
            # using the event in prev_events).
            redacted_event = prune_event(pdu)
            redacted_event.internal_metadata.soft_failed = True
            return redacted_event

        return pdu


@trace
async def _check_sigs_on_pdu(
    keyring: Keyring, room_version: RoomVersion, pdu: EventBase
) -> None:
    """Check that the given events are correctly signed

    Args:
        keyring: keyring object to do the checks
        room_version: the room version of the PDUs
        pdus: the events to be checked

    Raises:
        InvalidEventSignatureError if the event wasn't correctly signed.
    """

    # we want to check that the event is signed by:
    #
    # (a) the sender's server
    #
    #     - except in the case of invites created from a 3pid invite, which are exempt
    #     from this check, because the sender has to match that of the original 3pid
    #     invite, but the event may come from a different HS, for reasons that I don't
    #     entirely grok (why do the senders have to match? and if they do, why doesn't the
    #     joining server ask the inviting server to do the switcheroo with
    #     exchange_third_party_invite?).
    #
    #     That's pretty awful, since redacting such an invite will render it invalid
    #     (because it will then look like a regular invite without a valid signature),
    #     and signatures are *supposed* to be valid whether or not an event has been
    #     redacted. But this isn't the worst of the ways that 3pid invites are broken.
    #
    # (b) for V1 and V2 rooms, the server which created the event_id
    #
    # let's start by getting the domain for each pdu, and flattening the event back
    # to JSON.

    # First we check that the sender event is signed by the sender's domain
    # (except if its a 3pid invite, in which case it may be sent by any server)
    sender_domain = get_domain_from_id(pdu.sender)
    if not _is_invite_via_3pid(pdu):
        try:
            await keyring.verify_event_for_server(
                sender_domain,
                pdu,
                pdu.origin_server_ts if room_version.enforce_key_validity else 0,
            )
        except Exception as e:
            raise InvalidEventSignatureError(
                f"unable to verify signature for sender domain {sender_domain}: {e}",
                pdu.event_id,
            ) from None

    # now let's look for events where the sender's domain is different to the
    # event id's domain (normally only the case for joins/leaves), and add additional
    # checks. Only do this if the room version has a concept of event ID domain
    # (ie, the room version uses old-style non-hash event IDs).
    if room_version.event_format == EventFormatVersions.ROOM_V1_V2:
        event_domain = get_domain_from_id(pdu.event_id)
        if event_domain != sender_domain:
            try:
                await keyring.verify_event_for_server(
                    event_domain,
                    pdu,
                    pdu.origin_server_ts if room_version.enforce_key_validity else 0,
                )
            except Exception as e:
                raise InvalidEventSignatureError(
                    f"unable to verify signature for event domain {event_domain}: {e}",
                    pdu.event_id,
                ) from None

    # If this is a join event for a restricted room it may have been authorised
    # via a different server from the sending server. Check those signatures.
    if (
        room_version.msc3083_join_rules
        and pdu.type == EventTypes.Member
        and pdu.membership == Membership.JOIN
        and EventContentFields.AUTHORISING_USER in pdu.content
    ):
        authorising_server = get_domain_from_id(
            pdu.content[EventContentFields.AUTHORISING_USER]
        )
        try:
            await keyring.verify_event_for_server(
                authorising_server,
                pdu,
                pdu.origin_server_ts if room_version.enforce_key_validity else 0,
            )
        except Exception as e:
            raise InvalidEventSignatureError(
                f"unable to verify signature for authorising serve {authorising_server}: {e}",
                pdu.event_id,
            ) from None


def _is_invite_via_3pid(event: EventBase) -> bool:
    return (
        event.type == EventTypes.Member
        and event.membership == Membership.INVITE
        and "third_party_invite" in event.content
    )


def event_from_pdu_json(pdu_json: JsonDict, room_version: RoomVersion) -> EventBase:
    """Construct an EventBase from an event json received over federation

    Args:
        pdu_json: pdu as received over federation
        room_version: The version of the room this event belongs to

    Raises:
        SynapseError: if the pdu is missing required fields or is otherwise
            not a valid matrix event
    """
    # we could probably enforce a bunch of other fields here (room_id, sender,
    # origin, etc etc)
    assert_params_in_dict(pdu_json, ("type", "depth"))

    # Strip any unauthorized values from "unsigned" if they exist
    if "unsigned" in pdu_json:
        _strip_unsigned_values(pdu_json)

    depth = pdu_json["depth"]
    if not isinstance(depth, int):
        raise SynapseError(400, "Depth %r not an intger" % (depth,), Codes.BAD_JSON)

    if depth < 0:
        raise SynapseError(400, "Depth too small", Codes.BAD_JSON)
    elif depth > MAX_DEPTH:
        raise SynapseError(400, "Depth too large", Codes.BAD_JSON)

    # Validate that the JSON conforms to the specification.
    if room_version.strict_canonicaljson:
        validate_canonicaljson(pdu_json)

    event = make_event_from_dict(pdu_json, room_version)
    return event


def _strip_unsigned_values(pdu_dict: JsonDict) -> None:
    """
    Strip any unsigned values unless specifically allowed, as defined by the whitelist.

    pdu: the json dict to strip values from. Note that the dict is mutated by this
    function
    """
    unsigned = pdu_dict["unsigned"]

    if not isinstance(unsigned, dict):
        pdu_dict["unsigned"] = {}

    if pdu_dict["type"] == "m.room.member":
        whitelist = ["knock_room_state", "invite_room_state", "age"]
    else:
        whitelist = ["age"]

    filtered_unsigned = {k: v for k, v in unsigned.items() if k in whitelist}
    pdu_dict["unsigned"] = filtered_unsigned