visibility.py

# Copyright 2014 - 2016 OpenMarket Ltd
# Copyright (C) The Matrix.org Foundation C.I.C. 2022
#
# 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 enum import Enum, auto
from typing import Collection, Dict, FrozenSet, List, Optional, Tuple

import attr
from typing_extensions import Final

from synapse.api.constants import EventTypes, HistoryVisibility, Membership
from synapse.events import EventBase
from synapse.events.snapshot import EventContext
from synapse.events.utils import prune_event
from synapse.logging.opentracing import trace
from synapse.storage.controllers import StorageControllers
from synapse.storage.databases.main import DataStore
from synapse.types import RetentionPolicy, StateMap, get_domain_from_id
from synapse.types.state import StateFilter
from synapse.util import Clock

logger = logging.getLogger(__name__)


VISIBILITY_PRIORITY = (
    HistoryVisibility.WORLD_READABLE,
    HistoryVisibility.SHARED,
    HistoryVisibility.INVITED,
    HistoryVisibility.JOINED,
)


MEMBERSHIP_PRIORITY = (
    Membership.JOIN,
    Membership.INVITE,
    Membership.KNOCK,
    Membership.LEAVE,
    Membership.BAN,
)

_HISTORY_VIS_KEY: Final[Tuple[str, str]] = (EventTypes.RoomHistoryVisibility, "")


@trace
async def filter_events_for_client(
    storage: StorageControllers,
    user_id: str,
    events: List[EventBase],
    is_peeking: bool = False,
    always_include_ids: FrozenSet[str] = frozenset(),
    filter_send_to_client: bool = True,
) -> List[EventBase]:
    """
    Check which events a user is allowed to see. If the user can see the event but its
    sender asked for their data to be erased, prune the content of the event.

    Args:
        storage
        user_id: user id to be checked
        events: sequence of events to be checked
        is_peeking: should be True if:
          * the user is not currently a member of the room, and:
          * the user has not been a member of the room since the given
            events
        always_include_ids: set of event ids to specifically include, if present
            in events (unless sender is ignored)
        filter_send_to_client: Whether we're checking an event that's going to be
            sent to a client. This might not always be the case since this function can
            also be called to check whether a user can see the state at a given point.

    Returns:
        The filtered events.
    """
    # Filter out events that have been soft failed so that we don't relay them
    # to clients.
    events_before_filtering = events
    events = [e for e in events if not e.internal_metadata.is_soft_failed()]
    if len(events_before_filtering) != len(events):
        if logger.isEnabledFor(logging.DEBUG):
            logger.debug(
                "filter_events_for_client: Filtered out soft-failed events: Before=%s, After=%s",
                [event.event_id for event in events_before_filtering],
                [event.event_id for event in events],
            )

    types = (_HISTORY_VIS_KEY, (EventTypes.Member, user_id))

    # we exclude outliers at this point, and then handle them separately later
    event_id_to_state = await storage.state.get_state_for_events(
        frozenset(e.event_id for e in events if not e.internal_metadata.outlier),
        state_filter=StateFilter.from_types(types),
    )

    # Get the users who are ignored by the requesting user.
    ignore_list = await storage.main.ignored_users(user_id)

    erased_senders = await storage.main.are_users_erased(e.sender for e in events)

    if filter_send_to_client:
        room_ids = {e.room_id for e in events}
        retention_policies: Dict[str, RetentionPolicy] = {}

        for room_id in room_ids:
            retention_policies[
                room_id
            ] = await storage.main.get_retention_policy_for_room(room_id)

    def allowed(event: EventBase) -> Optional[EventBase]:
        return _check_client_allowed_to_see_event(
            user_id=user_id,
            event=event,
            clock=storage.main.clock,
            filter_send_to_client=filter_send_to_client,
            sender_ignored=event.sender in ignore_list,
            always_include_ids=always_include_ids,
            retention_policy=retention_policies[room_id],
            state=event_id_to_state.get(event.event_id),
            is_peeking=is_peeking,
            sender_erased=erased_senders.get(event.sender, False),
        )

    # Check each event: gives an iterable of None or (a potentially modified)
    # EventBase.
    filtered_events = map(allowed, events)

    # Turn it into a list and remove None entries before returning.
    return [ev for ev in filtered_events if ev]


async def filter_event_for_clients_with_state(
    store: DataStore,
    user_ids: Collection[str],
    event: EventBase,
    context: EventContext,
    is_peeking: bool = False,
    filter_send_to_client: bool = True,
) -> Collection[str]:
    """
    Checks to see if an event is visible to the users in the list at the time of
    the event.

    Note: This does *not* check if the sender of the event was erased.

    Args:
        store: databases
        user_ids: user_ids to be checked
        event: the event to be checked
        context: EventContext for the event to be checked
        is_peeking: Whether the users are peeking into the room, ie not
            currently joined
        filter_send_to_client: Whether we're checking an event that's going to be
            sent to a client. This might not always be the case since this function can
            also be called to check whether a user can see the state at a given point.

    Returns:
        Collection of user IDs for whom the event is visible
    """
    # None of the users should see the event if it is soft_failed
    if event.internal_metadata.is_soft_failed():
        return []

    # Fast path if we don't have any user IDs to check.
    if not user_ids:
        return ()

    # Make a set for all user IDs that haven't been filtered out by a check.
    allowed_user_ids = set(user_ids)

    # Only run some checks if these events aren't about to be sent to clients. This is
    # because, if this is not the case, we're probably only checking if the users can
    # see events in the room at that point in the DAG, and that shouldn't be decided
    # on those checks.
    if filter_send_to_client:
        ignored_by = await store.ignored_by(event.sender)
        retention_policy = await store.get_retention_policy_for_room(event.room_id)

        for user_id in user_ids:
            if (
                _check_filter_send_to_client(
                    event,
                    store.clock,
                    retention_policy,
                    sender_ignored=user_id in ignored_by,
                )
                == _CheckFilter.DENIED
            ):
                allowed_user_ids.discard(user_id)

    if event.internal_metadata.outlier:
        # Normally these can't be seen by clients, but we make an exception for
        # for out-of-band membership events (eg, incoming invites, or rejections of
        # said invite) for the user themselves.
        if event.type == EventTypes.Member and event.state_key in allowed_user_ids:
            logger.debug("Returning out-of-band-membership event %s", event)
            return {event.state_key}

        return set()

    # First we get just the history visibility in case its shared/world-readable
    # room.
    visibility_state_map = await _get_state_map(
        store, event, context, StateFilter.from_types([_HISTORY_VIS_KEY])
    )

    visibility = get_effective_room_visibility_from_state(visibility_state_map)
    if (
        _check_history_visibility(event, visibility, is_peeking=is_peeking)
        == _CheckVisibility.ALLOWED
    ):
        return allowed_user_ids

    # The history visibility isn't lax, so we now need to fetch the membership
    # events of all the users.

    filter_list = []
    for user_id in allowed_user_ids:
        filter_list.append((EventTypes.Member, user_id))
    filter_list.append((EventTypes.RoomHistoryVisibility, ""))

    state_filter = StateFilter.from_types(filter_list)
    state_map = await _get_state_map(store, event, context, state_filter)

    # Now we check whether the membership allows each user to see the event.
    return {
        user_id
        for user_id in allowed_user_ids
        if _check_membership(user_id, event, visibility, state_map, is_peeking).allowed
    }


async def _get_state_map(
    store: DataStore, event: EventBase, context: EventContext, state_filter: StateFilter
) -> StateMap[EventBase]:
    """Helper function for getting a `StateMap[EventBase]` from an `EventContext`"""
    state_map = await context.get_prev_state_ids(state_filter)

    # Use events rather than event ids as content from the events are needed in
    # _check_visibility
    event_map = await store.get_events(state_map.values(), get_prev_content=False)

    updated_state_map = {}
    for state_key, event_id in state_map.items():
        state_event = event_map.get(event_id)
        if state_event:
            updated_state_map[state_key] = state_event

    if event.is_state():
        current_state_key = (event.type, event.state_key)
        # Add current event to updated_state_map, we need to do this here as it
        # may not have been persisted to the db yet
        updated_state_map[current_state_key] = event

    return updated_state_map


def _check_client_allowed_to_see_event(
    user_id: str,
    event: EventBase,
    clock: Clock,
    filter_send_to_client: bool,
    is_peeking: bool,
    always_include_ids: FrozenSet[str],
    sender_ignored: bool,
    retention_policy: RetentionPolicy,
    state: Optional[StateMap[EventBase]],
    sender_erased: bool,
) -> Optional[EventBase]:
    """Check with the given user is allowed to see the given event

    See `filter_events_for_client` for details about args

    Args:
        user_id
        event
        clock
        filter_send_to_client
        is_peeking
        always_include_ids
        sender_ignored: Whether the user is ignoring the event sender
        retention_policy: The retention policy of the room
        state: The state at the event, unless its an outlier
        sender_erased: Whether the event sender has been marked as "erased"

    Returns:
        None if the user cannot see this event at all

        a redacted copy of the event if they can only see a redacted
        version

        the original event if they can see it as normal.
    """
    # Only run some checks if these events aren't about to be sent to clients. This is
    # because, if this is not the case, we're probably only checking if the users can
    # see events in the room at that point in the DAG, and that shouldn't be decided
    # on those checks.
    if filter_send_to_client:
        if (
            _check_filter_send_to_client(event, clock, retention_policy, sender_ignored)
            == _CheckFilter.DENIED
        ):
            logger.debug(
                "_check_client_allowed_to_see_event(event=%s): Filtered out event because `_check_filter_send_to_client` returned `_CheckFilter.DENIED`",
                event.event_id,
            )
            return None

    if event.event_id in always_include_ids:
        return event

    # we need to handle outliers separately, since we don't have the room state.
    if event.internal_metadata.outlier:
        # Normally these can't be seen by clients, but we make an exception for
        # for out-of-band membership events (eg, incoming invites, or rejections of
        # said invite) for the user themselves.
        if event.type == EventTypes.Member and event.state_key == user_id:
            logger.debug(
                "_check_client_allowed_to_see_event(event=%s): Returning out-of-band-membership event %s",
                event.event_id,
                event,
            )
            return event

        logger.debug(
            "_check_client_allowed_to_see_event(event=%s): Filtered out event because it's an outlier",
            event.event_id,
        )
        return None

    if state is None:
        raise Exception("Missing state for non-outlier event")

    # get the room_visibility at the time of the event.
    visibility = get_effective_room_visibility_from_state(state)

    # Check if the room has lax history visibility, allowing us to skip
    # membership checks.
    #
    # We can only do this check if the sender has *not* been erased, as if they
    # have we need to check the user's membership.
    if (
        not sender_erased
        and _check_history_visibility(event, visibility, is_peeking)
        == _CheckVisibility.ALLOWED
    ):
        return event

    membership_result = _check_membership(user_id, event, visibility, state, is_peeking)
    if not membership_result.allowed:
        logger.debug(
            "_check_client_allowed_to_see_event(event=%s): Filtered out event because the user can't see the event because of their membership, membership_result.allowed=%s membership_result.joined=%s",
            event.event_id,
            membership_result.allowed,
            membership_result.joined,
        )
        return None

    # If the sender has been erased and the user was not joined at the time, we
    # must only return the redacted form.
    if sender_erased and not membership_result.joined:
        logger.debug(
            "_check_client_allowed_to_see_event(event=%s): Returning pruned event because `sender_erased` and the user was not joined at the time",
            event.event_id,
        )
        event = prune_event(event)

    return event


@attr.s(frozen=True, slots=True, auto_attribs=True)
class _CheckMembershipReturn:
    "Return value of _check_membership"
    allowed: bool
    joined: bool


def _check_membership(
    user_id: str,
    event: EventBase,
    visibility: str,
    state: StateMap[EventBase],
    is_peeking: bool,
) -> _CheckMembershipReturn:
    """Check whether the user can see the event due to their membership

    Returns:
        True if they can, False if they can't, plus the membership of the user
        at the event.
    """
    # If the event is the user's own membership event, use the 'most joined'
    # membership
    membership = None
    if event.type == EventTypes.Member and event.state_key == user_id:
        membership = event.content.get("membership", None)
        if membership not in MEMBERSHIP_PRIORITY:
            membership = "leave"

        prev_content = event.unsigned.get("prev_content", {})
        prev_membership = prev_content.get("membership", None)
        if prev_membership not in MEMBERSHIP_PRIORITY:
            prev_membership = "leave"

        # Always allow the user to see their own leave events, otherwise
        # they won't see the room disappear if they reject the invite
        #
        # (Note this doesn't work for out-of-band invite rejections, which don't
        # have prev_state populated. They are handled above in the outlier code.)
        if membership == "leave" and (
            prev_membership == "join" or prev_membership == "invite"
        ):
            return _CheckMembershipReturn(True, membership == Membership.JOIN)

        new_priority = MEMBERSHIP_PRIORITY.index(membership)
        old_priority = MEMBERSHIP_PRIORITY.index(prev_membership)
        if old_priority < new_priority:
            membership = prev_membership

    # otherwise, get the user's membership at the time of the event.
    if membership is None:
        membership_event = state.get((EventTypes.Member, user_id), None)
        if membership_event:
            membership = membership_event.membership

    # if the user was a member of the room at the time of the event,
    # they can see it.
    if membership == Membership.JOIN:
        return _CheckMembershipReturn(True, True)

    # otherwise, it depends on the room visibility.

    if visibility == HistoryVisibility.JOINED:
        # we weren't a member at the time of the event, so we can't
        # see this event.
        return _CheckMembershipReturn(False, False)

    elif visibility == HistoryVisibility.INVITED:
        # user can also see the event if they were *invited* at the time
        # of the event.
        return _CheckMembershipReturn(membership == Membership.INVITE, False)

    elif visibility == HistoryVisibility.SHARED and is_peeking:
        # if the visibility is shared, users cannot see the event unless
        # they have *subsequently* joined the room (or were members at the
        # time, of course)
        #
        # XXX: if the user has subsequently joined and then left again,
        # ideally we would share history up to the point they left. But
        # we don't know when they left. We just treat it as though they
        # never joined, and restrict access.
        return _CheckMembershipReturn(False, False)

    # The visibility is either shared or world_readable, and the user was
    # not a member at the time. We allow it.
    return _CheckMembershipReturn(True, False)


class _CheckFilter(Enum):
    MAYBE_ALLOWED = auto()
    DENIED = auto()


def _check_filter_send_to_client(
    event: EventBase,
    clock: Clock,
    retention_policy: RetentionPolicy,
    sender_ignored: bool,
) -> _CheckFilter:
    """Apply checks for sending events to client

    Returns:
        True if might be allowed to be sent to clients, False if definitely not.
    """

    if event.type == EventTypes.Dummy:
        return _CheckFilter.DENIED

    if not event.is_state() and sender_ignored:
        return _CheckFilter.DENIED

    # Until MSC2261 has landed we can't redact malicious alias events, so for
    # now we temporarily filter out m.room.aliases entirely to mitigate
    # abuse, while we spec a better solution to advertising aliases
    # on rooms.
    if event.type == EventTypes.Aliases:
        return _CheckFilter.DENIED

    # Don't try to apply the room's retention policy if the event is a state
    # event, as MSC1763 states that retention is only considered for non-state
    # events.
    if not event.is_state():
        max_lifetime = retention_policy.max_lifetime

        if max_lifetime is not None:
            oldest_allowed_ts = clock.time_msec() - max_lifetime

            if event.origin_server_ts < oldest_allowed_ts:
                return _CheckFilter.DENIED

    return _CheckFilter.MAYBE_ALLOWED


class _CheckVisibility(Enum):
    ALLOWED = auto()
    MAYBE_DENIED = auto()


def _check_history_visibility(
    event: EventBase, visibility: str, is_peeking: bool
) -> _CheckVisibility:
    """Check if event is allowed to be seen due to lax history visibility.

    Returns:
        True if user can definitely see the event, False if maybe not.
    """
    # Always allow history visibility events on boundaries. This is done
    # by setting the effective visibility to the least restrictive
    # of the old vs new.
    if event.type == EventTypes.RoomHistoryVisibility:
        prev_content = event.unsigned.get("prev_content", {})
        prev_visibility = prev_content.get("history_visibility", None)

        if prev_visibility not in VISIBILITY_PRIORITY:
            prev_visibility = HistoryVisibility.SHARED

        new_priority = VISIBILITY_PRIORITY.index(visibility)
        old_priority = VISIBILITY_PRIORITY.index(prev_visibility)
        if old_priority < new_priority:
            visibility = prev_visibility

    if visibility == HistoryVisibility.SHARED and not is_peeking:
        return _CheckVisibility.ALLOWED
    elif visibility == HistoryVisibility.WORLD_READABLE:
        return _CheckVisibility.ALLOWED

    return _CheckVisibility.MAYBE_DENIED


def get_effective_room_visibility_from_state(state: StateMap[EventBase]) -> str:
    """Get the actual history vis, from a state map including the history_visibility event
    Handles missing and invalid history visibility events.
    """
    visibility_event = state.get(_HISTORY_VIS_KEY, None)
    if not visibility_event:
        return HistoryVisibility.SHARED

    visibility = visibility_event.content.get(
        "history_visibility", HistoryVisibility.SHARED
    )
    if visibility not in VISIBILITY_PRIORITY:
        visibility = HistoryVisibility.SHARED
    return visibility


async def filter_events_for_server(
    storage: StorageControllers,
    target_server_name: str,
    local_server_name: str,
    events: List[EventBase],
    redact: bool = True,
    check_history_visibility_only: bool = False,
) -> List[EventBase]:
    """Filter a list of events based on whether given server is allowed to
    see them.

    Args:
        storage
        server_name
        events
        redact: Whether to return a redacted version of the event, or
            to filter them out entirely.
        check_history_visibility_only: Whether to only check the
            history visibility, rather than things like if the sender has been
            erased. This is used e.g. during pagination to decide whether to
            backfill or not.

    Returns
        The filtered events.
    """

    def is_sender_erased(event: EventBase, erased_senders: Dict[str, bool]) -> bool:
        if erased_senders and erased_senders[event.sender]:
            logger.info("Sender of %s has been erased, redacting", event.event_id)
            return True
        return False

    def check_event_is_visible(
        visibility: str, memberships: StateMap[EventBase]
    ) -> bool:
        if visibility not in (HistoryVisibility.INVITED, HistoryVisibility.JOINED):
            return True

        # We now loop through all membership events looking for
        # membership states for the requesting server to determine
        # if the server is either in the room or has been invited
        # into the room.
        for ev in memberships.values():
            assert get_domain_from_id(ev.state_key) == target_server_name

            memtype = ev.membership
            if memtype == Membership.JOIN:
                return True
            elif memtype == Membership.INVITE:
                if visibility == HistoryVisibility.INVITED:
                    return True

        # server has no users in the room: redact
        return False

    if not check_history_visibility_only:
        erased_senders = await storage.main.are_users_erased(e.sender for e in events)
    else:
        # We don't want to check whether users are erased, which is equivalent
        # to no users having been erased.
        erased_senders = {}

    # Filter out non-local events when we are in the middle of a partial join, since our servers
    # list can be out of date and we could leak events to servers not in the room anymore.
    # This can also be true for local events but we consider it to be an acceptable risk.

    # We do this check as a first step and before retrieving membership events because
    # otherwise a room could be fully joined after we retrieve those, which would then bypass
    # this check but would base the filtering on an outdated view of the membership events.

    partial_state_invisible_events = set()
    if not check_history_visibility_only:
        for e in events:
            sender_domain = get_domain_from_id(e.sender)
            if (
                sender_domain != local_server_name
                and await storage.main.is_partial_state_room(e.room_id)
            ):
                partial_state_invisible_events.add(e)

    # Let's check to see if all the events have a history visibility
    # of "shared" or "world_readable". If that's the case then we don't
    # need to check membership (as we know the server is in the room).
    event_to_history_vis = await _event_to_history_vis(storage, events)

    # for any with restricted vis, we also need the memberships
    event_to_memberships = await _event_to_memberships(
        storage,
        [
            e
            for e in events
            if event_to_history_vis[e.event_id]
            not in (HistoryVisibility.SHARED, HistoryVisibility.WORLD_READABLE)
        ],
        target_server_name,
    )

    to_return = []
    for e in events:
        erased = is_sender_erased(e, erased_senders)
        visible = check_event_is_visible(
            event_to_history_vis[e.event_id], event_to_memberships.get(e.event_id, {})
        )

        if e in partial_state_invisible_events:
            visible = False

        if visible and not erased:
            to_return.append(e)
        elif redact:
            to_return.append(prune_event(e))

    return to_return


async def _event_to_history_vis(
    storage: StorageControllers, events: Collection[EventBase]
) -> Dict[str, str]:
    """Get the history visibility at each of the given events

    Returns a map from event id to history_visibility setting
    """

    # outliers get special treatment here. We don't have the state at that point in the
    # room (and attempting to look it up will raise an exception), so all we can really
    # do is assume that the requesting server is allowed to see the event. That's
    # equivalent to there not being a history_visibility event, so we just exclude
    # any outliers from the query.
    event_to_state_ids = await storage.state.get_state_ids_for_events(
        frozenset(e.event_id for e in events if not e.internal_metadata.is_outlier()),
        state_filter=StateFilter.from_types(types=(_HISTORY_VIS_KEY,)),
    )

    visibility_ids = {
        vis_event_id
        for vis_event_id in (
            state_ids.get(_HISTORY_VIS_KEY) for state_ids in event_to_state_ids.values()
        )
        if vis_event_id
    }
    vis_events = await storage.main.get_events(visibility_ids)

    result: Dict[str, str] = {}
    for event in events:
        vis = HistoryVisibility.SHARED
        state_ids = event_to_state_ids.get(event.event_id)

        # if we didn't find any state for this event, it's an outlier, and we assume
        # it's open
        visibility_id = None
        if state_ids:
            visibility_id = state_ids.get(_HISTORY_VIS_KEY)

        if visibility_id:
            vis_event = vis_events[visibility_id]
            vis = vis_event.content.get("history_visibility", HistoryVisibility.SHARED)
            assert isinstance(vis, str)

        result[event.event_id] = vis
    return result


async def _event_to_memberships(
    storage: StorageControllers, events: Collection[EventBase], server_name: str
) -> Dict[str, StateMap[EventBase]]:
    """Get the remote membership list at each of the given events

    Returns a map from event id to state map, which will contain only membership events
    for the given server.
    """

    if not events:
        return {}

    # for each event, get the event_ids of the membership state at those events.
    #
    # TODO: this means that we request the entire membership list. If there  are only
    #   one or two users on this server, and the room is huge, this is very wasteful
    #   (it means more db work, and churns the *stateGroupMembersCache*).
    #   It might be that we could extend StateFilter to specify "give me keys matching
    #   *:<server_name>", to avoid this.

    event_to_state_ids = await storage.state.get_state_ids_for_events(
        frozenset(e.event_id for e in events),
        state_filter=StateFilter.from_types(types=((EventTypes.Member, None),)),
    )

    # We only want to pull out member events that correspond to the
    # server's domain.
    #
    # event_to_state_ids contains lots of duplicates, so it turns out to be
    # cheaper to build a complete event_id => (type, state_key) dict, and then
    # filter out the ones we don't want
    #
    event_id_to_state_key = {
        event_id: key
        for key_to_eid in event_to_state_ids.values()
        for key, event_id in key_to_eid.items()
    }

    def include(state_key: str) -> bool:
        # we avoid using get_domain_from_id here for efficiency.
        idx = state_key.find(":")
        if idx == -1:
            return False
        return state_key[idx + 1 :] == server_name

    event_map = await storage.main.get_events(
        [
            e_id
            for e_id, (_, state_key) in event_id_to_state_key.items()
            if include(state_key)
        ]
    )

    return {
        e_id: {
            key: event_map[inner_e_id]
            for key, inner_e_id in key_to_eid.items()
            if inner_e_id in event_map
        }
        for e_id, key_to_eid in event_to_state_ids.items()
    }