fix(crypto): Take into account pending to-device room key sharing requests when collecting devices that have already received a session

This avoids conditions where a key may be shared with a device only
after we decided that it is fine to reuse (and not rotate) the session
based on the wrong assumption that that particular device does not have
the keys.

Signed-off-by: Niklas Baumstark
[niklas.baumstark@gmail.com](mailto:niklas.baumstark@gmail.com)

Author: procr1337

crates/matrix-sdk-crypto/CHANGELOG.md

@@ -24,6 +24,10 @@ All notable changes to this project will be documented in this file.
   presence of blacklisted/withheld devices in the room.
   ([#4954](https://github.com/matrix-org/matrix-rust-sdk/pull/4954))
 
+- Fix [#2729](https://github.com/matrix-org/matrix-rust-sdk/issues/2729) which in rare
+  cases can cause room key oversharing.
+  ([#4975](https://github.com/matrix-org/matrix-rust-sdk/pull/4975))
+
 ## [0.11.0] - 2025-04-11
 
 ### Features

crates/matrix-sdk-crypto/src/olm/group_sessions/outbound.rs

@@ -160,7 +160,7 @@ pub struct OutboundGroupSession {
     pub(crate) shared_with_set:
         Arc<StdRwLock<BTreeMap<OwnedUserId, BTreeMap<OwnedDeviceId, ShareInfo>>>>,
     #[allow(clippy::type_complexity)]
-    to_share_with_set:
+    pub(crate) to_share_with_set:
         Arc<StdRwLock<BTreeMap<OwnedTransactionId, (Arc<ToDeviceRequest>, ShareInfoSet)>>>,
 }
 

crates/matrix-sdk-crypto/src/session_manager/group_sessions/share_strategy.rs

@@ -427,22 +427,55 @@ fn is_session_overshared_for_user(
     let recipient_device_ids: BTreeSet<&DeviceId> =
         recipient_devices.iter().map(|d| d.device_id()).collect();
 
+    let mut shared: Vec<&DeviceId> = Vec::new();
+
+    // This duplicates a conservative subset of the logic in
+    // `OutboundGroupSession::is_shared_with`, because we
+    // don't have corresponding DeviceData at hand
+    fn is_actually_shared(info: &ShareInfo) -> bool {
+        match info {
+            ShareInfo::Shared(_) => true,
+            ShareInfo::Withheld(_) => false,
+        }
+    }
+
+    // Collect the devices that have definitely received the session already
     let guard = outbound_session.shared_with_set.read();
+    if let Some(for_user) = guard.get(user_id) {
+        shared.extend(for_user.iter().filter_map(|(d, info)| {
+            if is_actually_shared(info) {
+                Some(AsRef::<DeviceId>::as_ref(d))
+            } else {
+                None
+            }
+        }));
+    }
+
+    // To be conservative, also collect the devices that would still receive the
+    // session from a pending to-device request if we don't rotate beforehand
+    let guard = outbound_session.to_share_with_set.read();
+    for (_txid, share_infos) in guard.values() {
+        if let Some(for_user) = share_infos.get(user_id) {
+            shared.extend(for_user.iter().filter_map(|(d, info)| {
+                if is_actually_shared(info) {
+                    Some(AsRef::<DeviceId>::as_ref(d))
+                } else {
+                    None
+                }
+            }));
+        }
+    }
 
-    let Some(shared) = guard.get(user_id) else {
+    if shared.is_empty() {
         return false;
-    };
+    }
 
-    // Devices that received this session
-    let shared: BTreeSet<&DeviceId> = shared
-        .iter()
-        .filter(|(_, info)| matches!(info, ShareInfo::Shared(_)))
-        .map(|(d, _)| d.as_ref())
-        .collect();
+    let shared: BTreeSet<&DeviceId> = shared.into_iter().collect();
 
     // The set difference between
     //
-    // 1. Devices that had previously received the session, and
+    // 1. Devices that had previously received (or are queued to receive) the
+    //    session, and
     // 2. Devices that would now receive the session
     //
     // Represents newly deleted or blacklisted devices. If this
@@ -729,17 +762,21 @@ mod tests {
         },
     };
     use ruma::{
-        device_id, events::room::history_visibility::HistoryVisibility, room_id, TransactionId,
+        device_id,
+        events::{dummy::ToDeviceDummyEventContent, room::history_visibility::HistoryVisibility},
+        room_id, TransactionId,
     };
     use serde_json::json;
 
     use crate::{
         error::SessionRecipientCollectionError,
-        olm::OutboundGroupSession,
+        olm::{OutboundGroupSession, ShareInfo},
         session_manager::{
             group_sessions::share_strategy::collect_session_recipients, CollectStrategy,
         },
+        store::caches::SequenceNumber,
         testing::simulate_key_query_response_for_verification,
+        types::requests::ToDeviceRequest,
         CrossSigningKeyExport, EncryptionSettings, LocalTrust, OlmError, OlmMachine,
     };
 
@@ -2136,6 +2173,61 @@ mod tests {
         assert!(share_result.should_rotate);
     }
 
+    /// Test that the session is rotated if a devices has a pending
+    /// to-device request that would share the keys with it.
+    #[async_test]
+    async fn test_should_rotate_based_on_device_with_pending_request_excluded() {
+        let machine = test_machine().await;
+        import_known_users_to_test_machine(&machine).await;
+
+        let encryption_settings = all_devices_strategy_settings();
+        let group_session = create_test_outbound_group_session(&machine, &encryption_settings);
+        let sender_key = machine.identity_keys().curve25519;
+
+        let dan_user = KeyDistributionTestData::dan_id();
+        let dan_dev1 = KeyDistributionTestData::dan_signed_device_id();
+        let dan_dev2 = KeyDistributionTestData::dan_unsigned_device_id();
+
+        // Share the session with device 1
+        group_session.mark_shared_with(dan_user, dan_dev1, sender_key).await;
+
+        {
+            // Add a pending request to share with device 2
+            let share_infos = BTreeMap::from([(
+                dan_user.to_owned(),
+                BTreeMap::from([(
+                    dan_dev2.to_owned(),
+                    ShareInfo::new_shared(sender_key, 0, SequenceNumber::default()),
+                )]),
+            )]);
+
+            let txid = TransactionId::new();
+            let req = Arc::new(ToDeviceRequest::for_recipients(
+                dan_user,
+                vec![dan_dev2.to_owned()],
+                &ruma::events::AnyToDeviceEventContent::Dummy(ToDeviceDummyEventContent),
+                txid.clone(),
+            ));
+            group_session.add_request(txid, req, share_infos);
+        }
+
+        // Remove device 2
+        let keys_query = KeyDistributionTestData::dan_keys_query_response_device_loggedout();
+        machine.mark_request_as_sent(&TransactionId::new(), &keys_query).await.unwrap();
+
+        // Share again
+        let share_result = collect_session_recipients(
+            machine.store(),
+            vec![KeyDistributionTestData::dan_id()].into_iter(),
+            &encryption_settings,
+            &group_session,
+        )
+        .await
+        .unwrap();
+
+        assert!(share_result.should_rotate);
+    }
+
     /// Test that the session is not rotated if a devices is removed
     /// but was already withheld from receiving the session.
     #[async_test]