[PATCH net-next v6 10/12] net/rds: pin the connection across RDMA-CM event handling

[Allison Henderson <achender@kernel.org>]

rds_rdma_cm_event_handler_cmn() picks the connection up from
cm_id->context, which carries no reference, and holds c_cm_lock - a
mutex that lives in the connection's path array - across the transport
callbacks.  Before this series that was already a use-after-free
whenever a callback destroyed the connection, since rds_conn_destroy()
freed it synchronously and the handler's mutex_unlock() ran on freed
memory; the one such callback, rds_ib_cm_connect_complete() on a
protocol version below 3.1, has meanwhile been switched to
rds_conn_drop() by commit f97d8c7bab78 ("rds: ib: use rds_conn_drop()
on protocol version mismatch"), which also removed the deadlock that
destroy took on c_cm_lock.

Now that a connection is freed by its last reference, the remaining
exposure is a callback that drops the last reference other than the
handler's - which has none - and the reference handed out by
rds_conn_create() to rds_ib_cm_handle_connect() and dropped at its
end.  Take a reference for the duration of the handler, and ignore the
event if the connection is already being freed: its cm_id teardown is
what stops event delivery, so an event that still arrives belongs to a
connection whose shutdown has run and whose memory is on its way out.

rds_ib_cm_handle_connect() has the mirror-image hole: a connection
whose destroy has already quiesced it sits in RDS_CONN_DOWN with no
cm_id, which is exactly the state the DOWN -> CONNECTING transition
claims.  A connect request arriving then would install a new cm_id and
QP on a connection that is only waiting for its last reference to go
away, and nothing would tear them down again.  Re-check
rds_destroy_pending() under c_cm_lock and reject the request instead.

Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Allison Henderson <achender@kernel.org>
---
 net/rds/ib_cm.c          | 11 +++++++++--
 net/rds/rdma_transport.c | 16 +++++++++++++++-
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/net/rds/ib_cm.c b/net/rds/ib_cm.c
index 786ddcb45bcb..1b5491598433 100644
--- a/net/rds/ib_cm.c
+++ b/net/rds/ib_cm.c
@@ -874,6 +874,13 @@ int rds_ib_cm_handle_connect(struct rdma_cm_id *cm_id,
 	 * see the comment above rds_queue_reconnect()
 	 */
 	mutex_lock(&conn->c_cm_lock);
+	/* A destroy that has already quiesced this conn leaves it in
+	 * RDS_CONN_DOWN with no cm_id, exactly what the transition
+	 * below would happily claim; nothing would tear the new cm_id
+	 * and QP down again before the conn is freed.  Reject instead.
+	 */
+	if (rds_destroy_pending(conn))
+		goto out;
 	if (!rds_conn_transition(conn, RDS_CONN_DOWN, RDS_CONN_CONNECTING)) {
 		if (rds_conn_state(conn) == RDS_CONN_UP) {
 			rdsdebug("incoming connect while connecting\n");
@@ -928,8 +935,8 @@ int rds_ib_cm_handle_connect(struct rdma_cm_id *cm_id,
 		mutex_unlock(&conn->c_cm_lock);
 		/* Drop the reference rds_conn_create() handed us.  The
 		 * conn stays reachable through cm_id->context without a
-		 * reference of its own for now; the CM event handler is
-		 * given one of its own by a following patch.
+		 * reference of its own; rds_rdma_cm_event_handler_cmn()
+		 * takes one for the duration of each event it handles.
 		 */
 		rds_conn_put(conn);
 	}
diff --git a/net/rds/rdma_transport.c b/net/rds/rdma_transport.c
index b15cf316b23a..584e9867810f 100644
--- a/net/rds/rdma_transport.c
+++ b/net/rds/rdma_transport.c
@@ -63,6 +63,18 @@ static int rds_rdma_cm_event_handler_cmn(struct rdma_cm_id *cm_id,
 	if (cm_id->device->node_type == RDMA_NODE_IB_CA)
 		trans = &rds_ib_transport;
 
+	/* cm_id->context carries no reference of its own.  Pin the
+	 * connection for the duration of the handler: what the callbacks
+	 * below do may drop the last reference other than ours, and the
+	 * mutex released at out: lives in the connection's path array.
+	 * A connection already being freed gets no events handled.
+	 */
+	if (conn && !rds_conn_get_unless_zero(conn)) {
+		rdsdebug("conn %p id %p is being freed, ignoring event\n",
+			 conn, cm_id);
+		return 0;
+	}
+
 	/* Prevent shutdown from tearing down the connection
 	 * while we're executing. */
 	if (conn) {
@@ -171,8 +183,10 @@ static int rds_rdma_cm_event_handler_cmn(struct rdma_cm_id *cm_id,
 	}
 
 out:
-	if (conn)
+	if (conn) {
 		mutex_unlock(&conn->c_cm_lock);
+		rds_conn_put(conn);
+	}
 
 	rdsdebug("id %p event %u (%s) handling ret %d\n", cm_id, event->event,
 		 rdma_event_msg(event->event), ret);
-- 
2.25.1