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Jon Shallow
15b42e9faa
For any TLS implementation, it is possible to crash the coap-server by initiating (D)TLS handshakes but never completing them, thus continuing to use more RAM. Each partially completed handshake is left in the endpoint->sessions chain in a state of COAP_SESSION_STATE_HANDSHAKE and unless endpoint->max_idle_sessions is deliberately configured, these sessions will never get deleted. A cause of partially completed handshakes is when there is a PSK mismatch. Furthermore, unnecessary CPU usage is used to search down long lists held in endpoint->sessions. This fix limits the number of these sessions in a handshake state to the number defined by endpoint->max_handshake_sessions if non zero or 100 to prevent memory exhaustion. By default, endpoint->max_handshake_sessions is 0. Sessions in state COAP_SESSION_STATE_HANDSHAKE will get timed out after 30 seconds of inactivity. This does not prevent a DOS attack by creating max_handshake_sessions as a genuine new session will fail to get responded to until the number of current COAP_SESSION_STATE_HANDSHAKE sessions drop. However, the use of Cookie exchanges by the underlying TLS library prior to a new session being created and added into endpoint->sessions will filter out a lot of the DOS attacks. include/coap2/coap_session.h New default definition for COAP_DEFAULT_MAX_HANDSHAKE_SESSIONS New default definition for COAP_PARTIAL_SESSION_TIMEOUT_TICKS include/coap2/net.h Include max_handshake_sessions in coap_context_t src/coap_session.c In coap_endpoint_get_session() check the number of sessions in COAP_SESSION_STATE_HANDSHAKE and handle accordingly when limits are hit, or they need to be timed out.