Modified semantics of mbedtls_ssl_{get,set}_session()

This affects users who call mbedtls_ssl_get_session() or mbedtls_ssl_set_session() multiple times on the same SSL context representing an established TLS 1.2 connection. Those users will now observe the second call to fail with MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE.

Migration path:

• Exporting the same TLS 1.2 connection multiple times via mbedtls_ssl_get_session() leads to multiple copies of the same session. This use of mbedtls_ssl_get_session() is discouraged, and the following should be considered:
  • If the various session copies are later loaded into fresh SSL contexts via mbedtls_ssl_set_session(), export via mbedtls_ssl_get_session() only once and load the same session into different contexts via mbedtls_ssl_set_session(). Since mbedtls_ssl_set_session() makes a copy of the session that's being loaded, this is functionally equivalent.
  • If the various session copies are later serialized via mbedtls_ssl_session_save(), export and serialize the session only once via mbedtls_ssl_get_session() and mbedtls_ssl_session_save() and make copies of the raw data instead.
• Calling mbedtls_ssl_set_session() multiple times in Mbed TLS 2.x is not useful since subsequent calls overwrite the effect of previous calls. Applications achieve equivalent functional behaviour by issuing only the very last call to mbedtls_ssl_set_session().