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Implement 3.0-semantics for mbedtls_ssl_{get,set}_session()

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mbedtls_ssl_{get,set}_session() exhibited idempotent behaviour
in Mbed TLS 2.x. Multiple calls to those functions are not useful
in TLS 1.2, and the idempotent nature is unsuitable for support of
TLS 1.3 which introduces the availabilty to offer multiple tickets
for resumption, as well as receive multiple tickets.

In preparation for TLS 1.3 support, this commit relaxes the semantics
of `mbedtls_ssl_{get,set}_session()` by allowing implementations to
fail gracefully, and leveraging this freedom by modifying the
existing TLS 1.2 implementation to only accept one call to
`mbedtls_ssl_{get,set}_session()` per context, and non-fatally
failing all subsequent invocations.

For TLS 1.3, it will be leveraged by making multiple calls to
`mbedtls_ssl_get_session()` issue one ticket a time until no more
tickets are available, and by using multiple calls to
`mbedtls_ssl_set_session()` to allow the client to offer multiple
tickets to the server.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker
2021-05-14 16:01:05 +01:00
parent 494dc71de8
commit e810bbc1ac
2 changed files with 95 additions and 28 deletions
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27
library/ssl_tls.c
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@ -3788,6 +3788,9 @@ int mbedtls_ssl_set_session( mbedtls_ssl_context *ssl, const mbedtls_ssl_session
return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
}
if( ssl->handshake->resume == 1 )
return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
if( ( ret = mbedtls_ssl_session_copy( ssl->session_negotiate,
session ) ) != 0 )
return( ret );
@ -4789,6 +4792,8 @@ const mbedtls_x509_crt *mbedtls_ssl_get_peer_cert( const mbedtls_ssl_context *ss
int mbedtls_ssl_get_session( const mbedtls_ssl_context *ssl,
mbedtls_ssl_session *dst )
{
int ret;
if( ssl == NULL ||
dst == NULL ||
ssl->session == NULL ||
@ -4797,7 +4802,27 @@ int mbedtls_ssl_get_session( const mbedtls_ssl_context *ssl,
return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
}
return( mbedtls_ssl_session_copy( dst, ssl->session ) );
/* Since Mbed TLS 3.0, mbedtls_ssl_get_session() is no longer
* idempotent: Each session can only be exported once.
*
* (This is in preparation for TLS 1.3 support where we will
* need the ability to export multiple sessions (aka tickets),
* which will be achieved by calling mbedtls_ssl_get_session()
* multiple times until it fails.)
*
* Check whether we have already exported the current session,
* and fail if so.
*/
if( ssl->session->exported == 1 )
return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
ret = mbedtls_ssl_session_copy( dst, ssl->session );
if( ret != 0 )
return( ret );
/* Remember that we've exported the session. */
ssl->session->exported = 1;
return( 0 );
}
#endif /* MBEDTLS_SSL_CLI_C */