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Don't store the peer CRT chain twice during renegotiation

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Context: During a handshake, the SSL/TLS handshake logic constructs
an instance of ::mbedtls_ssl_session representing the SSL session
being established. This structure contains information such as the
session's master secret, the peer certificate, or the session ticket
issues by the server (if applicable).
During a renegotiation, the new session is constructed aside the existing
one and destroys and replaces the latter only when the renegotiation is
complete. While conceptually clear, this means that during the renegotiation,
large pieces of information such as the peer's CRT or the session ticket
exist twice in memory, even though the original versions are removed
eventually.

This commit removes the simultaneous presence of two peer CRT chains
in memory during renegotiation, in the following way:
- Unlike in the case of SessionTickets handled in the previous commit,
  we cannot simply free the peer's CRT chain from the previous handshake
  before parsing the new one, as we need to verify that the peer's end-CRT
  hasn't changed to mitigate the 'Triple Handshake Attack'.
- Instead, we perform a binary comparison of the original peer end-CRT
  with the one presented during renegotiation, and if it succeeds, we
  avoid re-parsing CRT by moving the corresponding CRT pointer from the
  old to the new session structure.
- The remaining CRTs in the peer's chain are not affected by the triple
  handshake attack protection, and for them we may employ the canonical
  approach of freeing them before parsing the remainder of the new chain.

Note that this commit intends to not change any observable behavior
of the stack. In particular:
- The peer's CRT chain is still verified during renegotiation.
- The tail of the peer's CRT chain may change during renegotiation.
This commit is contained in:
Hanno Becker 2019-01-30 14:46:46 +00:00
parent b2964cbe14
commit def9bdc152
1 changed files with 96 additions and 64 deletions
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160
library/ssl_tls.c
View file

@ -5435,6 +5435,21 @@ write_msg:
return( ret ); return( ret );
} }
static int ssl_check_peer_crt_unchanged( mbedtls_ssl_context *ssl,
unsigned char *crt_buf,
size_t crt_buf_len )
{
mbedtls_x509_crt const * const peer_crt = ssl->session->peer_cert;
if( peer_crt == NULL )
return( -1 );
if( peer_crt->raw.len != crt_buf_len )
return( -1 );
return( memcmp( peer_crt->raw.p, crt_buf, crt_buf_len) );
}
/* /*
* Once the certificate message is read, parse it into a cert chain and * Once the certificate message is read, parse it into a cert chain and
* perform basic checks, but leave actual verification to the caller * perform basic checks, but leave actual verification to the caller
@ -5525,35 +5540,29 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl )
return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE ); return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
} }
/* Make &ssl->in_msg[i] point to the beginning of the CRT chain. */
i += 3;
/* In case we tried to reuse a session but it failed */ /* In case we tried to reuse a session but it failed */
if( ssl->session_negotiate->peer_cert != NULL ) if( ssl->session_negotiate->peer_cert != NULL )
{ {
mbedtls_x509_crt_free( ssl->session_negotiate->peer_cert ); mbedtls_x509_crt_free( ssl->session_negotiate->peer_cert );
mbedtls_free( ssl->session_negotiate->peer_cert ); mbedtls_free( ssl->session_negotiate->peer_cert );
ssl->session_negotiate->peer_cert = NULL;
} }
if( ( ssl->session_negotiate->peer_cert = mbedtls_calloc( 1, /* Iterate through and parse the CRTs in the provided chain. */
sizeof( mbedtls_x509_crt ) ) ) == NULL )
{
MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%d bytes) failed",
sizeof( mbedtls_x509_crt ) ) );
mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
}
mbedtls_x509_crt_init( ssl->session_negotiate->peer_cert );
i += 3;
while( i < ssl->in_hslen ) while( i < ssl->in_hslen )
{ {
/* Check that there's room for the next CRT's length fields. */
if ( i + 3 > ssl->in_hslen ) { if ( i + 3 > ssl->in_hslen ) {
MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) ); MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL, mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR ); MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE ); return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
} }
/* In theory, the CRT can be up to 2**24 Bytes, but we don't support
* anything beyond 2**16 ~ 64K. */
if( ssl->in_msg[i] != 0 ) if( ssl->in_msg[i] != 0 )
{ {
MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) ); MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
@ -5562,6 +5571,7 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl )
return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE ); return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
} }
/* Read length of the next CRT in the chain. */
n = ( (unsigned int) ssl->in_msg[i + 1] << 8 ) n = ( (unsigned int) ssl->in_msg[i + 1] << 8 )
| (unsigned int) ssl->in_msg[i + 2]; | (unsigned int) ssl->in_msg[i + 2];
i += 3; i += 3;
@ -5574,67 +5584,89 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl )
return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE ); return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
} }
/* Check if we're handling the first CRT in the chain. */
if( ssl->session_negotiate->peer_cert == NULL )
{
/* During client-side renegotiation, check the server's end-CRTs
* hasn't changed compared to the initial handshake, mitigating
* the triple handshake attack. On success, reuse the original
* end-CRT instead of parsing it again. */
#if defined(MBEDTLS_SSL_RENEGOTIATION) && defined(MBEDTLS_SSL_CLI_C)
if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT &&
ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS )
{
MBEDTLS_SSL_DEBUG_MSG( 3, ( "Check that peer CRT hasn't changed during renegotiation" ) );
if( ssl_check_peer_crt_unchanged( ssl, &ssl->in_msg[i], n ) != 0 )
{
MBEDTLS_SSL_DEBUG_MSG( 1, ( "new server cert during renegotiation" ) );
mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
}
/* Move CRT chain structure to new session instance. */
ssl->session_negotiate->peer_cert = ssl->session->peer_cert;
ssl->session->peer_cert = NULL;
/* Delete all remaining CRTs from the original CRT chain. */
mbedtls_x509_crt_free( ssl->session_negotiate->peer_cert->next );
ssl->session_negotiate->peer_cert->next = NULL;
i += n;
continue;
}
#endif /* MBEDTLS_SSL_RENEGOTIATION && MBEDTLS_SSL_CLI_C */
/* Outside of client-side renegotiation, create a fresh X.509 CRT
* instance to parse the end-CRT into. */
ssl->session_negotiate->peer_cert =
mbedtls_calloc( 1, sizeof( mbedtls_x509_crt ) );
if( ssl->session_negotiate->peer_cert == NULL )
{
MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%d bytes) failed",
sizeof( mbedtls_x509_crt ) ) );
mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
}
mbedtls_x509_crt_init( ssl->session_negotiate->peer_cert );
/* Intentional fall through */
}
/* Parse the next certificate in the chain. */
ret = mbedtls_x509_crt_parse_der( ssl->session_negotiate->peer_cert, ret = mbedtls_x509_crt_parse_der( ssl->session_negotiate->peer_cert,
ssl->in_msg + i, n ); ssl->in_msg + i, n );
switch( ret ) switch( ret )
{ {
case 0: /*ok*/ case 0: /*ok*/
case MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG + MBEDTLS_ERR_OID_NOT_FOUND: case MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG + MBEDTLS_ERR_OID_NOT_FOUND:
/* Ignore certificate with an unknown algorithm: maybe a /* Ignore certificate with an unknown algorithm: maybe a
prior certificate was already trusted. */ prior certificate was already trusted. */
break; break;
case MBEDTLS_ERR_X509_ALLOC_FAILED: case MBEDTLS_ERR_X509_ALLOC_FAILED:
alert = MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR; alert = MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR;
goto crt_parse_der_failed; goto crt_parse_der_failed;
case MBEDTLS_ERR_X509_UNKNOWN_VERSION: case MBEDTLS_ERR_X509_UNKNOWN_VERSION:
alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT; alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
goto crt_parse_der_failed; goto crt_parse_der_failed;
default: default:
alert = MBEDTLS_SSL_ALERT_MSG_BAD_CERT; alert = MBEDTLS_SSL_ALERT_MSG_BAD_CERT;
crt_parse_der_failed: crt_parse_der_failed:
mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL, alert ); mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL, alert );
MBEDTLS_SSL_DEBUG_RET( 1, " mbedtls_x509_crt_parse_der", ret ); MBEDTLS_SSL_DEBUG_RET( 1, " mbedtls_x509_crt_parse_der", ret );
return( ret ); return( ret );
} }
i += n; i += n;
} }
MBEDTLS_SSL_DEBUG_CRT( 3, "peer certificate", ssl->session_negotiate->peer_cert ); MBEDTLS_SSL_DEBUG_CRT( 3, "peer certificate", ssl->session_negotiate->peer_cert );
/*
* On client, make sure the server cert doesn't change during renego to
* avoid "triple handshake" attack: https://secure-resumption.com/
*/
#if defined(MBEDTLS_SSL_RENEGOTIATION) && defined(MBEDTLS_SSL_CLI_C)
if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT &&
ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS )
{
if( ssl->session->peer_cert == NULL )
{
MBEDTLS_SSL_DEBUG_MSG( 1, ( "new server cert during renegotiation" ) );
mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
}
if( ssl->session->peer_cert->raw.len !=
ssl->session_negotiate->peer_cert->raw.len ||
memcmp( ssl->session->peer_cert->raw.p,
ssl->session_negotiate->peer_cert->raw.p,
ssl->session->peer_cert->raw.len ) != 0 )
{
MBEDTLS_SSL_DEBUG_MSG( 1, ( "server cert changed during renegotiation" ) );
mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
}
}
#endif /* MBEDTLS_SSL_RENEGOTIATION && MBEDTLS_SSL_CLI_C */
return( 0 ); return( 0 );
} }