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Don't send empty fragments of nonempty handshake messages

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This for example lead to the following corner case bug:
The code attempted to piggy-back a Finished message at
the end of a datagram where precisely 12 bytes of payload
were still available. This lead to an empty Finished fragment
being sent, and when  mbedtls_ssl_flight_transmit() was called
again, it believed that it was just starting to send the
Finished message, thereby calling ssl_swap_epochs() which
had already happened in the call sending the empty fragment.
Therefore, the second call would send the 'rest' of the
Finished message with wrong epoch.
This commit is contained in:
Hanno Becker 2018-08-17 16:47:58 +01:00
parent 4a9d006f5f
commit e1dcb03557
1 changed files with 7 additions and 8 deletions
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15
library/ssl_tls.c
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@ -2923,15 +2923,17 @@ int mbedtls_ssl_flight_transmit( mbedtls_ssl_context *ssl )
size_t max_frag_len;
const mbedtls_ssl_flight_item * const cur = ssl->handshake->cur_msg;
int const is_finished =
( cur->type == MBEDTLS_SSL_MSG_HANDSHAKE &&
cur->p[0] == MBEDTLS_SSL_HS_FINISHED );
uint8_t const force_flush = ssl->disable_datagram_packing == 1 ?
SSL_FORCE_FLUSH : SSL_DONT_FORCE_FLUSH;
/* Swap epochs before sending Finished: we can't do it after
* sending ChangeCipherSpec, in case write returns WANT_READ.
* Must be done before copying, may change out_msg pointer */
if( cur->type == MBEDTLS_SSL_MSG_HANDSHAKE &&
cur->p[0] == MBEDTLS_SSL_HS_FINISHED &&
ssl->handshake->cur_msg_p == ( cur->p + 12 ) )
if( is_finished && ssl->handshake->cur_msg_p == ( cur->p + 12 ) )
{
MBEDTLS_SSL_DEBUG_MSG( 2, ( "swap epochs to send finished message" ) );
ssl_swap_epochs( ssl );
@ -2968,13 +2970,10 @@ int mbedtls_ssl_flight_transmit( mbedtls_ssl_context *ssl )
const size_t rem_len = hs_len - frag_off;
size_t cur_hs_frag_len, max_hs_frag_len;
if( max_frag_len < 12 )
if( ( max_frag_len < 12 ) || ( max_frag_len == 12 && hs_len != 0 ) )
{
if( cur->type == MBEDTLS_SSL_MSG_HANDSHAKE &&
cur->p[0] == MBEDTLS_SSL_HS_FINISHED )
{
if( is_finished )
ssl_swap_epochs( ssl );
}
if( ( ret = mbedtls_ssl_flush_output( ssl ) ) != 0 )
return( ret );