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Improve readability and efficiency of SSL cache reference impl

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The reference session cache implementation may end up storing multiple
sessions associated to the same session ID if the set()-call for the
second session finds an outdated cache entry prior to noticing the entry
with the matching session ID. While this logically overwrites the existing
entry since we always search the cache in order, this is at least a waste
of resources.

This commit fixes this by always checking first whether the given ID is
already present in the cache.

It also restructures the code for easier readability.

Fixes #4509.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2021-05-13 07:05:07 +01:00
parent f47199da26
commit 845ceb7cc8
1 changed files with 83 additions and 85 deletions
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168
library/ssl_cache.c
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@ -129,7 +129,6 @@ static int ssl_cache_pick_writing_slot( mbedtls_ssl_cache_context *cache,
size_t session_id_len,
mbedtls_ssl_cache_entry **dst )
{
int ret = 1;
#if defined(MBEDTLS_HAVE_TIME)
mbedtls_time_t t = mbedtls_time( NULL ), oldest = 0;
mbedtls_ssl_cache_entry *old = NULL;
@ -140,117 +139,116 @@ static int ssl_cache_pick_writing_slot( mbedtls_ssl_cache_context *cache,
cur = cache->chain;
prv = NULL;
/* Check 1: Is there already an entry with the given session ID?
*
* If yes, overwrite it.
*
* If not, `count` will hold the size of the session cache
* at the end of this loop, and `prv` will point to the last
* entry, both of which will be used later. */
while( cur != NULL )
{
count++;
#if defined(MBEDTLS_HAVE_TIME)
if( cache->timeout != 0 &&
(int) ( t - cur->timestamp ) > cache->timeout )
{
cur->timestamp = t;
break; /* expired, reuse this slot, update timestamp */
}
#endif
if( session_id_len == cur->session_id_len &&
memcmp( session_id, cur->session_id, cur->session_id_len ) == 0 )
{
break; /* client reconnected, keep timestamp for session id */
goto found;
}
#if defined(MBEDTLS_HAVE_TIME)
if( oldest == 0 || cur->timestamp < oldest )
{
oldest = cur->timestamp;
old = cur;
}
#endif
prv = cur;
cur = cur->next;
}
if( cur == NULL )
{
/* Check 2: Is there an outdated entry in the cache?
*
* If so, overwrite it.
*
* If not, remember the oldest entry in `old` for later.
*/
#if defined(MBEDTLS_HAVE_TIME)
/*
* Reuse oldest entry if max_entries reached
*/
if( count >= cache->max_entries )
while( cur != NULL )
{
if( cache->timeout != 0 &&
(int) ( t - cur->timestamp ) > cache->timeout )
{
if( old == NULL )
{
ret = 1;
goto exit;
}
cur = old;
goto found;
}
#else /* MBEDTLS_HAVE_TIME */
/*
* Reuse first entry in chain if max_entries reached,
* but move to last place
*/
if( count >= cache->max_entries )
if( oldest == 0 || cur->timestamp < oldest )
{
if( cache->chain == NULL )
{
ret = 1;
goto exit;
}
cur = cache->chain;
cache->chain = cur->next;
cur->next = NULL;
prv->next = cur;
oldest = cur->timestamp;
old = cur;
}
prv = cur;
cur = cur->next;
}
#endif /* MBEDTLS_HAVE_TIME */
else
{
/*
* max_entries not reached, create new entry
*/
cur = mbedtls_calloc( 1, sizeof(mbedtls_ssl_cache_entry) );
if( cur == NULL )
{
ret = 1;
goto exit;
}
if( prv == NULL )
cache->chain = cur;
else
prv->next = cur;
}
/* Check 3: Is there free space in the cache? */
if( count < cache->max_entries )
{
/* Create new entry */
cur = mbedtls_calloc( 1, sizeof(mbedtls_ssl_cache_entry) );
if( cur == NULL )
return( 1 );
/* Append to the end of the linked list. */
if( prv == NULL )
cache->chain = cur;
else
prv->next = cur;
goto found;
}
/* Last resort: The cache is full and doesn't contain any outdated
* elements. In this case, we evict the oldest one, judged by timestamp
* (if present) or cache-order. */
#if defined(MBEDTLS_HAVE_TIME)
cur->timestamp = t;
#endif
}
if( cur != NULL )
if( old == NULL )
{
*dst = cur;
/* This should only happen on an ill-configured cache
* with max_entries == 0. */
return( 1 );
}
#else /* MBEDTLS_HAVE_TIME */
/* Reuse first entry in chain, but move to last place. */
if( cache->chain == NULL )
return( 1 );
/*
* If we're reusing an entry, free it first.
*/
if( cur->session != NULL )
{
mbedtls_free( cur->session );
cur->session = NULL;
cur->session_len = 0;
memset( cur->session_id, 0, sizeof( cur->session_id ) );
cur->session_id_len = 0;
}
old = cache->chain;
cache->chain = old->next;
cur->next = NULL;
prv->next = old;
#endif /* MBEDTLS_HAVE_TIME */
ret = 0;
/* Now `old` points to the oldest entry to be overwritten. */
cur = old;
found:
#if defined(MBEDTLS_HAVE_TIME)
cur->timestamp = t;
#endif
/* If we're reusing an entry, free it first. */
if( cur->session != NULL )
{
mbedtls_free( cur->session );
cur->session = NULL;
cur->session_len = 0;
memset( cur->session_id, 0, sizeof( cur->session_id ) );
cur->session_id_len = 0;
}
exit:
return( ret );
*dst = cur;
return( 0 );
}
int mbedtls_ssl_cache_set( void *data,