library/ssl_cache.c

/*
 *  SSL session cache implementation
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0
 *
 *  Licensed under the Apache License, Version 2.0 (the "License"); you may
 *  not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
/*
 * These session callbacks use a simple chained list
 * to store and retrieve the session information.
 */

#include "common.h"

#if defined(MBEDTLS_SSL_CACHE_C)

#include "mbedtls/platform.h"

#include "mbedtls/ssl_cache.h"
#include "ssl_misc.h"

#include <string.h>

void mbedtls_ssl_cache_init( mbedtls_ssl_cache_context *cache )
{
    memset( cache, 0, sizeof( mbedtls_ssl_cache_context ) );

    cache->timeout = MBEDTLS_SSL_CACHE_DEFAULT_TIMEOUT;
    cache->max_entries = MBEDTLS_SSL_CACHE_DEFAULT_MAX_ENTRIES;

#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_init( &cache->mutex );
#endif
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_cache_find_entry( mbedtls_ssl_cache_context *cache,
                                 unsigned char const *session_id,
                                 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 );
#endif
    mbedtls_ssl_cache_entry *cur;

    for( cur = cache->chain; cur != NULL; cur = cur->next )
    {
#if defined(MBEDTLS_HAVE_TIME)
        if( cache->timeout != 0 &&
            (int) ( t - cur->timestamp ) > cache->timeout )
            continue;
#endif

        if( session_id_len != cur->session_id_len ||
            memcmp( session_id, cur->session_id,
                    cur->session_id_len ) != 0 )
        {
            continue;
        }

        break;
    }

    if( cur != NULL )
    {
        *dst = cur;
        ret = 0;
    }

    return( ret );
}


int mbedtls_ssl_cache_get( void *data,
                           unsigned char const *session_id,
                           size_t session_id_len,
                           mbedtls_ssl_session *session )
{
    int ret = 1;
    mbedtls_ssl_cache_context *cache = (mbedtls_ssl_cache_context *) data;
    mbedtls_ssl_cache_entry *entry;

#if defined(MBEDTLS_THREADING_C)
    if( mbedtls_mutex_lock( &cache->mutex ) != 0 )
        return( 1 );
#endif

    ret = ssl_cache_find_entry( cache, session_id, session_id_len, &entry );
    if( ret != 0 )
        goto exit;

    ret = mbedtls_ssl_session_load( session,
                                    entry->session,
                                    entry->session_len );
    if( ret != 0 )
        goto exit;

    ret = 0;

exit:
#if defined(MBEDTLS_THREADING_C)
    if( mbedtls_mutex_unlock( &cache->mutex ) != 0 )
        ret = 1;
#endif

    return( ret );
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_cache_pick_writing_slot( mbedtls_ssl_cache_context *cache,
                                        unsigned char const *session_id,
                                        size_t session_id_len,
                                        mbedtls_ssl_cache_entry **dst )
{
#if defined(MBEDTLS_HAVE_TIME)
    mbedtls_time_t t = mbedtls_time( NULL ), oldest = 0;
#endif /* MBEDTLS_HAVE_TIME */

    mbedtls_ssl_cache_entry *old = NULL;
    int count = 0;
    mbedtls_ssl_cache_entry *cur, *last;

    /* 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 `last` will point to the last
     * entry, both of which will be used later. */

    last = NULL;
    for( cur = cache->chain; cur != NULL; cur = cur->next )
    {
        count++;
        if( session_id_len == cur->session_id_len &&
            memcmp( session_id, cur->session_id, cur->session_id_len ) == 0 )
        {
            goto found;
        }
        last = cur;
    }

    /* 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)
    for( cur = cache->chain; cur != NULL; cur = cur->next )
    {
        if( cache->timeout != 0 &&
            (int) ( t - cur->timestamp ) > cache->timeout )
        {
            goto found;
        }

        if( oldest == 0 || cur->timestamp < oldest )
        {
            oldest = cur->timestamp;
            old = cur;
        }
    }
#endif /* MBEDTLS_HAVE_TIME */

    /* 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( last == NULL )
            cache->chain = cur;
        else
            last->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)
    if( old == NULL )
    {
        /* 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 );

    old = cache->chain;
    cache->chain = old->next;
    old->next = NULL;
    last->next = old;
#endif /* MBEDTLS_HAVE_TIME */

    /* 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;
    }

    *dst = cur;
    return( 0 );
}

int mbedtls_ssl_cache_set( void *data,
                           unsigned char const *session_id,
                           size_t session_id_len,
                           const mbedtls_ssl_session *session )
{
    int ret = 1;
    mbedtls_ssl_cache_context *cache = (mbedtls_ssl_cache_context *) data;
    mbedtls_ssl_cache_entry *cur;

    size_t session_serialized_len;
    unsigned char *session_serialized = NULL;

#if defined(MBEDTLS_THREADING_C)
    if( ( ret = mbedtls_mutex_lock( &cache->mutex ) ) != 0 )
        return( ret );
#endif

    ret = ssl_cache_pick_writing_slot( cache,
                                       session_id, session_id_len,
                                       &cur );
    if( ret != 0 )
        goto exit;

    /* Check how much space we need to serialize the session
     * and allocate a sufficiently large buffer. */
    ret = mbedtls_ssl_session_save( session, NULL, 0, &session_serialized_len );
    if( ret != MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL )
    {
        ret = 1;
        goto exit;
    }

    session_serialized = mbedtls_calloc( 1, session_serialized_len );
    if( session_serialized == NULL )
    {
        ret = MBEDTLS_ERR_SSL_ALLOC_FAILED;
        goto exit;
    }

    /* Now serialize the session into the allocated buffer. */
    ret = mbedtls_ssl_session_save( session,
                                    session_serialized,
                                    session_serialized_len,
                                    &session_serialized_len );
    if( ret != 0 )
        goto exit;

    if( session_id_len > sizeof( cur->session_id ) )
    {
        ret = 1;
        goto exit;
    }
    cur->session_id_len = session_id_len;
    memcpy( cur->session_id, session_id, session_id_len );

    cur->session = session_serialized;
    cur->session_len = session_serialized_len;
    session_serialized = NULL;

    ret = 0;

exit:
#if defined(MBEDTLS_THREADING_C)
    if( mbedtls_mutex_unlock( &cache->mutex ) != 0 )
        ret = 1;
#endif

    if( session_serialized != NULL )
    {
        mbedtls_platform_zeroize( session_serialized, session_serialized_len );
        mbedtls_free( session_serialized );
        session_serialized = NULL;
    }

    return( ret );
}

#if defined(MBEDTLS_HAVE_TIME)
void mbedtls_ssl_cache_set_timeout( mbedtls_ssl_cache_context *cache, int timeout )
{
    if( timeout < 0 ) timeout = 0;

    cache->timeout = timeout;
}
#endif /* MBEDTLS_HAVE_TIME */

void mbedtls_ssl_cache_set_max_entries( mbedtls_ssl_cache_context *cache, int max )
{
    if( max < 0 ) max = 0;

    cache->max_entries = max;
}

void mbedtls_ssl_cache_free( mbedtls_ssl_cache_context *cache )
{
    mbedtls_ssl_cache_entry *cur, *prv;

    cur = cache->chain;

    while( cur != NULL )
    {
        prv = cur;
        cur = cur->next;

        mbedtls_free( prv->session );
        mbedtls_free( prv );
    }

#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_free( &cache->mutex );
#endif
    cache->chain = NULL;
}

#endif /* MBEDTLS_SSL_CACHE_C */