SSL: optimized rotation of session ticket keys.

Instead of syncing keys with shared memory on each ticket operation,
the code now does this only when the worker is going to change expiration
of the current key, or going to switch to a new key: that is, usually
at most once per second.

To do so without races, the code maintains 3 keys: current, previous,
and next.  If a worker will switch to the next key earlier, other workers
will still be able to decrypt new tickets, since they will be encrypted
with the next key.

src/event/ngx_event_openssl.c

@@ -3773,6 +3773,7 @@ ngx_ssl_session_cache_init(ngx_shm_zone_t *shm_zone, void *data)
 
     cache->ticket_keys[0].expire = 0;
     cache->ticket_keys[1].expire = 0;
+    cache->ticket_keys[2].expire = 0;
 
     cache->fail_time = 0;
 
@@ -4250,7 +4251,7 @@ ngx_ssl_session_ticket_keys(ngx_conf_t *cf, ngx_ssl_t *ssl, ngx_array_t *paths)
         return NGX_OK;
     }
 
-    keys = ngx_array_create(cf->pool, paths ? paths->nelts : 2,
+    keys = ngx_array_create(cf->pool, paths ? paths->nelts : 3,
                             sizeof(ngx_ssl_ticket_key_t));
     if (keys == NULL) {
         return NGX_ERROR;
@@ -4285,9 +4286,13 @@ ngx_ssl_session_ticket_keys(ngx_conf_t *cf, ngx_ssl_t *ssl, ngx_array_t *paths)
 
         /* placeholder for keys in shared memory */
 
-        key = ngx_array_push_n(keys, 2);
+        key = ngx_array_push_n(keys, 3);
         key[0].shared = 1;
+        key[0].expire = 0;
         key[1].shared = 1;
+        key[1].expire = 0;
+        key[2].shared = 1;
+        key[2].expire = 0;
 
         return NGX_OK;
     }
@@ -4347,6 +4352,7 @@ ngx_ssl_session_ticket_keys(ngx_conf_t *cf, ngx_ssl_t *ssl, ngx_array_t *paths)
         }
 
         key->shared = 0;
+        key->expire = 1;
 
         if (size == 48) {
             key->size = 48;
@@ -4514,7 +4520,7 @@ ngx_ssl_ticket_key_callback(ngx_ssl_conn_t *ssl_conn,
 
         /* renew if non-default key */
 
-        if (i != 0) {
+        if (i != 0 && key[i].expire) {
             return 2;
         }
 
@@ -4545,9 +4551,21 @@ ngx_ssl_rotate_ticket_keys(SSL_CTX *ssl_ctx, ngx_log_t *log)
         return NGX_OK;
     }
 
+    /*
+     * if we don't need to update expiration of the current key
+     * and the previous key is still needed, don't sync with shared
+     * memory to save some work; in the worst case other worker process
+     * will switch to the next key, but this process will still be able
+     * to decrypt tickets encrypted with it
+     */
+
     now = ngx_time();
     expire = now + SSL_CTX_get_timeout(ssl_ctx);
 
+    if (key[0].expire >= expire && key[1].expire >= now) {
+        return NGX_OK;
+    }
+
     shm_zone = SSL_CTX_get_ex_data(ssl_ctx, ngx_ssl_session_cache_index);
 
     cache = shm_zone->data;
@@ -4567,47 +4585,57 @@ ngx_ssl_rotate_ticket_keys(SSL_CTX *ssl_ctx, ngx_log_t *log)
             return NGX_ERROR;
         }
 
-        key->shared = 1;
-        key->expire = expire;
-        key->size = 80;
-        ngx_memcpy(key->name, buf, 16);
-        ngx_memcpy(key->hmac_key, buf + 16, 32);
-        ngx_memcpy(key->aes_key, buf + 48, 32);
+        key[0].shared = 1;
+        key[0].expire = expire;
+        key[0].size = 80;
+        ngx_memcpy(key[0].name, buf, 16);
+        ngx_memcpy(key[0].hmac_key, buf + 16, 32);
+        ngx_memcpy(key[0].aes_key, buf + 48, 32);
 
         ngx_explicit_memzero(&buf, 80);
 
         ngx_log_debug2(NGX_LOG_DEBUG_EVENT, log, 0,
                        "ssl ticket key: \"%*xs\"",
-                       (size_t) 16, key->name);
+                       (size_t) 16, key[0].name);
+
+        /*
+         * copy the current key to the next key, as initialization of
+         * the previous key will replace the current key with the next
+         * key
+         */
+
+        key[2] = key[0];
     }
 
     if (key[1].expire < now) {
 
         /*
          * if the previous key is no longer needed (or not initialized),
-         * replace it with the current key and generate new current key
+         * replace it with the current key, replace the current key with
+         * the next key, and generate new next key
          */
 
         key[1] = key[0];
+        key[0] = key[2];
 
         if (RAND_bytes(buf, 80) != 1) {
             ngx_ssl_error(NGX_LOG_ALERT, log, 0, "RAND_bytes() failed");
             ngx_shmtx_unlock(&shpool->mutex);
             return NGX_ERROR;
         }
 
-        key->shared = 1;
-        key->expire = expire;
-        key->size = 80;
-        ngx_memcpy(key->name, buf, 16);
-        ngx_memcpy(key->hmac_key, buf + 16, 32);
-        ngx_memcpy(key->aes_key, buf + 48, 32);
+        key[2].shared = 1;
+        key[2].expire = 0;
+        key[2].size = 80;
+        ngx_memcpy(key[2].name, buf, 16);
+        ngx_memcpy(key[2].hmac_key, buf + 16, 32);
+        ngx_memcpy(key[2].aes_key, buf + 48, 32);
 
         ngx_explicit_memzero(&buf, 80);
 
         ngx_log_debug2(NGX_LOG_DEBUG_EVENT, log, 0,
                        "ssl ticket key: \"%*xs\"",
-                       (size_t) 16, key->name);
+                       (size_t) 16, key[2].name);
     }
 
     /*

src/event/ngx_event_openssl.h

@@ -160,7 +160,7 @@ typedef struct {
     ngx_rbtree_t                session_rbtree;
     ngx_rbtree_node_t           sentinel;
     ngx_queue_t                 expire_queue;
-    ngx_ssl_ticket_key_t        ticket_keys[2];
+    ngx_ssl_ticket_key_t        ticket_keys[3];
     time_t                      fail_time;
 } ngx_ssl_session_cache_t;