HTTP/2: per-iteration stream handling limit.

To ensure that attempts to flood servers with many streams are detected
early, a limit of no more than 2 * max_concurrent_streams new streams per one
event loop iteration was introduced.  This limit is applied even if
max_concurrent_streams is not yet reached - for example, if corresponding
streams are handled synchronously or reset.

Further, refused streams are now limited to maximum of max_concurrent_streams
and 100, similarly to priority_limit initial value, providing some tolerance
to clients trying to open several streams at the connection start, yet
low tolerance to flooding attempts.

--HG--
branch : 1.3

src/http/v2/ngx_http_v2.c

@@ -347,6 +347,7 @@ ngx_http_v2_read_handler(ngx_event_t *rev)
     ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0, "http2 read handler");
 
     h2c->blocked = 1;
+    h2c->new_streams = 0;
 
     if (c->close) {
         c->close = 0;
@@ -1284,6 +1285,14 @@ ngx_http_v2_state_headers(ngx_http_v2_connection_t *h2c, u_char *pos,
         goto rst_stream;
     }
 
+    if (h2c->new_streams++ >= 2 * h2scf->concurrent_streams) {
+        ngx_log_error(NGX_LOG_INFO, h2c->connection->log, 0,
+                      "client sent too many streams at once");
+
+        status = NGX_HTTP_V2_REFUSED_STREAM;
+        goto rst_stream;
+    }
+
     if (!h2c->settings_ack
         && !(h2c->state.flags & NGX_HTTP_V2_END_STREAM_FLAG)
         && h2scf->preread_size < NGX_HTTP_V2_DEFAULT_WINDOW)
@@ -1349,6 +1358,12 @@ ngx_http_v2_state_headers(ngx_http_v2_connection_t *h2c, u_char *pos,
 
 rst_stream:
 
+    if (h2c->refused_streams++ > ngx_max(h2scf->concurrent_streams, 100)) {
+        ngx_log_error(NGX_LOG_INFO, h2c->connection->log, 0,
+                      "client sent too many refused streams");
+        return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_NO_ERROR);
+    }
+
     if (ngx_http_v2_send_rst_stream(h2c, h2c->state.sid, status) != NGX_OK) {
         return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_INTERNAL_ERROR);
     }

src/http/v2/ngx_http_v2.h

@@ -131,6 +131,8 @@ struct ngx_http_v2_connection_s {
     ngx_uint_t                       processing;
     ngx_uint_t                       frames;
     ngx_uint_t                       idle;
+    ngx_uint_t                       new_streams;
+    ngx_uint_t                       refused_streams;
     ngx_uint_t                       priority_limit;
 
     size_t                           send_window;