HTTP/2: limit the number of idle state switches.

An attack that continuously switches HTTP/2 connection between
idle and active states can result in excessive CPU usage.
This is because when a connection switches to the idle state,
all of its memory pool caches are freed.

This change limits the maximum allowed number of idle state
switches to 10 * http2_max_requests (i.e., 10000 by default).
This limits possible CPU usage in one connection, and also
imposes a limit on the maximum lifetime of a connection.

Initially reported by Gal Goldshtein from F5 Networks.

src/http/v2/ngx_http_v2.c

diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index 8089ddd..9571e71 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -4511,12 +4511,19 @@
 
 #endif
 
-    c->destroyed = 0;
-    ngx_reusable_connection(c, 0);
-
     h2scf = ngx_http_get_module_srv_conf(h2c->http_connection->conf_ctx,
                                          ngx_http_v2_module);
 
+    if (h2c->idle++ > 10 * h2scf->max_requests) {
+        ngx_log_error(NGX_LOG_INFO, h2c->connection->log, 0,
+                      "http2 flood detected");
+        ngx_http_v2_finalize_connection(h2c, NGX_HTTP_V2_NO_ERROR);
+        return;
+    }
+
+    c->destroyed = 0;
+    ngx_reusable_connection(c, 0);
+
     h2c->pool = ngx_create_pool(h2scf->pool_size, h2c->connection->log);
     if (h2c->pool == NULL) {
         ngx_http_v2_finalize_connection(h2c, NGX_HTTP_V2_INTERNAL_ERROR);

src/http/v2/ngx_http_v2.h

diff --git a/src/http/v2/ngx_http_v2.h b/src/http/v2/ngx_http_v2.h
index e8eaebb..bec2216 100644
--- a/src/http/v2/ngx_http_v2.h
+++ b/src/http/v2/ngx_http_v2.h
@@ -121,6 +121,7 @@
 
     ngx_uint_t                       processing;
     ngx_uint_t                       frames;
+    ngx_uint_t                       idle;
 
     ngx_uint_t                       pushing;
     ngx_uint_t                       concurrent_pushes;