[PATCH] HTTP/2: per-iteration stream handling limit

[Sergey Kandaurov]

> On 10 Oct 2023, at 16:29, Maxim Dounin <mdounin at mdounin.ru> wrote:
> 
> # HG changeset patch
> # User Maxim Dounin <mdounin at mdounin.ru>
> # Date 1696940019 -10800
> #      Tue Oct 10 15:13:39 2023 +0300
> # Node ID cdda286c0f1b4b10f30d4eb6a63fefb9b8708ecc
> # Parent  3db945fda515014d220151046d02f3960bcfca0a
> 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.
> 
> diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
> --- a/src/http/v2/ngx_http_v2.c
> +++ b/src/http/v2/ngx_http_v2.c
> @@ -347,6 +347,7 @@ ngx_http_v2_read_handler(ngx_event_t *re
>     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_co
>         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_co
> 
> 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);
>     }
> diff --git a/src/http/v2/ngx_http_v2.h b/src/http/v2/ngx_http_v2.h
> --- a/src/http/v2/ngx_http_v2.h
> +++ b/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;

Looks good.

-- 
Sergey Kandaurov