--- /dev/null
+commit d07b601a5549798f8e500582336756e04dfd25c5
+Author: Remi Gacogne <remi.gacogne@powerdns.com>
+Date: Tue Oct 10 15:47:57 2023 +0200
+
+ [http2] delay processing requests upon observing suspicious behavior
+
+ Backport of 94fbc54b6c9309912fe3d53e7b63408bbe9a1b0d to v2.2.x
+
+--- a/include/h2o.h
++++ b/include/h2o.h
+@@ -378,6 +378,10 @@ struct st_h2o_globalconf_t {
+ * list of callbacks
+ */
+ h2o_protocol_callbacks_t callbacks;
++ /**
++ * milliseconds to delay processing requests when suspicious behavior is detected
++ */
++ uint64_t dos_delay;
+ } http2;
+
+ struct {
+@@ -590,6 +594,10 @@ struct st_h2o_context_t {
+ * timeout entry used for graceful shutdown
+ */
+ h2o_timeout_entry_t _graceful_shutdown_timeout;
++ /*
++ * dos timeout
++ */
++ h2o_timeout_t dos_delay_timeout;
+ struct {
+ /**
+ * counter for http2 errors internally emitted by h2o
+--- a/include/h2o/http2_internal.h
++++ b/include/h2o/http2_internal.h
+@@ -179,6 +179,7 @@ struct st_h2o_http2_stream_t {
+ h2o_linklist_t link;
+ h2o_http2_scheduler_openref_t scheduler;
+ } _refs;
++ unsigned reset_by_peer : 1;
+ h2o_send_state_t send_state; /* state of the ostream, only used in push mode */
+ /* placed at last since it is large and has it's own ctor */
+ h2o_req_t req;
+@@ -232,6 +233,13 @@ struct st_h2o_http2_conn_t {
+ } _write;
+ h2o_cache_t *push_memo;
+ h2o_http2_casper_t *casper;
++ /**
++ * DoS mitigation; the idea here is to delay processing requests when observing suspicious behavior
++ */
++ struct {
++ h2o_timeout_entry_t process_delay;
++ size_t reset_budget; /* RST_STREAM frames are considered suspicious when this value goes down to zero */
++ } dos_mitigation;
+ };
+
+ int h2o_http2_update_peer_settings(h2o_http2_settings_t *settings, const uint8_t *src, size_t len, const char **err_desc);
+--- a/lib/core/config.c
++++ b/lib/core/config.c
+@@ -189,6 +189,7 @@ void h2o_config_init(h2o_globalconf_t *c
+ config->http2.latency_optimization.min_rtt = 50; // milliseconds
+ config->http2.latency_optimization.max_additional_delay = 10;
+ config->http2.latency_optimization.max_cwnd = 65535;
++ config->http2.dos_delay = 100; /* 100ms processing delay when observing suspicious behavior */
+ config->http2.callbacks = H2O_HTTP2_CALLBACKS;
+ config->mimemap = h2o_mimemap_create();
+
+--- a/lib/core/configurator.c
++++ b/lib/core/configurator.c
+@@ -531,6 +531,12 @@ static int on_config_http2_casper(h2o_co
+ return 0;
+ }
+
++
++static int on_config_http2_dos_delay(h2o_configurator_command_t *cmd, h2o_configurator_context_t *ctx, yoml_t *node)
++{
++ return config_timeout(cmd, node, &ctx->globalconf->http2.dos_delay);
++}
++
+ static int assert_is_mimetype(h2o_configurator_command_t *cmd, yoml_t *node)
+ {
+ if (node->type != YOML_TYPE_SCALAR) {
+@@ -910,6 +916,9 @@ void h2o_configurator__init_core(h2o_glo
+ on_config_http2_push_preload);
+ h2o_configurator_define_command(&c->super, "http2-casper", H2O_CONFIGURATOR_FLAG_GLOBAL | H2O_CONFIGURATOR_FLAG_HOST,
+ on_config_http2_casper);
++ h2o_configurator_define_command(&c->super, "http2-dos-delay",
++ H2O_CONFIGURATOR_FLAG_GLOBAL | H2O_CONFIGURATOR_FLAG_EXPECT_SCALAR,
++ on_config_http2_dos_delay);
+ h2o_configurator_define_command(&c->super, "file.mime.settypes",
+ (H2O_CONFIGURATOR_FLAG_ALL_LEVELS & ~H2O_CONFIGURATOR_FLAG_EXTENSION) |
+ H2O_CONFIGURATOR_FLAG_EXPECT_MAPPING,
+--- a/lib/core/context.c
++++ b/lib/core/context.c
+@@ -101,6 +101,7 @@ void h2o_context_init(h2o_context_t *ctx
+ h2o_linklist_init_anchor(&ctx->http1._conns);
+ h2o_timeout_init(ctx->loop, &ctx->http2.idle_timeout, config->http2.idle_timeout);
+ h2o_timeout_init(ctx->loop, &ctx->http2.graceful_shutdown_timeout, config->http2.graceful_shutdown_timeout);
++ h2o_timeout_init(ctx->loop, &ctx->http2.dos_delay_timeout, config->http2.dos_delay);
+ h2o_linklist_init_anchor(&ctx->http2._conns);
+ ctx->proxy.client_ctx.loop = loop;
+ h2o_timeout_init(ctx->loop, &ctx->proxy.io_timeout, config->proxy.io_timeout);
+@@ -146,6 +147,7 @@ void h2o_context_dispose(h2o_context_t *
+ h2o_timeout_dispose(ctx->loop, &ctx->http1.req_timeout);
+ h2o_timeout_dispose(ctx->loop, &ctx->http2.idle_timeout);
+ h2o_timeout_dispose(ctx->loop, &ctx->http2.graceful_shutdown_timeout);
++ h2o_timeout_dispose(ctx->loop, &ctx->http2.dos_delay_timeout);
+ h2o_timeout_dispose(ctx->loop, &ctx->proxy.io_timeout);
+ /* what should we do here? assert(!h2o_linklist_is_empty(&ctx->http2._conns); */
+
+--- a/lib/http2/connection.c
++++ b/lib/http2/connection.c
+@@ -161,7 +161,6 @@ static void update_idle_timeout(h2o_http
+ h2o_timeout_unlink(&conn->_timeout_entry);
+
+ if (conn->num_streams.pull.half_closed + conn->num_streams.push.half_closed == 0) {
+- assert(h2o_linklist_is_empty(&conn->_pending_reqs));
+ conn->_timeout_entry.cb = on_idle_timeout;
+ h2o_timeout_link(conn->super.ctx->loop, &conn->super.ctx->http2.idle_timeout, &conn->_timeout_entry);
+ }
+@@ -175,6 +174,9 @@ static int can_run_requests(h2o_http2_co
+
+ static void run_pending_requests(h2o_http2_conn_t *conn)
+ {
++ if (h2o_timeout_is_linked(&conn->dos_mitigation.process_delay))
++ return;
++
+ while (!h2o_linklist_is_empty(&conn->_pending_reqs) && can_run_requests(conn)) {
+ /* fetch and detach a pending stream */
+ h2o_http2_stream_t *stream = H2O_STRUCT_FROM_MEMBER(h2o_http2_stream_t, _refs.link, conn->_pending_reqs.next);
+@@ -226,6 +228,16 @@ void h2o_http2_conn_unregister_stream(h2
+ assert(h2o_http2_scheduler_is_open(&stream->_refs.scheduler));
+ h2o_http2_scheduler_close(&stream->_refs.scheduler);
+
++ /* Decrement reset_budget if the stream was reset by peer, otherwise increment. By doing so, we penalize connections that
++ * generate resets for >50% of requests. */
++ if (stream->reset_by_peer) {
++ if (conn->dos_mitigation.reset_budget > 0)
++ --conn->dos_mitigation.reset_budget;
++ } else {
++ if (conn->dos_mitigation.reset_budget < conn->super.ctx->globalconf->http2.max_concurrent_requests_per_connection)
++ ++conn->dos_mitigation.reset_budget;
++ }
++
+ switch (stream->state) {
+ case H2O_HTTP2_STREAM_STATE_IDLE:
+ case H2O_HTTP2_STREAM_STATE_RECV_HEADERS:
+@@ -272,6 +284,8 @@ void close_connection_now(h2o_http2_conn
+ h2o_hpack_dispose_header_table(&conn->_output_header_table);
+ assert(h2o_linklist_is_empty(&conn->_pending_reqs));
+ h2o_timeout_unlink(&conn->_timeout_entry);
++ if (h2o_timeout_is_linked(&conn->dos_mitigation.process_delay))
++ h2o_timeout_unlink(&conn->dos_mitigation.process_delay);
+ h2o_buffer_dispose(&conn->_write.buf);
+ if (conn->_write.buf_in_flight != NULL)
+ h2o_buffer_dispose(&conn->_write.buf_in_flight);
+@@ -797,11 +811,19 @@ static int handle_rst_stream_frame(h2o_h
+ return H2O_HTTP2_ERROR_PROTOCOL;
+ }
+
+- stream = h2o_http2_conn_get_stream(conn, frame->stream_id);
+- if (stream != NULL) {
++ if ((stream = h2o_http2_conn_get_stream(conn, frame->stream_id)) == NULL)
++ return 0;
++
+ /* reset the stream */
++ stream->reset_by_peer = 1;
+ h2o_http2_stream_reset(conn, stream);
+- }
++
++ /* setup process delay if we've just ran out of reset budget */
++ if (conn->dos_mitigation.reset_budget == 0 && conn->super.ctx->globalconf->http2.dos_delay != 0 &&
++ !h2o_timeout_is_linked(&conn->dos_mitigation.process_delay))
++ h2o_timeout_link(conn->super.ctx->loop, &conn->super.ctx->http2.dos_delay_timeout,
++ &conn->dos_mitigation.process_delay);
++
+ /* TODO log */
+
+ return 0;
+@@ -1204,6 +1226,14 @@ static h2o_iovec_t log_priority_actual_w
+ return h2o_iovec_init(s, len);
+ }
+
++static void on_dos_process_delay(h2o_timeout_entry_t *timer)
++{
++ h2o_http2_conn_t *conn = H2O_STRUCT_FROM_MEMBER(h2o_http2_conn_t, dos_mitigation.process_delay, timer);
++
++ assert(!h2o_timeout_is_linked(&conn->dos_mitigation.process_delay));
++ run_pending_requests(conn);
++}
++
+ static h2o_http2_conn_t *create_conn(h2o_context_t *ctx, h2o_hostconf_t **hosts, h2o_socket_t *sock, struct timeval connected_at)
+ {
+ static const h2o_conn_callbacks_t callbacks = {
+@@ -1240,6 +1270,9 @@ static h2o_http2_conn_t *create_conn(h2o
+ conn->_write.timeout_entry.cb = emit_writereq;
+ h2o_http2_window_init(&conn->_write.window, &conn->peer_settings);
+
++ conn->dos_mitigation.process_delay.cb = on_dos_process_delay;
++ conn->dos_mitigation.reset_budget = conn->super.ctx->globalconf->http2.max_concurrent_requests_per_connection;
++
+ return conn;
+ }
+
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