h2: Add a rate limit facility for h/2 RST handling

This adds parameters h2_rst_allowance and h2_rst_allowance_period,
which govern the rate of which we allow clients to reset h/2 streams.

If the limit is exceeded the connection is closed.

Mitigates: #3996

bin/varnishd/http2/cache_http2.h

@@ -193,6 +193,8 @@ struct h2_sess {
 	VTAILQ_HEAD(,h2_req)		txqueue;
 
 	h2_error			error;
+	double				rst_budget;
+	vtim_real			last_rst;
 };
 
 #define ASSERT_RXTHR(h2) do {assert(h2->rxthr == pthread_self());} while(0)

bin/varnishd/http2/cache_http2_proto.c

@@ -320,9 +320,41 @@ h2_rx_push_promise(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
 /**********************************************************************
  */
 
+static h2_error
+h2_rapid_reset(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
+{
+	vtim_real now;
+	vtim_dur d;
+
+	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
+	ASSERT_RXTHR(h2);
+	CHECK_OBJ_NOTNULL(r2, H2_REQ_MAGIC);
+
+	if (cache_param->h2_rapid_reset_limit == 0)
+		return (0);
+
+	now = VTIM_real();
+	d = now - h2->last_rst;
+	h2->rst_budget += cache_param->h2_rapid_reset_limit * d /
+	    cache_param->h2_rapid_reset_period;
+	h2->rst_budget = vmin_t(double, h2->rst_budget,
+	    cache_param->h2_rapid_reset_limit);
+	h2->last_rst = now;
+
+	if (h2->rst_budget < 1.0) {
+		Lck_Lock(&h2->sess->mtx);
+		VSLb(h2->vsl, SLT_Error, "H2: Hit RST limit. Closing session.");
+		Lck_Unlock(&h2->sess->mtx);
+		return (H2CE_ENHANCE_YOUR_CALM);
+	}
+	h2->rst_budget -= 1.0;
+	return (0);
+}
+
 static h2_error v_matchproto_(h2_rxframe_f)
 h2_rx_rst_stream(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
 {
+	h2_error h2e;
 
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	ASSERT_RXTHR(h2);
@@ -332,8 +364,9 @@ h2_rx_rst_stream(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
 		return (H2CE_FRAME_SIZE_ERROR);
 	if (r2 == NULL)
 		return (0);
+	h2e = h2_rapid_reset(wrk, h2, r2);
 	h2_kill_req(wrk, h2, r2, h2_streamerror(vbe32dec(h2->rxf_data)));
-	return (0);
+	return (h2e);
 }
 
 /**********************************************************************

bin/varnishd/http2/cache_http2_session.c

@@ -127,6 +127,9 @@ h2_init_sess(struct sess *sp,
 	h2_local_settings(&h2->local_settings);
 	h2->remote_settings = H2_proto_settings;
 	h2->decode = decode;
+	h2->rst_budget = cache_param->h2_rapid_reset_limit;
+	h2->last_rst = sp->t_open;
+	AZ(isnan(h2->last_rst));
 
 	AZ(VHT_Init(h2->dectbl, h2->local_settings.header_table_size));
 

bin/varnishtest/tests/r03996.vtc

@@ -0,0 +1,49 @@
+varnishtest "h2 rapid reset"
+
+barrier b1 sock 5
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -cliok "param.set feature +http2"
+varnish v1 -cliok "param.set debug +syncvsl"
+varnish v1 -cliok "param.set h2_rapid_reset_limit 3"
+
+varnish v1 -vcl+backend {
+	import vtc;
+
+	sub vcl_recv {
+		vtc.barrier_sync("${b1_sock}");
+	}
+
+} -start
+
+client c1 {
+	stream 0 {
+		rxgoaway
+		expect goaway.err == ENHANCE_YOUR_CALM
+	} -start
+
+	stream 1 {
+		txreq
+		txrst
+	} -run
+	stream 3 {
+		txreq
+		txrst
+	} -run
+	stream 5 {
+		txreq
+		txrst
+	} -run
+	stream 7 {
+		txreq
+		txrst
+	} -run
+
+	barrier b1 sync
+	stream 0 -wait
+} -run
+

include/tbl/params.h

@@ -1257,6 +1257,34 @@ PARAM_SIMPLE(
 	"HTTP2 maximum size of an uncompressed header list."
 )
 
+PARAM_SIMPLE(
+	/* name */	h2_rapid_reset_limit,
+	/* typ */	uint,
+	/* min */	"0",
+	/* max */	NULL,
+	/* def */	"0",
+	/* units */	NULL,
+	/* descr */
+	"HTTP2 RST Allowance.\n"
+	"Specifies the maximum number of allowed stream resets issued by\n"
+	"a client over a time period before the connection is closed.\n"
+	"Setting this parameter to 0 disables the limit.",
+	/* flags */	EXPERIMENTAL,
+)
+
+PARAM_SIMPLE(
+	/* name */	h2_rapid_reset_period,
+	/* typ */	timeout,
+	/* min */	"1.000",
+	/* max */	NULL,
+	/* def */	"60.000",
+	/* units */	"seconds",
+	/* descr */
+	"HTTP2 sliding window duration for h2_rapid_reset_limit.",
+	/* flags */	EXPERIMENTAL|WIZARD,
+)
+
+
 /*--------------------------------------------------------------------
  * Memory pool parameters
  */