[PATCH 2 of 6] QUIC: fixed anti-amplification with explicit send

[Roman Arutyunyan]

# HG changeset patch
# User Roman Arutyunyan <arut at nginx.com>
# Date 1700650341 -14400
#      Wed Nov 22 14:52:21 2023 +0400
# Node ID 87290bcf25a6fc62448722f5a72327a30fdf31d9
# Parent  088ee5449fdc67ee8ab983910d98599652683434
QUIC: fixed anti-amplification with explicit send.

Previously, when using ngx_quic_frame_sendto() to explicitly send a packet with
a single frame, anti-amplification limit was not properly enforced.  Even when
there was no quota left for the packet, it was sent anyway, but with no padding.
Now the packet is not sent at all.

This function is called to send PATH_CHALLENGE/PATH_RESPONSE, PMTUD and probe
packets.  For all these cases packet send is retried later in case the send was
not successful.

diff --git a/src/event/quic/ngx_event_quic_migration.c b/src/event/quic/ngx_event_quic_migration.c
--- a/src/event/quic/ngx_event_quic_migration.c
+++ b/src/event/quic/ngx_event_quic_migration.c
@@ -872,6 +872,7 @@ ngx_quic_expire_path_mtu_discovery(ngx_c
 static ngx_int_t
 ngx_quic_send_path_mtu_probe(ngx_connection_t *c, ngx_quic_path_t *path)
 {
+    size_t                  mtu;
     ngx_int_t               rc;
     ngx_uint_t              log_error;
     ngx_quic_frame_t        frame;
@@ -895,7 +896,12 @@ ngx_quic_send_path_mtu_probe(ngx_connect
     log_error = c->log_error;
     c->log_error = NGX_ERROR_IGNORE_EMSGSIZE;
 
+    mtu = path->mtu;
+    path->mtu = path->mtud;
+
     rc = ngx_quic_frame_sendto(c, &frame, path->mtud, path);
+
+    path->mtu = mtu;
     c->log_error = log_error;
 
     if (rc == NGX_ERROR) {
diff --git a/src/event/quic/ngx_event_quic_output.c b/src/event/quic/ngx_event_quic_output.c
--- a/src/event/quic/ngx_event_quic_output.c
+++ b/src/event/quic/ngx_event_quic_output.c
@@ -1181,7 +1181,7 @@ ngx_int_t
 ngx_quic_frame_sendto(ngx_connection_t *c, ngx_quic_frame_t *frame,
     size_t min, ngx_quic_path_t *path)
 {
-    size_t                  min_payload, pad;
+    size_t                  max, max_payload, min_payload, pad;
     ssize_t                 len, sent;
     ngx_str_t               res;
     ngx_quic_header_t       pkt;
@@ -1194,15 +1194,25 @@ ngx_quic_frame_sendto(ngx_connection_t *
     qc = ngx_quic_get_connection(c);
     ctx = ngx_quic_get_send_ctx(qc, frame->level);
 
+    max = ngx_quic_path_limit(c, path, path->mtu);
+
+    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                   "quic sendto %s packet max:%uz min:%uz",
+                   ngx_quic_level_name(ctx->level), max, min);
+
     ngx_quic_init_packet(c, ctx, &pkt, path);
 
-    min = ngx_quic_path_limit(c, path, min);
+    min_payload = ngx_quic_payload_size(&pkt, min);
+    max_payload = ngx_quic_payload_size(&pkt, max);
 
-    min_payload = min ? ngx_quic_payload_size(&pkt, min) : 0;
-
+    /* RFC 9001, 5.4.2.  Header Protection Sample */
     pad = 4 - pkt.num_len;
     min_payload = ngx_max(min_payload, pad);
 
+    if (min_payload > max_payload) {
+        return NGX_AGAIN;
+    }
+
 #if (NGX_DEBUG)
     frame->pnum = pkt.number;
 #endif
@@ -1210,8 +1220,8 @@ ngx_quic_frame_sendto(ngx_connection_t *
     ngx_quic_log_frame(c->log, frame, 1);
 
     len = ngx_quic_create_frame(NULL, frame);
-    if (len > NGX_QUIC_MAX_UDP_PAYLOAD_SIZE) {
-        return NGX_ERROR;
+    if ((size_t) len > max_payload) {
+        return NGX_AGAIN;
     }
 
     len = ngx_quic_create_frame(src, frame);
@@ -1258,8 +1268,6 @@ ngx_quic_path_limit(ngx_connection_t *c,
         max = (path->sent >= max) ? 0 : max - path->sent;
 
         if ((off_t) size > max) {
-            ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
-                           "quic path limit %uz - %O", size, max);
             return max;
         }
     }