[nginx] QUIC: fixed anti-amplification with explicit send.

[Roman Arutyunyan]

details:   https://hg.nginx.org/nginx/rev/3a67dd34b6cc
branches:  
changeset: 9190:3a67dd34b6cc
user:      Roman Arutyunyan <arut at nginx.com>
date:      Wed Nov 22 14:52:21 2023 +0400
description:
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.

diffstat:

 src/event/quic/ngx_event_quic_migration.c |   6 ++++++
 src/event/quic/ngx_event_quic_output.c    |  24 ++++++++++++++++--------
 2 files changed, 22 insertions(+), 8 deletions(-)

diffs (85 lines):

diff -r fcec773dd249 -r 3a67dd34b6cc src/event/quic/ngx_event_quic_migration.c
--- a/src/event/quic/ngx_event_quic_migration.c	Wed Nov 29 18:13:25 2023 +0400
+++ b/src/event/quic/ngx_event_quic_migration.c	Wed Nov 22 14:52:21 2023 +0400
@@ -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 -r fcec773dd249 -r 3a67dd34b6cc src/event/quic/ngx_event_quic_output.c
--- a/src/event/quic/ngx_event_quic_output.c	Wed Nov 29 18:13:25 2023 +0400
+++ b/src/event/quic/ngx_event_quic_output.c	Wed Nov 22 14:52:21 2023 +0400
@@ -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;
         }
     }