[PATCH 3 of 6] QUIC: ignore duplicate PATH_CHALLENGE frames

[Roman Arutyunyan]

# HG changeset patch
# User Roman Arutyunyan <arut at nginx.com>
# Date 1700650092 -14400
#      Wed Nov 22 14:48:12 2023 +0400
# Node ID fcfe832f2590ad81fc74bc479e1b3ccf8c14683a
# Parent  87290bcf25a6fc62448722f5a72327a30fdf31d9
QUIC: ignore duplicate PATH_CHALLENGE frames.

According to RFC 9000, an endpoint SHOULD NOT send multiple PATH_CHALLENGE
frames in a single packet.  The change adds a check to enforce this claim to
optimize server behavior.  Previously each PATH_CHALLENGE always resulted in a
single response datagram being sent to client.  The effect of this was however
limited by QUIC flood protection.

Also, PATH_CHALLENGE is explicitly disabled in Initial and Handshake levels,
see RFC 9000, Table 3.  However, technically it may be sent by client in 0-RTT
over a new path without actual migration, even though the migration itself is
prohibited during handshake.  This allows client to coalesce multiple 0-RTT
packets each carrying a PATH_CHALLENGE and end up with multiple PATH_CHALLENGEs
per datagram.  This again leads to suboptimal behavior, see above.  Since the
purpose of sending PATH_CHALLENGE frames in 0-RTT is unclear, these frames are
now only allowed in 1-RTT.  For 0-RTT they are silently ignored.

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
@@ -40,6 +40,14 @@ ngx_quic_handle_path_challenge_frame(ngx
     ngx_quic_frame_t        frame, *fp;
     ngx_quic_connection_t  *qc;
 
+    if (pkt->level != ssl_encryption_application || pkt->path_challenged) {
+        ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                       "quic ignoring PATH_CHALLENGE");
+        return NGX_OK;
+    }
+
+    pkt->path_challenged = 1;
+
     qc = ngx_quic_get_connection(c);
 
     ngx_memzero(&frame, sizeof(ngx_quic_frame_t));
diff --git a/src/event/quic/ngx_event_quic_transport.h b/src/event/quic/ngx_event_quic_transport.h
--- a/src/event/quic/ngx_event_quic_transport.h
+++ b/src/event/quic/ngx_event_quic_transport.h
@@ -336,6 +336,7 @@ typedef struct {
     unsigned                                    retried:1;
     unsigned                                    first:1;
     unsigned                                    rebound:1;
+    unsigned                                    path_challenged:1;
 } ngx_quic_header_t;