QUIC: avoid partial expansion of PATH_CHALLENGE/PATH_RESPONSE.

By default packets with these frames are expanded to 1200 bytes.  Previously,
if anti-amplification limit did not allow this expansion, it was limited to
whatever size was allowed.  However RFC 9000 clearly states no partial
expansion should happen in both cases.

    Section 8.2.1.  Initiating Path Validation:

    An endpoint MUST expand datagrams that contain a PATH_CHALLENGE frame
    to at least the smallest allowed maximum datagram size of 1200 bytes,
    unless the anti-amplification limit for the path does not permit
    sending a datagram of this size.

    Section 8.2.2. Path Validation Responses:

    An endpoint MUST expand datagrams that contain a PATH_RESPONSE frame
    to at least the smallest allowed maximum datagram size of 1200 bytes.
    ...
    However, an endpoint MUST NOT expand the datagram containing the
    PATH_RESPONSE if the resulting data exceeds the anti-amplification limit.

src/event/quic/ngx_event_quic_connection.h

@@ -106,8 +106,7 @@ struct ngx_quic_path_s {
     size_t                            max_mtu;
     off_t                             sent;
     off_t                             received;
-    u_char                            challenge1[8];
+    u_char                            challenge[2][8];
-    u_char                            challenge2[8];
     uint64_t                          seqnum;
     uint64_t                          mtu_pnum[NGX_QUIC_PATH_RETRIES];
     ngx_str_t                         addr_text;

src/event/quic/ngx_event_quic_migration.c

@@ -36,6 +36,7 @@ ngx_int_t
 ngx_quic_handle_path_challenge_frame(ngx_connection_t *c,
     ngx_quic_header_t *pkt, ngx_quic_path_challenge_frame_t *f)
 {
+    size_t                  min;
     ngx_quic_frame_t        frame, *fp;
     ngx_quic_connection_t  *qc;
 
@@ -57,8 +58,14 @@ ngx_quic_handle_path_challenge_frame(ngx_connection_t *c,
     /*
      * An endpoint MUST expand datagrams that contain a PATH_RESPONSE frame
      * to at least the smallest allowed maximum datagram size of 1200 bytes.
+     * ...
+     * However, an endpoint MUST NOT expand the datagram containing the
+     * PATH_RESPONSE if the resulting data exceeds the anti-amplification limit.
      */
-    if (ngx_quic_frame_sendto(c, &frame, 1200, pkt->path) == NGX_ERROR) {
+
+    min = (ngx_quic_path_limit(c, pkt->path, 1200) < 1200) ? 0 : 1200;
+
+    if (ngx_quic_frame_sendto(c, &frame, min, pkt->path) == NGX_ERROR) {
         return NGX_ERROR;
     }
 
@@ -113,8 +120,8 @@ ngx_quic_handle_path_response_frame(ngx_connection_t *c,
             continue;
         }
 
-        if (ngx_memcmp(path->challenge1, f->data, sizeof(f->data)) == 0
+        if (ngx_memcmp(path->challenge[0], f->data, sizeof(f->data)) == 0
-            || ngx_memcmp(path->challenge2, f->data, sizeof(f->data)) == 0)
+            || ngx_memcmp(path->challenge[1], f->data, sizeof(f->data)) == 0)
         {
             goto valid;
         }
@@ -510,11 +517,7 @@ ngx_quic_validate_path(ngx_connection_t *c, ngx_quic_path_t *path)
 
     path->tries = 0;
 
-    if (RAND_bytes(path->challenge1, 8) != 1) {
+    if (RAND_bytes((u_char *) path->challenge, sizeof(path->challenge)) != 1) {
-        return NGX_ERROR;
-    }
-
-    if (RAND_bytes(path->challenge2, 8) != 1) {
         return NGX_ERROR;
     }
 
@@ -535,6 +538,8 @@ ngx_quic_validate_path(ngx_connection_t *c, ngx_quic_path_t *path)
 static ngx_int_t
 ngx_quic_send_path_challenge(ngx_connection_t *c, ngx_quic_path_t *path)
 {
+    size_t            min;
+    ngx_uint_t        n;
     ngx_quic_frame_t  frame;
 
     ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
@@ -546,26 +551,24 @@ ngx_quic_send_path_challenge(ngx_connection_t *c, ngx_quic_path_t *path)
     frame.level = ssl_encryption_application;
     frame.type = NGX_QUIC_FT_PATH_CHALLENGE;
 
-    ngx_memcpy(frame.u.path_challenge.data, path->challenge1, 8);
+    for (n = 0; n < 2; n++) {
 
-    /*
+        ngx_memcpy(frame.u.path_challenge.data, path->challenge[n], 8);
-     * RFC 9000, 8.2.1.  Initiating Path Validation
-     *
-     * An endpoint MUST expand datagrams that contain a PATH_CHALLENGE frame
-     * to at least the smallest allowed maximum datagram size of 1200 bytes,
-     * unless the anti-amplification limit for the path does not permit
-     * sending a datagram of this size.
-     */
 
-     /* same applies to PATH_RESPONSE frames */
+        /*
-    if (ngx_quic_frame_sendto(c, &frame, 1200, path) == NGX_ERROR) {
+         * RFC 9000, 8.2.1.  Initiating Path Validation
-        return NGX_ERROR;
+         *
-    }
+         * An endpoint MUST expand datagrams that contain a PATH_CHALLENGE frame
+         * to at least the smallest allowed maximum datagram size of 1200 bytes,
+         * unless the anti-amplification limit for the path does not permit
+         * sending a datagram of this size.
+         */
 
-    ngx_memcpy(frame.u.path_challenge.data, path->challenge2, 8);
+        min = (ngx_quic_path_limit(c, path, 1200) < 1200) ? 0 : 1200;
 
-    if (ngx_quic_frame_sendto(c, &frame, 1200, path) == NGX_ERROR) {
+        if (ngx_quic_frame_sendto(c, &frame, min, path) == NGX_ERROR) {
-        return NGX_ERROR;
+            return NGX_ERROR;
+        }
     }
 
     return NGX_OK;

src/event/quic/ngx_event_quic_output.c

@@ -63,8 +63,6 @@ static ssize_t ngx_quic_send(ngx_connection_t *c, u_char *buf, size_t len,
     struct sockaddr *sockaddr, socklen_t socklen);
 static void ngx_quic_set_packet_number(ngx_quic_header_t *pkt,
     ngx_quic_send_ctx_t *ctx);
-static size_t ngx_quic_path_limit(ngx_connection_t *c, ngx_quic_path_t *path,
-    size_t size);
 
 
 ngx_int_t
@@ -1250,7 +1248,7 @@ ngx_quic_frame_sendto(ngx_connection_t *c, ngx_quic_frame_t *frame,
 }
 
 
-static size_t
+size_t
 ngx_quic_path_limit(ngx_connection_t *c, ngx_quic_path_t *path, size_t size)
 {
     off_t  max;

src/event/quic/ngx_event_quic_output.h

@@ -34,5 +34,7 @@ ngx_int_t ngx_quic_send_ack_range(ngx_connection_t *c,
 
 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 ngx_quic_path_limit(ngx_connection_t *c, ngx_quic_path_t *path,
+    size_t size);
 
 #endif /* _NGX_EVENT_QUIC_OUTPUT_H_INCLUDED_ */