Comply with the received Record Size Limit extension
Fixes #7010
Signed-off-by: Jan Bruckner <jan@janbruckner.de>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index eae192b..6b799ee 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -439,6 +439,24 @@
size_t mbedtls_ssl_get_input_max_frag_len(const mbedtls_ssl_context *ssl);
#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
+#if defined(MBEDTLS_SSL_RECORD_SIZE_LIMIT)
+/**
+ * \brief Return the record size limit (in bytes) for
+ * the output buffer. This is less than the value requested by the
+ * peer (using RFC 8449), since it subtracts the space required for the
+ * content type and padding of the TLSInnerPlaintext struct (RFC 8446).
+ * Returns MBEDTLS_SSL_OUT_CONTENT_LEN if no limit was requested by the peer.
+ *
+ * \sa mbedtls_ssl_get_max_out_record_payload()
+ * ssl_compute_internal_record_size_limit()
+ *
+ * \param ssl SSL context
+ *
+ * \return Current record size limit for the output buffer.
+ */
+size_t mbedtls_ssl_get_output_record_size_limit(const mbedtls_ssl_context *ssl);
+#endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */
+
#if defined(MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH)
static inline size_t mbedtls_ssl_get_output_buflen(const mbedtls_ssl_context *ctx)
{
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 4daf2e7..7a8c759 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2455,6 +2455,7 @@
* uint8 ticket_flags;
* opaque resumption_key<0..255>;
* uint32 max_early_data_size;
+ * uint16 record_size_limit;
* select ( endpoint ) {
* case client: ClientOnlyData;
* case server: uint64 ticket_creation_time;
@@ -2490,6 +2491,9 @@
#if defined(MBEDTLS_SSL_EARLY_DATA)
needed += 4; /* max_early_data_size */
#endif
+#if defined(MBEDTLS_SSL_RECORD_SIZE_LIMIT)
+ needed += 2; /* record_size_limit */
+#endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */
#if defined(MBEDTLS_HAVE_TIME)
needed += 8; /* ticket_creation_time or ticket_reception_time */
@@ -2534,6 +2538,10 @@
MBEDTLS_PUT_UINT32_BE(session->max_early_data_size, p, 0);
p += 4;
#endif
+#if defined(MBEDTLS_SSL_RECORD_SIZE_LIMIT)
+ MBEDTLS_PUT_UINT16_BE(session->record_size_limit, p, 0);
+ p += 2;
+#endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */
#if defined(MBEDTLS_HAVE_TIME) && defined(MBEDTLS_SSL_SRV_C)
if (session->endpoint == MBEDTLS_SSL_IS_SERVER) {
@@ -2610,6 +2618,13 @@
session->max_early_data_size = MBEDTLS_GET_UINT32_BE(p, 0);
p += 4;
#endif
+#if defined(MBEDTLS_SSL_RECORD_SIZE_LIMIT)
+ if (end - p < 2) {
+ return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
+ }
+ session->record_size_limit = MBEDTLS_GET_UINT16_BE(p, 0);
+ p += 2;
+#endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */
#if defined(MBEDTLS_HAVE_TIME) && defined(MBEDTLS_SSL_SRV_C)
if (session->endpoint == MBEDTLS_SSL_IS_SERVER) {
@@ -3458,6 +3473,7 @@
size_t max_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
#if !defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH) && \
+ !defined(MBEDTLS_SSL_RECORD_SIZE_LIMIT) && \
!defined(MBEDTLS_SSL_PROTO_DTLS)
(void) ssl;
#endif
@@ -3470,6 +3486,14 @@
}
#endif
+#if defined(MBEDTLS_SSL_RECORD_SIZE_LIMIT)
+ const size_t record_size_limit = mbedtls_ssl_get_output_record_size_limit(ssl);
+
+ if (max_len > record_size_limit) {
+ max_len = record_size_limit;
+ }
+#endif
+
#if defined(MBEDTLS_SSL_PROTO_DTLS)
if (mbedtls_ssl_get_current_mtu(ssl) != 0) {
const size_t mtu = mbedtls_ssl_get_current_mtu(ssl);
diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index 052df7e..1a246c4 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -2113,12 +2113,11 @@
ret = mbedtls_ssl_tls13_parse_record_size_limit_ext(
ssl, p, p + extension_data_len);
-
- /* TODO: Return unconditionally here until we handle the record
- * size limit correctly. Once handled correctly, only return in
- * case of errors. */
- return ret;
-
+ if (ret != 0) {
+ MBEDTLS_SSL_DEBUG_RET(
+ 1, ("mbedtls_ssl_tls13_parse_record_size_limit_ext"), ret);
+ return ret;
+ }
break;
#endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index cc77a94..7c7aac8 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -1714,7 +1714,7 @@
MBEDTLS_SSL_DEBUG_MSG(2, ("RecordSizeLimit: %u Bytes", record_size_limit));
- /* RFC 8449, section 4
+ /* RFC 8449, section 4:
*
* Endpoints MUST NOT send a "record_size_limit" extension with a value
* smaller than 64. An endpoint MUST treat receipt of a smaller value
@@ -1727,13 +1727,47 @@
return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
}
- MBEDTLS_SSL_DEBUG_MSG(
- 2, ("record_size_limit extension is still in development. Aborting handshake."));
+ ssl->session_negotiate->record_size_limit = record_size_limit;
- MBEDTLS_SSL_PEND_FATAL_ALERT(
- MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_EXT,
- MBEDTLS_ERR_SSL_UNSUPPORTED_EXTENSION);
- return MBEDTLS_ERR_SSL_UNSUPPORTED_EXTENSION;
+ return 0;
+}
+
+static inline size_t ssl_compute_internal_record_size_limit(size_t record_size_limit)
+{
+ /* RFC 8449, section 4:
+ *
+ * This value [record_size_limit] is the length of the plaintext of a protected record.
+ * The value includes the content type and padding added in TLS 1.3 (that is, the complete
+ * length of TLSInnerPlaintext).
+ *
+ * Thus, round down to a multiple of MBEDTLS_SSL_CID_TLS1_3_PADDING_GRANULARITY
+ * and subtract 1 (for the content type that will be added later)
+ */
+ return ((record_size_limit / MBEDTLS_SSL_CID_TLS1_3_PADDING_GRANULARITY) *
+ MBEDTLS_SSL_CID_TLS1_3_PADDING_GRANULARITY) - 1;
+}
+
+size_t mbedtls_ssl_get_output_record_size_limit(const mbedtls_ssl_context *ssl)
+{
+ const size_t max_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
+ size_t record_size_limit = max_len;
+
+ if (ssl->session != NULL &&
+ ssl->session->record_size_limit >= MBEDTLS_SSL_RECORD_SIZE_LIMIT_MIN &&
+ ssl->session->record_size_limit < max_len) {
+ record_size_limit = ssl_compute_internal_record_size_limit(ssl->session->record_size_limit);
+ }
+
+ // TODO: this is currently untested
+ /* During a handshake, use the value being negotiated */
+ if (ssl->session_negotiate != NULL &&
+ ssl->session_negotiate->record_size_limit >= MBEDTLS_SSL_RECORD_SIZE_LIMIT_MIN &&
+ ssl->session_negotiate->record_size_limit < max_len) {
+ record_size_limit = ssl_compute_internal_record_size_limit(
+ ssl->session_negotiate->record_size_limit);
+ }
+
+ return record_size_limit;
}
#endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index bfe805f..9e2cbbf 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -1699,14 +1699,11 @@
ret = mbedtls_ssl_tls13_parse_record_size_limit_ext(
ssl, p, extension_data_end);
-
- /*
- * TODO: Return unconditionally here until we handle the record
- * size limit correctly.
- * Once handled correctly, only return in case of errors.
- */
- return ret;
-
+ if (ret != 0) {
+ MBEDTLS_SSL_DEBUG_RET(
+ 1, ("mbedtls_ssl_tls13_parse_record_size_limit_ext"), ret);
+ return ret;
+ }
break;
#endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */