Valentin V. Bartenev
89a59d5a2e
Rename arch/esp32/tls_cipher_restrict.c → arch/esp32/tls_server_config.c. The file now handles two aspects of TLS server configuration via the existing __wrap_mbedtls_ssl_config_defaults() intercept, so the old name no longer described its full scope. Add TLS session ticket support (RFC 5077). Browsers can now resume HTTPS sessions without a full RSA handshake, reducing connection latency on repeated connections. A static mbedtls_ssl_ticket_context is initialized once on first server start using the ESP32 hardware RNG (esp_fill_random), then registered via mbedtls_ssl_conf_session_tickets_cb(). The context must be static because mbedTLS stores the pointer for the lifetime of the server, analogous to kServerOnlyCipherSuites. If mbedtls_ssl_ticket_setup() fails, the flag remains unset and session tickets are silently skipped rather than registering callbacks against an uninitialized context. CONFIG_MBEDTLS_SSL_SESSION_TICKETS is enabled by default in the pre-built arduino-esp32 framework, so no build system changes are required beyond the filename update in platformio.ini.
116 lines
4.1 KiB
C
116 lines
4.1 KiB
C
/*
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* arch/esp32/tls_server_config.c
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*
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* Configures the TLS server via a linker-wrap of mbedtls_ssl_config_defaults():
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*
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* 1. Restricts cipher suites to RSA key exchange, eliminating the ECDH
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* computation that starves IDLE0 and triggers the task watchdog during
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* HTTPS handshakes.
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*
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* 2. Enables TLS session tickets so that browsers can resume sessions
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* without a full RSA handshake.
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*
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* Background (cipher suite restriction):
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* Browsers negotiate ECDHE cipher suites by default. On ESP32-S3 the
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* hardware RSA accelerator handles RSA key exchange efficiently, but
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* there is no ECP hardware accelerator. ECDHE requires the server to
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* compute an ephemeral key pair: ecp_precompute_comb() builds a comb
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* table through many sequential ECP point doublings, each dispatched
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* to the hardware bignum unit (esp_bignum.c), but the ECP layer has
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* no RTOS yield points between iterations. The entire computation runs
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* to completion on CPU 0 without ever resetting the task watchdog.
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*
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* A single handshake does not exceed the watchdog timeout on its own,
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* but two consecutive handshakes (e.g. a browser retry after a failed
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* attempt) accumulate enough uninterrupted runtime to starve IDLE0:
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*
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* E (54924) esp-tls-mbedtls: mbedtls_ssl_handshake returned -0x0050
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* E (57208) esp-tls-mbedtls: mbedtls_ssl_handshake returned -0x7280
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* E (57638) task_wdt: Task watchdog got triggered.
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* E (57638) task_wdt: - IDLE0 (CPU 0)
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* E (57638) task_wdt: Tasks currently running:
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* E (57638) task_wdt: CPU 0: httpd
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*
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* The crash occurs in ecp_precompute_comb() → ecp_double_jac() →
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* mbedtls_mpi_mul_mpi() during the ServerKeyExchange step.
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*
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* There is no sdkconfig knob accessible at runtime through
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* esp_https_server. The user_cb hook fires after the handshake, too
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* late to change cipher suites. The only pre-handshake intercept point
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* is mbedtls_ssl_config_defaults(), called once per ssl_config init.
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*
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* MQTT uses MBEDTLS_SSL_IS_CLIENT; the HTTPS server uses
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* MBEDTLS_SSL_IS_SERVER — this is the discriminator.
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*
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* Note: this file is compiled only for [esp32_base] targets (IDF v4).
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* CONFIG_MBEDTLS_SSL_SESSION_TICKETS is enabled by default in the
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* arduino-esp32 2.x pre-built framework, so no sdkconfig changes are needed.
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*/
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#include "mbedtls/ssl.h"
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#include "mbedtls/ssl_ticket.h"
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#include "esp_random.h"
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/*
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* RSA key exchange cipher suites only.
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* Must be static — mbedTLS stores the pointer, does not copy the array.
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* Terminated with 0.
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*/
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static const int kServerOnlyCipherSuites[] = {
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MBEDTLS_TLS_RSA_WITH_AES_128_GCM_SHA256,
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MBEDTLS_TLS_RSA_WITH_AES_256_GCM_SHA384,
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MBEDTLS_TLS_RSA_WITH_AES_128_CBC_SHA256,
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MBEDTLS_TLS_RSA_WITH_AES_256_CBC_SHA256,
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0
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};
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/*
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* Session ticket context. Must be static — mbedTLS stores the pointer for
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* the lifetime of the server, analogous to kServerOnlyCipherSuites.
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* Initialized once on first server start.
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*/
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static mbedtls_ssl_ticket_context s_ticket_ctx;
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static int s_ticket_ctx_ready = 0;
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/* Thin wrapper around the ESP32 hardware RNG for mbedtls_ssl_ticket_setup(). */
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static int esp_rng(void *ctx, unsigned char *buf, size_t len) {
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(void)ctx;
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esp_fill_random(buf, len);
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return 0;
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}
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extern int __real_mbedtls_ssl_config_defaults(
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mbedtls_ssl_config *conf,
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int endpoint,
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int transport,
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int preset
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);
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int __wrap_mbedtls_ssl_config_defaults(
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mbedtls_ssl_config *conf,
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int endpoint,
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int transport,
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int preset
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) {
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int ret = __real_mbedtls_ssl_config_defaults(conf, endpoint, transport, preset);
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if (ret == 0 && endpoint == MBEDTLS_SSL_IS_SERVER) {
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mbedtls_ssl_conf_ciphersuites(conf, kServerOnlyCipherSuites);
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if (!s_ticket_ctx_ready) {
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mbedtls_ssl_ticket_init(&s_ticket_ctx);
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int ticket_ret = mbedtls_ssl_ticket_setup(&s_ticket_ctx, esp_rng, NULL,
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MBEDTLS_CIPHER_AES_256_GCM, 86400);
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if (ticket_ret == 0) {
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s_ticket_ctx_ready = 1;
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}
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}
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if (s_ticket_ctx_ready) {
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mbedtls_ssl_conf_session_tickets_cb(conf,
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mbedtls_ssl_ticket_write,
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mbedtls_ssl_ticket_parse,
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&s_ticket_ctx);
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}
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}
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return ret;
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}
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