Browsers negotiate ECDHE cipher suites by default. On ESP32-S3 the
hardware RSA accelerator handles RSA key exchange efficiently, but
there is no ECP hardware accelerator. ECDHE requires the server to
compute an ephemeral key pair: ecp_precompute_comb() builds a comb
table through many sequential ECP point doublings, each dispatched
to the hardware bignum unit (esp_bignum.c), but the ECP layer has
no RTOS yield points between iterations. The entire computation runs
to completion on CPU 0 without ever resetting the task watchdog.
A single handshake does not exceed the watchdog timeout on its own,
but two consecutive handshakes (e.g. a browser retry after a failed
attempt) accumulate enough uninterrupted runtime to starve IDLE0:
E (54924) esp-tls-mbedtls: mbedtls_ssl_handshake returned -0x0050
E (57208) esp-tls-mbedtls: mbedtls_ssl_handshake returned -0x7280
E (57638) task_wdt: Task watchdog got triggered.
E (57638) task_wdt: - IDLE0 (CPU 0)
E (57638) task_wdt: Tasks currently running:
E (57638) task_wdt: CPU 0: httpd
The crash occurs in ecp_precompute_comb() → ecp_double_jac() →
mbedtls_mpi_mul_mpi() during the ServerKeyExchange step.
Fix by wrapping mbedtls_ssl_config_defaults() via the linker --wrap
mechanism. The wrapper intercepts server-side SSL config init
(MBEDTLS_SSL_IS_SERVER) and replaces the cipher suite list with
RSA key exchange only, routing handshakes through the hardware RSA
accelerator and eliminating the ECDH path entirely. MQTT connections
(MBEDTLS_SSL_IS_CLIENT) are unaffected.
Also switch the self-signed cert generator from EC (prime256v1) to
RSA 2048 so the generated certificate matches the restricted cipher
suites.
Changes:
- arch/esp32/tls_cipher_restrict.c: new file implementing the
mbedtls_ssl_config_defaults wrap; restricts server cipher suites
to RSA_WITH_AES_{128,256}_{GCM,CBC}_SHA{256,384}
- platformio.ini: add -Wl,--wrap=mbedtls_ssl_config_defaults and
include tls_cipher_restrict.c in the esp32_base build
- arch/esp32/extra_scripts/generate_web_panel_cert.py: switch key
generation from `openssl ecparam -name prime256v1` to `openssl
genrsa 2048`
