MBEDTLS_ENTROPY_HARDWARE_ALT, mbedtls_ctr_drbg_seed() freezes ? - mbed-tls

17 Aug 2020


      Greetings,
I am new to the list, please do excuse me, in case of any list
specific etiquette issues.
Trying to use a 1.6.1 release with a Cortex M7 port, specifically a STM32H7.
After enabling MBEDTLS_ENTROPY_HARDWARE_ALT, did implement
mbedtls_hardware_poll()
It looks thus, and it does appear to work from a hardware perspective:
/**
 * mbedtls_hardware_poll()
 * Read random data from the Hardware RNG for entropy applications
 */
int mbedtls_hardware_poll(void *arg,
              unsigned char *ent_buf,
              size_t count,
              size_t *ent_len)
{
    register uint8_t i = 0;
    uint32_t rand;
if (!LL_RNG_IsEnabled(RNG))
        LL_RNG_Enable(RNG);                /* Enable Random Number Generator */
for (i = 0; i < count; i++) {
        while (!LL_RNG_IsActiveFlag_DRDY(RNG)) { }    /* Wait for DRDY
flag to be raised */
        if ((LL_RNG_IsActiveFlag_CECS(RNG))    ||
            (LL_RNG_IsActiveFlag_SECS(RNG))) {        /* Check error, if any */
/* Clock or Seed Error detected. Set Error */
            printf(" (%d) %s: Clock/Seed Error!\r\n", __LINE__, __FUNCTION__);
        }
        rand = LL_RNG_ReadRandData32(RNG);        /* Read RNG data */
        memcpy(&(ent_buf[i * 4]), &rand, 4);        /* *ent_len += 4 */
    }
    LL_RNG_Disable(RNG);                    /* Stop random numbers generation */
    *ent_len = ((i + 1) * 4);
    printf(" (%d) %s: Random Words: %d Word: %04d\r\n",
        __LINE__,
        __FUNCTION__,
        count,
        rand);
return 0;
}
The code which causes the problem is this, in my tls_init()
int tls_init(void)
{
    int ret;
/* inspired by https://tls.mbed.org/kb/how-to/mbedtls-tutorial */
    const char *pers = "SYS-LWH7";
printf(" (%d) %s: Initializing\r\n", __LINE__, __FUNCTION__);
    /* initialize descriptors */
mbedtls_ssl_init(&ssl);
    printf(" (%d) %s: SSL initialize\r\n", __LINE__, __FUNCTION__);
mbedtls_ssl_config_init(&conf);
    printf(" (%d) %s: SSL Config initialized\r\n", __LINE__, __FUNCTION__);
mbedtls_x509_crt_init(&cacert);
    printf(" (%d) %s: x509 CRT initialized\r\n", __LINE__, __FUNCTION__);
mbedtls_ctr_drbg_init(&ctr_drbg);
    printf(" (%d) %s: DRBG initialized\r\n", __LINE__, __FUNCTION__);
mbedtls_entropy_init(&entropy);
    printf(" (%d) %s: Entropy initialized\r\n", __LINE__, __FUNCTION__);
ret = mbedtls_ctr_drbg_seed(&ctr_drbg,
                    mbedtls_entropy_func,
                    &entropy,
                    (const unsigned char *) pers,
                    strlen(pers));
    if (ret) {
LWIP_DEBUGF(MQTT_APP_DEBUG_TRACE,
                ("failed !\n mbedtls_ctr_drbg_seed returned %d\n",
                ret));
printf(" (%d) %s: DRBG seed failed, ret=%d\r\n", __LINE__,
__FUNCTION__, ret);
        return -1;
    }
    printf(" (%d) %s: DRBG seed returned:%d\r\n", __LINE__, __FUNCTION__, ret);
/**
     * The transport type determines if we are using
     * TLS (MBEDTLS_SSL_TRANSPORT_STREAM) or
     * DTLS (MBEDTLS_SSL_TRANSPORT_DATAGRAM).
     */
    ret = mbedtls_ssl_config_defaults(&conf,
                      MBEDTLS_SSL_IS_CLIENT,
                      MBEDTLS_SSL_TRANSPORT_STREAM,
                      MBEDTLS_SSL_PRESET_DEFAULT);
    if (ret) {
        LWIP_DEBUGF(MQTT_APP_DEBUG_TRACE,
                ("failed !\n mbedtls_ssl_config_defaults returned %d\n\n",
                ret));
printf("(%d) %s: SSL config defaults failed, ret=%d\r\n",
__LINE__, __FUNCTION__, ret);
        return -1;
    }
    printf("(%d) %s: SSL config defaults returned:%d\r\n", __LINE__,
__FUNCTION__, ret);
ret = mbedtls_x509_crt_parse(&cacert,
                     (const unsigned char *)test_ca_crt,
                     test_ca_crt_len);
    if (ret)
        printf(" (%d) %s: failed!\n  mbedtls_x509_crt_parse returned
%d\r\n", __LINE__, __FUNCTION__, ret);
    else
        printf(" (%d) %s: mbedtls_x509_crt_parse returned %d\r\n",
__LINE__, __FUNCTION__, ret);
mbedtls_ssl_conf_ca_chain(&conf, &cacert, NULL);
    mbedtls_ssl_conf_authmode(&conf, MBEDTLS_SSL_VERIFY_REQUIRED);
/**
     * The library needs to know which random engine
     * to use and which debug function to use as callback.
     */
    mbedtls_ssl_conf_rng(&conf, mbedtls_ctr_drbg_random, &ctr_drbg);
    mbedtls_ssl_conf_dbg(&conf, my_debug, stdout);
    mbedtls_ssl_setup(&ssl, &conf);
}
The output of which looks thus, in a serial terminal:
(1217) print_dhcp_state: Try connect to Broker
(174) tls_init: Initializing
(178) tls_init: SSL initialize
(181) tls_init: SSL Config initialized
(184) tls_init: x509 CRT initialized
(187) tls_init: DRBG initialized
(190) tls_init: Entropy initialized
 (1027) mbedtls_hardware_poll: Random Words: 128 Word: -558876895
Any thoughts/ideas, what could be wrong ?
Any kind soul in here ?
Thanks,
Manu