Re: [mbed-tls] MBEDTLS_ENTROPY_HARDWARE_ALT, mbedtls_ctr_drbg_seed() freezes ? - mbed-tls

12 Feb 2021


      Hi Manu,
f_entropy is the argument passed to mbedtls_ctr_drbg_seed, which is
mbedtls_entropy_func. This function obtains entropy from all configured
sources, which depends on the library configuration. Check if it might
be calling other sources (see what sources are added in
mbedtls_entropy_init in library/entropy.c).
I think there's a bug in your mbedtls_hardware_poll implementation. It's
receiving a buffer ent_buf with room for count BYTES, but you're filling
it with count WORDS. You need to pass the exact number of bytes. In
practice, unless the library is configured in a weird way, count will be
a multiple of 4, which can make your code a bit simpler.
-- 
Gilles Peskine
Mbed TLS developer

On 17/08/2020 19:16, Manu Abraham via mbed-tls wrote:
> 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);work
>     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