The PSA RNG is not thread-safe

[gilles-peskine-arm]

The global RNG used inside PSA crypto is not thread-safe if more than one thread can execute PSA crypto code at the same time. It should be protected by a mutex.

[gilles-peskine-arm]

Actually CTR_DRBG and HMAC_DRBG objects are protected by a mutex. So the PSA RNG is safe if either of these is used. This leaves the case of MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG. The call to mbedtls_psa_external_get_random should be protected by a mutex.

[yanesca]

It makes sense to handle MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG consistently with drivers. In the MVP thread safety is the responsibility of the drivers and the core does not make any guarantees.