Document PSA's need for threading
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 84af7f7..078d5c5 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -2290,6 +2290,10 @@
* That is, the APIs enabled by this option are not covered by the usual
* promises of API stability.
*
+ * \warning In multithreaded applications, you must also enable
+ * #MBEDTLS_THREADING_C, unless only one thread ever calls PSA functions
+ * (`psa_xxx()`), including indirect calls through SSL/TLS, X.509 or PK.
+ *
* Requires: MBEDTLS_PSA_CRYPTO_C.
*
* Uncomment this to enable internal use of PSA Crypto and new associated APIs.
@@ -3389,6 +3393,14 @@
*
* Enable the Platform Security Architecture cryptography API.
*
+ * \note In multithreaded applications, you must enable #MBEDTLS_THREADING_C,
+ * unless only one thread ever calls `psa_xxx()` functions.
+ * That includes indirect calls, such as:
+ * - indirect calls from PK, X.509 or SSL functions when
+ * #MBEDTLS_USE_PSA_CRYPTO is enabled;
+ * - any other call to a function that requires calling psa_crypto_init()
+ * beforehand.
+ *
* Module: library/psa_crypto.c
*
* Requires: either MBEDTLS_CTR_DRBG_C and MBEDTLS_ENTROPY_C,
@@ -3605,11 +3617,29 @@
/**
* \def MBEDTLS_THREADING_C
*
- * Enable the threading abstraction layer.
- * By default Mbed TLS assumes it is used in a non-threaded environment or that
- * contexts are not shared between threads. If you do intend to use contexts
+ * Traditionally, Mbed TLS assumes it is used in a non-threaded environment or
+ * that contexts are not shared between threads. If you do intend to use contexts
* between threads, you will need to enable this layer to prevent race
- * conditions. See also our Knowledge Base article about threading:
+ * conditions.
+ *
+ * The PSA subsystem has an implicit shared context. Therefore, you must
+ * enable this option if more than one thread may use any part of
+ * Mbed TLS that is implemented on top of the PSA subsystem.
+ *
+ * You must enable this option in multithreaded applications where more than
+ * one thread performs any of the following operations:
+ *
+ * - Any call to a PSA function (`psa_xxx()`).
+ * - Any call to a TLS, X.509 or PK function (`mbedtls_ssl_xxx()`,
+ * `mbedtls_x509_xxx()`, `mbedtls_pkcs7_xxx()`, `mbedtls_pk_xxx()`)
+ * if `MBEDTLS_USE_PSA_CRYPTO` is enabled (regardless of whether individual
+ * TLS, X.509 or PK contexts are shared between threads).
+ * - Any use of a cryptographic context if the same context is used in
+ * multiple threads.
+ * - Any call to a function where the documentation specifies that
+ * psa_crypto_init() must be called prior to that function.
+ *
+ * See also our Knowledge Base article about threading:
* https://mbed-tls.readthedocs.io/en/latest/kb/development/thread-safety-and-multi-threading
*
* Module: library/threading.c