Merge pull request #10004 from gilles-peskine-arm/doc-threading-needed-by-psa-3.6
Backport 3.6: Document PSA's need for threading
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index ebc9276..e16f31a 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -1807,6 +1807,11 @@
* running handshake hash) only use PSA crypto if
* #MBEDTLS_USE_PSA_CRYPTO is enabled.
*
+ * \note In multithreaded applications, you must also enable
+ * #MBEDTLS_THREADING_C, even if individual TLS contexts are not
+ * shared between threads, unless only one thread ever calls
+ * TLS functions.
+ *
* Uncomment this macro to enable the support for TLS 1.3.
*/
#define MBEDTLS_SSL_PROTO_TLS1_3
@@ -2125,6 +2130,10 @@
* before calling any function from the SSL/TLS, X.509 or PK modules, except
* for the various mbedtls_xxx_init() functions which can be called at any time.
*
+ * \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.
+ *
* \note An important and desirable effect of this option is that it allows
* PK, X.509 and TLS to take advantage of PSA drivers. For example, enabling
* this option is what allows use of drivers for ECDSA, ECDH and EC J-PAKE in
@@ -3211,7 +3220,18 @@
/**
* \def MBEDTLS_PSA_CRYPTO_C
*
- * Enable the Platform Security Architecture cryptography API.
+ * Enable the Platform Security Architecture (PSA) 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:
+ * - performing a TLS handshake if support for TLS 1.3 is enabled;
+ * - using a TLS 1.3 connection;
+ * - indirect calls from PK, X.509 or SSL functions when
+ * #MBEDTLS_USE_PSA_CRYPTO is enabled;
+ * - indirect calls to calculate a hash when #MBEDTLS_MD_C is disabled;
+ * - any other call to a function that requires calling psa_crypto_init()
+ * beforehand.
*
* Module: library/psa_crypto.c
*
@@ -3631,10 +3651,38 @@
* \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).
+ * - A TLS 1.3 connection, regardless of the compile-time configuration.
+ * - Any library feature that calculates a hash, if `MBEDTLS_MD_C` is disabled.
+ * As an exception, algorithm-specific low-level modules do not require
+ * threading protection unless the contexts are shared between threads.
+ * - Any library feature that performs symmetric encryption or decryption,
+ * if `MBEDTLS_CIPHER_C` is disabled.
+ * As an exception, algorithm-specific low-level modules do not require
+ * threading protection unless the 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