psa: let mbedtls_psa_get_random() always use psa_generate_random()

It means that mbedtls_psa_get_random() goes through the PSA interface
all the times. Fallbacks to CTR_DRBG or HMAC_DRBG are still possible,
but that depends on how the crypto provider is built.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

include/mbedtls/psa_util.h

@@ -23,42 +23,22 @@
 
 #if defined(MBEDTLS_PSA_CRYPTO_C)
 
-/* Expose whatever RNG the PSA subsystem uses to applications using the
- * mbedtls_xxx API. The declarations and definitions here need to be
- * consistent with the implementation in library/psa_crypto_random_impl.h.
- * See that file for implementation documentation. */
-
-
-/* The type of a `f_rng` random generator function that many library functions
- * take.
- *
- * This type name is not part of the Mbed TLS stable API. It may be renamed
- * or moved without warning.
- */
-typedef int mbedtls_f_rng_t(void *p_rng, unsigned char *output, size_t output_size);
-
-#if defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
-
 /** The random generator function for the PSA subsystem.
  *
  * This function is suitable as the `f_rng` random generator function
- * parameter of many `mbedtls_xxx` functions. Use #MBEDTLS_PSA_RANDOM_STATE
+ * parameter of many `mbedtls_xxx` functions.
- * to obtain the \p p_rng parameter.
  *
  * The implementation of this function depends on the configuration of the
  * library.
  *
- * \note Depending on the configuration, this may be a function or
- *       a pointer to a function.
- *
  * \note This function may only be used if the PSA crypto subsystem is active.
  *       This means that you must call psa_crypto_init() before any call to
  *       this function, and you must not call this function after calling
  *       mbedtls_psa_crypto_free().
  *
- * \param p_rng         The random generator context. This must be
+ * \param p_rng         This parameter is only kept for backward compatibility
- *                      #MBEDTLS_PSA_RANDOM_STATE. No other state is
+ *                      reasons with legacy `f_rng` functions and it's ignored.
- *                      supported.
+ *                      Set to #MBEDTLS_PSA_RANDOM_STATE or NULL.
  * \param output        The buffer to fill. It must have room for
  *                      \c output_size bytes.
  * \param output_size   The number of bytes to write to \p output.
@@ -80,32 +60,11 @@ int mbedtls_psa_get_random(void *p_rng,
 
 /** The random generator state for the PSA subsystem.
  *
- * This macro expands to an expression which is suitable as the `p_rng`
+ * This macro always expands to NULL because the `p_rng` parameter is unused
- * random generator state parameter of many `mbedtls_xxx` functions.
+ * in mbedtls_psa_get_random(), but it's kept for interface's backward
- * It must be used in combination with the random generator function
+ * compatibility.
- * mbedtls_psa_get_random().
- *
- * The implementation of this macro depends on the configuration of the
- * library. Do not make any assumption on its nature.
  */
-#define MBEDTLS_PSA_RANDOM_STATE NULL
+#define MBEDTLS_PSA_RANDOM_STATE    NULL
-
-#else /* !defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG) */
-
-#if defined(MBEDTLS_CTR_DRBG_C)
-#include "mbedtls/ctr_drbg.h"
-typedef mbedtls_ctr_drbg_context mbedtls_psa_drbg_context_t;
-static mbedtls_f_rng_t *const mbedtls_psa_get_random = mbedtls_ctr_drbg_random;
-#elif defined(MBEDTLS_HMAC_DRBG_C)
-#include "mbedtls/hmac_drbg.h"
-typedef mbedtls_hmac_drbg_context mbedtls_psa_drbg_context_t;
-static mbedtls_f_rng_t *const mbedtls_psa_get_random = mbedtls_hmac_drbg_random;
-#endif
-extern mbedtls_psa_drbg_context_t *const mbedtls_psa_random_state;
-
-#define MBEDTLS_PSA_RANDOM_STATE mbedtls_psa_random_state
-
-#endif /* !defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG) */
 
 /** \defgroup psa_tls_helpers TLS helper functions
  * @{

library/psa_crypto.c

@@ -102,7 +102,7 @@ typedef struct {
 static psa_global_data_t global_data;
 
 #if !defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
-mbedtls_psa_drbg_context_t *const mbedtls_psa_random_state =
+mbedtls_psa_drbg_context_t *const mbedtls_psa_drbg_ctx =
     &global_data.rng.drbg;
 #endif
 
@@ -7322,7 +7322,7 @@ static void mbedtls_psa_random_init(mbedtls_psa_random_context_t *rng)
                                MBEDTLS_ENTROPY_SOURCE_STRONG);
 #endif
 
-    mbedtls_psa_drbg_init(MBEDTLS_PSA_RANDOM_STATE);
+    mbedtls_psa_drbg_init(MBEDTLS_PSA_DRBG_CTX);
 #endif /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */
 }
 
@@ -7333,7 +7333,7 @@ static void mbedtls_psa_random_free(mbedtls_psa_random_context_t *rng)
 #if defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
     memset(rng, 0, sizeof(*rng));
 #else /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */
-    mbedtls_psa_drbg_free(MBEDTLS_PSA_RANDOM_STATE);
+    mbedtls_psa_drbg_free(MBEDTLS_PSA_DRBG_CTX);
     rng->entropy_free(&rng->entropy);
 #endif /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */
 }
@@ -7382,8 +7382,8 @@ psa_status_t psa_generate_random(uint8_t *output,
             (output_size > MBEDTLS_PSA_RANDOM_MAX_REQUEST ?
              MBEDTLS_PSA_RANDOM_MAX_REQUEST :
              output_size);
-        int ret = mbedtls_psa_get_random(MBEDTLS_PSA_RANDOM_STATE,
+        int ret = mbedtls_psa_legacy_get_random(MBEDTLS_PSA_DRBG_CTX,
-                                         output, request_size);
+                                                output, request_size);
         if (ret != 0) {
             return mbedtls_to_psa_error(ret);
         }

library/psa_crypto_random_impl.h

@@ -20,23 +20,9 @@
 
 #include "psa_util_internal.h"
 
-#if defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
+#if !defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
 
-#include <string.h>
+#include "mbedtls/entropy.h"
-#include <mbedtls/entropy.h> // only for error codes
-#include <psa/crypto.h>
-
-typedef mbedtls_psa_external_random_context_t mbedtls_psa_random_context_t;
-
-/* Trivial wrapper around psa_generate_random(). */
-int mbedtls_psa_get_random(void *p_rng,
-                           unsigned char *output,
-                           size_t output_size);
-
-/* The PSA RNG API doesn't need any externally maintained state. */
-#define MBEDTLS_PSA_RANDOM_STATE NULL
-
-#else /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */
 
 /* Choose a DRBG based on configuration and availability */
 #if defined(MBEDTLS_PSA_HMAC_DRBG_MD_TYPE)
@@ -67,11 +53,23 @@ int mbedtls_psa_get_random(void *p_rng,
 #error "No hash algorithm available for HMAC_DBRG."
 #endif
 
-#else
+#else /* !MBEDTLS_PSA_HMAC_DRBG_MD_TYPE && !MBEDTLS_CTR_DRBG_C && !MBEDTLS_HMAC_DRBG_C*/
 #error "No DRBG module available for the psa_crypto module."
-#endif
+#endif /* !MBEDTLS_PSA_HMAC_DRBG_MD_TYPE && !MBEDTLS_CTR_DRBG_C && !MBEDTLS_HMAC_DRBG_C*/
 
-#include "mbedtls/entropy.h"
+#if defined(MBEDTLS_CTR_DRBG_C)
+#include "mbedtls/ctr_drbg.h"
+#elif defined(MBEDTLS_HMAC_DRBG_C)
+#include "mbedtls/hmac_drbg.h"
+#endif /* !MBEDTLS_CTR_DRBG_C && !MBEDTLS_HMAC_DRBG_C */
+
+#if defined(MBEDTLS_CTR_DRBG_C)
+#define mbedtls_psa_legacy_get_random       mbedtls_ctr_drbg_random
+typedef mbedtls_ctr_drbg_context            mbedtls_psa_drbg_context_t;
+#elif defined(MBEDTLS_HMAC_DRBG_C)
+#define mbedtls_psa_legacy_get_random       mbedtls_hmac_drbg_random
+typedef mbedtls_hmac_drbg_context           mbedtls_psa_drbg_context_t;
+#endif /* !MBEDTLS_CTR_DRBG_C && !MBEDTLS_HMAC_DRBG_C */
 
 /** Initialize the PSA DRBG.
  *
@@ -111,20 +109,6 @@ typedef struct {
     mbedtls_psa_drbg_context_t drbg;
 } mbedtls_psa_random_context_t;
 
-/* Defined in include/psa_util_internal.h so that it's visible to
- * application code. The declaration here is redundant, but included
- * as a safety net to make it more likely that a future change that
- * accidentally causes the implementation to diverge from the interface
- * will be noticed. */
-/* Do not include the declaration under MSVC because it doesn't accept it
- * ("error C2370: 'mbedtls_psa_get_random' : redefinition; different storage class").
- * Observed with Visual Studio 2013. A known bug apparently:
- * https://stackoverflow.com/questions/8146541/duplicate-external-static-declarations-not-allowed-in-visual-studio
- */
-#if !defined(_MSC_VER)
-static mbedtls_f_rng_t *const mbedtls_psa_get_random;
-#endif
-
 /** The maximum number of bytes that mbedtls_psa_get_random() is expected to
  * return.
  */
@@ -134,27 +118,13 @@ static mbedtls_f_rng_t *const mbedtls_psa_get_random;
 #define MBEDTLS_PSA_RANDOM_MAX_REQUEST MBEDTLS_HMAC_DRBG_MAX_REQUEST
 #endif
 
-/** A pointer to the PSA DRBG state.
+/** A pointer to the PSA DRBG context.
  *
  * This variable is only intended to be used through the macro
- * #MBEDTLS_PSA_RANDOM_STATE.
+ * #MBEDTLS_PSA_DRBG_CTX.
  */
-/* psa_crypto.c sets this variable to a pointer to the DRBG state in the
+extern mbedtls_psa_drbg_context_t *const mbedtls_psa_drbg_ctx;
- * global PSA crypto state. */
+#define MBEDTLS_PSA_DRBG_CTX    mbedtls_psa_drbg_ctx
-/* The type `mbedtls_psa_drbg_context_t` is defined in
- * include/psa_util_internal.h so that `mbedtls_psa_random_state` can be
- * declared there and be visible to application code. */
-extern mbedtls_psa_drbg_context_t *const mbedtls_psa_random_state;
-
-/** A pointer to the PSA DRBG state.
- *
- * This macro expands to an expression that is suitable as the \c p_rng
- * parameter to pass to mbedtls_psa_get_random().
- *
- * This macro exists in all configurations where the psa_crypto module is
- * enabled. Its expansion depends on the configuration.
- */
-#define MBEDTLS_PSA_RANDOM_STATE mbedtls_psa_random_state
 
 /** Seed the PSA DRBG.
  *
@@ -172,14 +142,14 @@ static inline int mbedtls_psa_drbg_seed(
     const unsigned char *custom, size_t len)
 {
 #if defined(MBEDTLS_CTR_DRBG_C)
-    return mbedtls_ctr_drbg_seed(MBEDTLS_PSA_RANDOM_STATE,
+    return mbedtls_ctr_drbg_seed(MBEDTLS_PSA_DRBG_CTX,
                                  mbedtls_entropy_func,
                                  entropy,
                                  custom, len);
 #elif defined(MBEDTLS_HMAC_DRBG_C)
     const mbedtls_md_info_t *md_info =
         mbedtls_md_info_from_type(MBEDTLS_PSA_HMAC_DRBG_MD_TYPE);
-    return mbedtls_hmac_drbg_seed(MBEDTLS_PSA_RANDOM_STATE,
+    return mbedtls_hmac_drbg_seed(MBEDTLS_PSA_DRBG_CTX,
                                   md_info,
                                   mbedtls_entropy_func,
                                   entropy,

library/psa_util.c

@@ -46,6 +46,7 @@
 #if defined(MBEDTLS_BLOCK_CIPHER_SOME_PSA)
 #include <mbedtls/cipher.h>
 #endif
+#include <mbedtls/entropy.h>
 
 /* PSA_SUCCESS is kept at the top of each error table since
  * it's the most common status when everything functions properly. */
@@ -343,15 +344,7 @@ mbedtls_ecp_group_id mbedtls_ecc_group_from_psa(psa_ecc_family_t family,
  * `mbedtls_psa_get_random(MBEDTLS_PSA_RANDOM_STATE, ...)` calls
  * `psa_generate_random(...)`. The state parameter is ignored since the
  * PSA API doesn't support passing an explicit state.
- *
- * In the non-external case, psa_generate_random() calls an
- * `mbedtls_xxx_drbg_random` function which has exactly the same signature
- * and semantics as mbedtls_psa_get_random(). As an optimization,
- * instead of doing this back-and-forth between the PSA API and the
- * classic API, psa_crypto_random_impl.h defines `mbedtls_psa_get_random`
- * as a constant function pointer to `mbedtls_xxx_drbg_random`.
  */
-#if defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
 int mbedtls_psa_get_random(void *p_rng,
                            unsigned char *output,
                            size_t output_size)
@@ -369,7 +362,6 @@ int mbedtls_psa_get_random(void *p_rng,
         return MBEDTLS_ERR_ENTROPY_SOURCE_FAILED;
     }
 }
-#endif /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */
 
 #endif /* MBEDTLS_PSA_CRYPTO_C */