Prevent mbedtls_psa_register_se_key with volatile keys

mbedtls_psa_register_se_key() is not usable with volatile keys, since there
is no way to return the implementation-chosen key identifier which would be
needed to use the key. Document this limitation. Reject an attempt to create
such an unusable key. Fixes Mbed-TLS#9253.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

ChangeLog.d/mbedtls_psa_register_se_key.txt

@@ -0,0 +1,3 @@
+Bugfix
+   * Document and enforce the limitation of mbedtls_psa_register_se_key()
+     to persistent keys. Resolves #9253.

include/psa/crypto.h

@@ -129,6 +129,9 @@ static psa_key_attributes_t psa_key_attributes_init(void);
  *
  * \param[out] attributes  The attribute structure to write to.
  * \param key              The persistent identifier for the key.
+ *                         This can be any value in the range from
+ *                         #PSA_KEY_ID_USER_MIN to #PSA_KEY_ID_USER_MAX
+ *                         inclusive.
  */
 static void psa_set_key_id(psa_key_attributes_t *attributes,
                            mbedtls_svc_key_id_t key);

include/psa/crypto_extra.h

@@ -154,6 +154,14 @@ static inline void psa_clear_key_slot_number(
  * specified in \p attributes.
  *
  * \param[in] attributes        The attributes of the existing key.
+ *                              - The lifetime must be a persistent lifetime
+ *                                in a secure element. Volatile lifetimes are
+ *                                not currently supported.
+ *                              - The key identifier must be in the valid
+ *                                range for persistent keys.
+ *                              - The key type and size must be specified and
+ *                                must be consistent with the key material
+ *                                in the secure element.
  *
  * \retval #PSA_SUCCESS
  *         The key was successfully registered.

library/psa_crypto.c

@@ -2149,6 +2149,14 @@ psa_status_t mbedtls_psa_register_se_key(
         return PSA_ERROR_NOT_SUPPORTED;
     }
 
+    /* Not usable with volatile keys, even with an appropriate location,
+     * due to the API design.
+     * https://github.com/Mbed-TLS/mbedtls/issues/9253
+     */
+    if (PSA_KEY_LIFETIME_IS_VOLATILE(psa_get_key_lifetime(attributes))) {
+        return PSA_ERROR_INVALID_ARGUMENT;
+    }
+
     status = psa_start_key_creation(PSA_KEY_CREATION_REGISTER, attributes,
                                     &slot, &driver);
     if (status != PSA_SUCCESS) {

tests/suites/test_suite_psa_crypto_se_driver_hal.data

@@ -148,7 +148,16 @@ generate_key_smoke:PSA_KEY_TYPE_HMAC:256:PSA_ALG_HMAC( PSA_ALG_SHA_256 )
 Key registration: smoke test
 register_key_smoke_test:TEST_SE_PERSISTENT_LIFETIME:7:1:1:PSA_SUCCESS
 
-Key registration: invalid lifetime (volatile internal storage)
+Key registration: invalid lifetime (volatile, in SE, id=0)
+register_key_smoke_test:TEST_SE_VOLATILE_LIFETIME:7:0:0:PSA_ERROR_INVALID_ARGUMENT
+
+Key registration: invalid lifetime (volatile, in SE, id=1)
+register_key_smoke_test:TEST_SE_VOLATILE_LIFETIME:7:1:1:PSA_ERROR_INVALID_ARGUMENT
+
+Key registration: invalid lifetime (volatile, internal, id=0)
+register_key_smoke_test:PSA_KEY_LIFETIME_VOLATILE:7:0:0:PSA_ERROR_INVALID_ARGUMENT
+
+Key registration: invalid lifetime (volatile, internal, id=1)
 register_key_smoke_test:PSA_KEY_LIFETIME_VOLATILE:7:1:1:PSA_ERROR_INVALID_ARGUMENT
 
 Key registration: invalid lifetime (internal storage)