Pass the key creation method to drivers

Pass the key creation method (import/generate/derive/copy) to the
driver methods to allocate or validate a slot number. This allows
drivers to enforce policies such as "this key slot can only be used
for keys generated inside the secure element".

Author: gilles-peskine-arm

include/psa/crypto_se_driver.h

@@ -810,12 +810,23 @@ typedef struct {
  */
 /**@{*/
 
+/** An enumeration indicating how a key is created.
+ */
+typedef enum
+{
+    PSA_KEY_CREATION_IMPORT, /**< During psa_import_key() */
+    PSA_KEY_CREATION_GENERATE, /**< During psa_generate_key() */
+    PSA_KEY_CREATION_DERIVE, /**< During psa_key_derivation_output_key() */
+    PSA_KEY_CREATION_COPY, /**< During psa_copy_key() */
+} psa_key_creation_method_t;
+
 /** \brief A function that allocates a slot for a key.
  *
  * \param[in,out] drv_context       The driver context structure.
  * \param[in,out] persistent_data   A pointer to the persistent data
  *                                  that allows writing.
  * \param[in] attributes            Attributes of the key.
+ * \param method                    The way in which the key is being created.
  * \param[out] key_slot             Slot where the key will be stored.
  *                                  This must be a valid slot for a key of the
  *                                  chosen type. It must be unoccupied.
@@ -831,14 +842,16 @@ typedef psa_status_t (*psa_drv_se_allocate_key_t)(
     psa_drv_se_context_t *drv_context,
     void *persistent_data,
     const psa_key_attributes_t *attributes,
+    psa_key_creation_method_t method,
     psa_key_slot_number_t *key_slot);
 
 /** \brief A function that determines whether a slot number is valid
  * for a key.
  *
- * \param[in,out] drv_context       The driver context structure.
- * \param[in] attributes    Attributes of the key.
- * \param[in] key_slot      Slot where the key is to be stored.
+ * \param[in,out] drv_context   The driver context structure.
+ * \param[in] attributes        Attributes of the key.
+ * \param method                The way in which the key is being created.
+ * \param[in] key_slot          Slot where the key is to be stored.
  *
  * \retval #PSA_SUCCESS
  *         The given slot number is valid for a key with the given
@@ -855,6 +868,7 @@ typedef psa_status_t (*psa_drv_se_allocate_key_t)(
 typedef psa_status_t (*psa_drv_se_validate_slot_number_t)(
     psa_drv_se_context_t *drv_context,
     const psa_key_attributes_t *attributes,
+    psa_key_creation_method_t method,
     psa_key_slot_number_t key_slot);
 
 /** \brief A function that imports a key into a secure element in binary format
@@ -1004,9 +1018,12 @@ typedef struct {
     /** Function that allocates a slot for a key.
      *
      * The core calls this function to determine a slot number, then
-     * calls the actual creation function (such as
-     * psa_drv_se_key_management_t::p_import or
-     * psa_drv_se_key_management_t::p_generate).
+     * calls the actual creation function. The `method` parameter passed
+     * to this function indicates which creation function is called next:
+     * - #PSA_KEY_CREATION_IMPORT is followed by psa_drv_se_key_management_t::p_import
+     * - #PSA_KEY_CREATION_GENERATE is followed by psa_drv_se_key_management_t::p_generate
+     * - #PSA_KEY_CREATION_DERIVE is followed by psa_drv_se_key_derivation_t::p_derive
+     * - #PSA_KEY_CREATION_COPY is followed by psa_drv_se_key_management_t::p_export
      *
      * If this function succeeds, the next call that the core makes to the
      * driver is either the creation function or
@@ -1022,9 +1039,7 @@ typedef struct {
      * The core calls this function instead of
      * psa_drv_se_key_management_t::p_allocate to create
      * a key in a specific slot. It then calls the actual creation function
-     * (such as psa_drv_se_key_management_t::p_import or
-     * psa_drv_se_key_management_t::p_generate) or
-     * psa_drv_se_key_management_t::p_destroy.
+     * as for `p_allocate`.
      */
     psa_drv_se_validate_slot_number_t p_validate_slot_number;
     /** Function that performs a key import operation */

library/psa_crypto.c

@@ -1505,16 +1505,22 @@ static psa_status_t psa_validate_key_attributes(
     return( PSA_SUCCESS );
 }
 
-/** An enumeration indicating how a key is created.
+#if !defined(MBEDTLS_PSA_CRYPTO_SE_C)
+/* Key creation methods only make a difference in this module when
+ * secure element support is enabled. To avoid cluttering the code
+ * with too many #ifdef, we do use the type, and since crypto_se_driver.h
+ * is not #include'd, we need to define it. The type does not need to
+ * be compatible with the definition in crypto_se_driver.h, it just
+ * needs to be sufficient to get the code to build. So do something
+ * that gives compilers a chance of optimizing the variables of this type
+ * away: give all constants the same value.
  */
-typedef enum
-{
-    PSA_KEY_CREATION_IMPORT,
-    PSA_KEY_CREATION_GENERATE,
-    PSA_KEY_CREATION_DERIVE,
-    PSA_KEY_CREATION_COPY,
-    PSA_KEY_CREATION_REGISTER,
-} psa_key_creation_method_t;
+#define PSA_KEY_CREATION_IMPORT 0
+#define PSA_KEY_CREATION_GENERATE 0
+#define PSA_KEY_CREATION_DERIVE 0
+#define PSA_KEY_CREATION_COPY 0
+typedef int psa_key_creation_method_t;
+#endif /* !MBEDTLS_PSA_CRYPTO_SE_C */
 
 /** Prepare a key slot to receive key material.
  *
@@ -1592,7 +1598,7 @@ static psa_status_t psa_start_key_creation(
      * we can roll back to a state where the key doesn't exist. */
     if( *p_drv != NULL && method != PSA_KEY_CREATION_REGISTER )
     {
-        status = psa_find_se_slot_for_key( attributes, *p_drv,
+        status = psa_find_se_slot_for_key( attributes, method, *p_drv,
                                            &slot->data.se.slot_number );
         if( status != PSA_SUCCESS )
             return( status );

library/psa_crypto_se.c

@@ -191,6 +191,7 @@ psa_status_t psa_destroy_se_persistent_data( psa_key_lifetime_t lifetime )
 
 psa_status_t psa_find_se_slot_for_key(
     const psa_key_attributes_t *attributes,
+    psa_key_creation_method_t method,
     psa_se_drv_table_entry_t *driver,
     psa_key_slot_number_t *slot_number )
 {
@@ -214,7 +215,8 @@ psa_status_t psa_find_se_slot_for_key(
             driver->methods->key_management->p_validate_slot_number;
         if( p_validate_slot_number == NULL )
             return( PSA_ERROR_NOT_SUPPORTED );
-        status = p_validate_slot_number( &driver->context, attributes,
+        status = p_validate_slot_number( &driver->context,
+                                         attributes, method,
                                          *slot_number );
     }
     else
@@ -227,7 +229,7 @@ psa_status_t psa_find_se_slot_for_key(
             return( PSA_ERROR_NOT_SUPPORTED );
         status = p_allocate( &driver->context,
                              driver->internal.persistent_data,
-                             attributes,
+                             attributes, method,
                              slot_number );
     }
     return( status );

library/psa_crypto_se.h

@@ -135,6 +135,7 @@ psa_drv_se_context_t *psa_get_se_driver_context(
  */
 psa_status_t psa_find_se_slot_for_key(
     const psa_key_attributes_t *attributes,
+    psa_key_creation_method_t method,
     psa_se_drv_table_entry_t *driver,
     psa_key_slot_number_t *slot_number );
 

tests/suites/test_suite_psa_crypto_se_driver_hal.function

@@ -41,10 +41,12 @@
 static psa_status_t counter_allocate( psa_drv_se_context_t *context,
                                       void *persistent_data,
                                       const psa_key_attributes_t *attributes,
+                                      psa_key_creation_method_t method,
                                       psa_key_slot_number_t *slot_number )
 {
     psa_key_slot_number_t *p_counter = persistent_data;
     (void) attributes;
+    (void) method;
     if( context->persistent_data_size != sizeof( psa_key_slot_number_t ) )
         return( PSA_ERROR_DETECTED_BY_DRIVER );
     ++*p_counter;
@@ -162,10 +164,12 @@ static psa_status_t ram_destroy( psa_drv_se_context_t *context,
 static psa_status_t ram_allocate( psa_drv_se_context_t *context,
                                   void *persistent_data,
                                   const psa_key_attributes_t *attributes,
+                                  psa_key_creation_method_t method,
                                   psa_key_slot_number_t *slot_number )
 {
     ram_slot_usage_t *slot_usage = persistent_data;
     (void) attributes;
+    (void) method;
     DRIVER_ASSERT( context->persistent_data_size == sizeof( ram_slot_usage_t ) );
     for( *slot_number = ram_min_slot;
          *slot_number < ARRAY_LENGTH( ram_slots );
@@ -180,10 +184,12 @@ static psa_status_t ram_allocate( psa_drv_se_context_t *context,
 static psa_status_t ram_validate_slot_number(
     psa_drv_se_context_t *context,
     const psa_key_attributes_t *attributes,
+    psa_key_creation_method_t method,
     psa_key_slot_number_t slot_number )
 {
     (void) context;
     (void) attributes;
+    (void) method;
     if( slot_number >= ARRAY_LENGTH( ram_slots ) )
         return( PSA_ERROR_INVALID_ARGUMENT );
     return( PSA_SUCCESS );