Add infrastructure for key attribute flags

Add infrastructure for internal, external and dual-use flags, with a
compile-time check (if static_assert is available) to ensure that the
same numerical value doesn't get declared for two different purposes
in crypto_struct.h (external or dual-use) and
psa_crypto_core.h (internal).

Author: gilles-peskine-arm

include/psa/crypto_struct.h

@@ -322,14 +322,35 @@ typedef uint16_t psa_key_bits_t;
  * conditionals. */
 #define PSA_MAX_KEY_BITS 0xfff8
 
+/** A mask of flags that can be stored in key attributes.
+ *
+ * This type is also used internally to store flags in slots. Internal
+ * flags are defined in library/psa_crypto_core.h. Internal flags may have
+ * the same value as external flags if they are properly handled during
+ * key creation and in psa_get_key_attributes.
+ */
+typedef uint16_t psa_key_attributes_flag_t;
+
+#define MBEDLTS_PSA_KA_FLAG_SLOT_NUMBER ( (psa_key_attributes_flag_t) 0x0001 )
+
+/* A mask of key attribute flags used externally only.
+ * Only meant for internal checks inside the library. */
+#define MBEDTLS_PSA_KA_MASK_EXTERNAL_ONLY (      \
+        0 )
+
+/* A mask of key attribute flags used both internally and externally.
+ * Currently there aren't any. */
+#define MBEDTLS_PSA_KA_MASK_DUAL_USE (          \
+        0 )
+
 typedef struct
 {
     psa_key_type_t type;
     psa_key_lifetime_t lifetime;
     psa_key_id_t id;
     psa_key_policy_t policy;
     psa_key_bits_t bits;
-    uint16_t flags;
+    psa_key_attributes_flag_t flags;
 } psa_core_key_attributes_t;
 
 #define PSA_CORE_KEY_ATTRIBUTES_INIT {0, 0, 0, {0, 0, 0}, 0, 0}

library/psa_crypto.c

@@ -1407,6 +1407,15 @@ psa_status_t psa_export_public_key( psa_key_handle_t handle,
                                      data_length, 1 ) );
 }
 
+#if defined(static_assert)
+static_assert( ( MBEDTLS_PSA_KA_MASK_EXTERNAL_ONLY & MBEDTLS_PSA_KA_MASK_DUAL_USE ) == 0,
+               "One or more key attribute flag is listed as both external-only and dual-use" );
+static_assert( ( MBEDTLS_PSA_KA_MASK_INTERNAL_ONLY & MBEDTLS_PSA_KA_MASK_DUAL_USE ) == 0,
+               "One or more key attribute flag is listed as both external-only and dual-use" );
+static_assert( ( MBEDTLS_PSA_KA_MASK_INTERNAL_ONLY & MBEDTLS_PSA_KA_MASK_EXTERNAL_ONLY ) == 0,
+               "One or more key attribute flag is listed as both internal-only and external-only" );
+#endif
+
 /** Validate that a key policy is internally well-formed.
  *
  * This function only rejects invalid policies. It does not validate the
@@ -1466,6 +1475,11 @@ static psa_status_t psa_validate_key_attributes(
     if( psa_get_key_bits( attributes ) > PSA_MAX_KEY_BITS )
         return( PSA_ERROR_NOT_SUPPORTED );
 
+    /* Reject invalid flags. These should not be reachable through the API. */
+    if( attributes->core.flags & ~ ( MBEDTLS_PSA_KA_MASK_EXTERNAL_ONLY |
+                                     MBEDTLS_PSA_KA_MASK_DUAL_USE ) )
+        return( PSA_ERROR_INVALID_ARGUMENT );
+
     return( PSA_SUCCESS );
 }
 
@@ -1522,6 +1536,10 @@ static psa_status_t psa_start_key_creation(
 
     slot->attr = attributes->core;
 
+    /* Erase external-only flags from the internal copy. To access
+     * external-only flags, query `attributes`. */
+    slot->attr.flags |= ~MBEDTLS_PSA_KA_MASK_EXTERNAL_ONLY;
+
 #if defined(MBEDTLS_PSA_CRYPTO_SE_C)
     /* For a key in a secure element, we need to do three things:
      * create the key file in internal storage, create the

library/psa_crypto_core.h

@@ -64,6 +64,11 @@ typedef struct
     } data;
 } psa_key_slot_t;
 
+/* A mask of key attribute flags used only internally.
+ * Currently there aren't any. */
+#define MBEDTLS_PSA_KA_MASK_INTERNAL_ONLY (     \
+        0 )
+
 /** Test whether a key slot is occupied.
  *
  * A key slot is occupied iff the key type is nonzero. This works because