psa_export_key: Always set a valid data length

Make psa_export_key() always set a valid data_length when exporting,
even when there are errors. This makes the API easier to use for buggy
programs (like our test code).

Our test code previously used exported_length uninitialized when
checking to see that the buffer returned was all zero in import_export()
in the case where an error was returned from psa_export_key().
Initialize exported_length to an invalid length, and check that it gets
set properly by psa_export_key(), to avoid this using export_length
uninitialized. Note that the mem_is_zero() check is still valid when
psa_export_key() returns an error, e.g. where exported_length is 0, as
we want to check that nothing was written to the buffer on error.

Out test code also previous passed NULL for the data_length parameter of
psa_export_key() when it expected a failure (in key_policy_fail()).
However, data_length is not allowed to be NULL, especially now that we
write to data_length from psa_export_key() even when there are errors.
Update the test code to not pass in a NULL data_length.

library/psa_crypto.c

@@ -636,6 +636,12 @@ static  psa_status_t psa_internal_export_key( psa_key_slot_t key,
 {
     key_slot_t *slot;
 
+    /* Set the key to empty now, so that even when there are errors, we always
+     * set data_length to a value between 0 and data_size. On error, setting
+     * the key to empty is a good choice because an empty key representation is
+     * unlikely to be accepted anywhere. */
+    *data_length = 0;
+
     if( key == 0 || key > PSA_KEY_SLOT_COUNT )
         return( PSA_ERROR_EMPTY_SLOT );
     slot = &global_data.key_slots[key];

tests/suites/test_suite_psa_crypto.function

@@ -8,6 +8,9 @@
 #define PSA_CRYPTO_TEST_SIZE_T_RANGE( x ) 1
 #endif
 
+/** An invalid export length that will never be set by psa_export_key(). */
+static const size_t INVALID_EXPORT_LENGTH = ~0U;
+
 /** Test if a buffer is not all-bits zero.
  *
  * \param buffer    Pointer to the beginning of the buffer.
@@ -320,7 +323,7 @@ void import_export( data_t *data,
     unsigned char *exported = NULL;
     unsigned char *reexported = NULL;
     size_t export_size;
-    size_t exported_length;
+    size_t exported_length = INVALID_EXPORT_LENGTH;
     size_t reexported_length;
     psa_key_type_t got_type;
     size_t got_bits;
@@ -358,6 +361,13 @@ void import_export( data_t *data,
                              exported, export_size,
                              &exported_length );
     TEST_ASSERT( status == expected_export_status );
+
+    /* The exported length must be set by psa_export_key() to a value between 0
+     * and export_size. On errors, the exported length must be 0. */
+    TEST_ASSERT( exported_length != INVALID_EXPORT_LENGTH );
+    TEST_ASSERT( status == PSA_SUCCESS || exported_length == 0 );
+    TEST_ASSERT( exported_length <= export_size );
+
     TEST_ASSERT( mem_is_zero( exported + exported_length,
                               export_size - exported_length ) );
     if( status != PSA_SUCCESS )
@@ -536,13 +546,14 @@ void key_policy_fail( int usage_arg, int alg_arg, int expected_status,
 
     if( usage & PSA_KEY_USAGE_SIGN )
     {
+        size_t data_length;
         TEST_ASSERT( keypair != NULL );
         TEST_ASSERT( PSA_CRYPTO_TEST_SIZE_T_RANGE( keypair->len ) );
         TEST_ASSERT( psa_import_key( key_slot,
                                      PSA_KEY_TYPE_RSA_KEYPAIR,
                                      keypair->x,
                                      keypair->len ) == PSA_SUCCESS );
-        actual_status = psa_export_key( key_slot, NULL, 0, NULL );
+        actual_status = psa_export_key( key_slot, NULL, 0, &data_length );
     }
 
     TEST_ASSERT( actual_status == expected_status );