From 154bd95131470b5d58efc60829dbb8d371b8cd63 Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Thu, 19 Apr 2018 08:38:16 +0200
Subject: [PATCH] psa_destroy_key: return SUCCESS on an empty slot

Do wipe the slot even if it doesn't contain a key, to erase any metadata.
---
 include/psa/crypto.h | 12 +++++++++++-
 library/psa_crypto.c |  8 +++++---
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index c880586fe..982cca701 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -539,7 +539,17 @@ psa_status_t psa_import_key(psa_key_slot_t key,
                             size_t data_length);
 
 /**
- * \brief Destroy a key.
+ * \brief Destroy a key and restore the slot to its default state.
+ *
+ * This function destroys the content of the key slot from both volatile
+ * memory and, if applicable, non-volatile storage. Implementations shall
+ * make a best effort to ensure that any previous content of the slot is
+ * unrecoverable.
+ *
+ * This function also erases any metadata such as policies. It returns the
+ * specified slot to its default state.
+ *
+ * \param key           The key slot to erase.
  *
  * \retval PSA_SUCCESS
  *         The slot's content, if any, has been erased.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index b66862c50..deeffa3b8 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -373,9 +373,11 @@ psa_status_t psa_destroy_key(psa_key_slot_t key)
         return( PSA_ERROR_INVALID_ARGUMENT );
     slot = &global_data.key_slots[key];
     if( slot->type == PSA_KEY_TYPE_NONE )
-        return( PSA_ERROR_EMPTY_SLOT );
-
-    if( PSA_KEY_TYPE_IS_RAW_BYTES( slot->type ) )
+    {
+        /* No key material to clean, but do zeroize the slot below to wipe
+         * metadata such as policies. */
+    }
+    else if( PSA_KEY_TYPE_IS_RAW_BYTES( slot->type ) )
     {
         mbedtls_free( slot->data.raw.data );
     }