Dynamic key store: implementation
When MBEDTLS_PSA_KEY_STORE_DYNAMIC is enabled, key slots are now organized in
multiple slices. The slices are allocated on demand, which allows the key
store to grow. The size of slices grows exponentially, which allows reaching
a large number of slots with a small (static) number of slices without too
much overhead.
Maintain a linked list of free slots in each slice. This way, allocating a
slot takes O(1) time unless a slice needs to be allocated.
In this commit, slices are only ever freed when deinitializing the key
store. This should be improved in the future to free empty slices.
To avoid growing the persistent key cache without control, the persistent
key cache has a fixed size (reusing MBEDTLS_PSA_KEY_SLOT_COUNT to avoid
creating yet another option).
When MBEDTLS_PSA_KEY_STORE_DYNAMIC is disabled. no semantic change and
minimal changes to the code.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/psa_crypto_slot_management.h b/library/psa_crypto_slot_management.h
index cccf8f2..1e6e935 100644
--- a/library/psa_crypto_slot_management.h
+++ b/library/psa_crypto_slot_management.h
@@ -17,8 +17,10 @@
*
* The first #MBEDTLS_PSA_KEY_SLOT_COUNT identifiers of the implementation
* range of key identifiers are reserved for volatile key identifiers.
- * A volatile key identifier is equal to #PSA_KEY_ID_VOLATILE_MIN plus the
- * index of the key slot containing the volatile key definition.
+ *
+ * If \c id is a a volatile key identifier, #PSA_KEY_ID_VOLATILE_MIN - \c id
+ * indicates the key slot containing the volatile key definition. See
+ * psa_crypto_slot_management.c for details.
*/
/** The minimum value for a volatile key identifier.
@@ -27,8 +29,12 @@
/** The maximum value for a volatile key identifier.
*/
+#if defined(MBEDTLS_PSA_KEY_STORE_DYNAMIC)
+#define PSA_KEY_ID_VOLATILE_MAX (MBEDTLS_PSA_KEY_ID_BUILTIN_MIN - 1)
+#else /* MBEDTLS_PSA_KEY_STORE_DYNAMIC */
#define PSA_KEY_ID_VOLATILE_MAX \
(PSA_KEY_ID_VOLATILE_MIN + MBEDTLS_PSA_KEY_SLOT_COUNT - 1)
+#endif /* MBEDTLS_PSA_KEY_STORE_DYNAMIC */
/** Test whether a key identifier is a volatile key identifier.
*
@@ -113,13 +119,20 @@
* If multi-threading is enabled, the caller must hold the
* global key slot mutex.
*
- * \param[out] volatile_key_id On success, volatile key identifier
- * associated to the returned slot.
+ * \param[out] volatile_key_id If null, reserve a cache slot for
+ * a persistent or built-in key.
+ * If non-null, allocate a slot for
+ * a volatile key.
+ * If non-null, on success, the volatile key
+ * identifier corresponding with the
+ * returned slot.
* \param[out] p_slot On success, a pointer to the slot.
*
* \retval #PSA_SUCCESS \emptydescription
* \retval #PSA_ERROR_INSUFFICIENT_MEMORY
* There were no free key slots.
+ * When #MBEDTLS_PSA_KEY_STORE_DYNAMIC is enabled, there was not
+ * enough memory to allocate more slots.
* \retval #PSA_ERROR_BAD_STATE \emptydescription
* \retval #PSA_ERROR_CORRUPTION_DETECTED
* This function attempted to operate on a key slot which was in an
@@ -128,6 +141,29 @@
psa_status_t psa_reserve_free_key_slot(psa_key_id_t *volatile_key_id,
psa_key_slot_t **p_slot);
+#if defined(MBEDTLS_PSA_KEY_STORE_DYNAMIC)
+/** Return a key slot to the free list.
+ *
+ * Call this function when a slot obtained from psa_reserve_free_key_slot()
+ * is no longer in use.
+ *
+ * If multi-threading is enabled, the caller must hold the
+ * global key slot mutex.
+ *
+ * \param slice_idx The slice containing the slot.
+ * This is `slot->slice_index` when the slot
+ * is obtained from psa_reserve_free_key_slot().
+ * \param slot The key slot.
+ *
+ * \retval #PSA_SUCCESS \emptydescription
+ * \retval #PSA_ERROR_CORRUPTION_DETECTED
+ * This function attempted to operate on a key slot which was in an
+ * unexpected state.
+ */
+psa_status_t psa_free_key_slot(size_t slice_idx,
+ psa_key_slot_t *slot);
+#endif /* MBEDTLS_PSA_KEY_STORE_DYNAMIC */
+
/** Change the state of a key slot.
*
* This function changes the state of the key slot from expected_state to