Add early exit if zero length AEAD AD passed in.

With multipart AEAD, if we attempt to add zero length additional data,
then with the buffer sharing fixes this can now lead to undefined
behaviour when using gcm. Fix this by returning early, as there is
nothing to do if the input length is zero.

Signed-off-by: Paul Elliott <[email protected]>

Author: paul-elliott-arm

library/psa_crypto.c

@@ -5194,6 +5194,12 @@ psa_status_t psa_aead_update_ad(psa_aead_operation_t *operation,
         goto exit;
     }
 
+    /* No input to add (zero length), nothing to do. */
+    if (input_length == 0) {
+        status = PSA_SUCCESS;
+        goto exit;
+    }
+
     if (operation->lengths_set) {
         if (operation->ad_remaining < input_length) {
             status = PSA_ERROR_INVALID_ARGUMENT;