fix: don't iterate over certs if not validating certs

tests/unit/s2n_config_test.c

@@ -1147,6 +1147,43 @@ int main(int argc, char **argv)
             EXPECT_ERROR_WITH_ERRNO(s2n_config_validate_loaded_certificates(config, &rfc9151_applied_locally),
                     S2N_ERR_SECURITY_POLICY_INCOMPATIBLE_CERT);
         };
+
+        /* when cert preferences don't apply locally, certs in domain map are not iterated over
+         *
+         * Some customers load large numbers of certificates, so even iterating
+         * over every certificate without performing any validation is expensive.
+         */
+        {
+            struct s2n_security_policy non_local_rfc9151 = security_policy_rfc9151;
+
+            /* Assert that the security policy WOULD apply,
+             * if certificate_preferences_apply_locally was true.
+             */
+            EXPECT_NOT_NULL(non_local_rfc9151.certificate_key_preferences);
+            EXPECT_NOT_NULL(non_local_rfc9151.certificate_signature_preferences);
+            EXPECT_TRUE(non_local_rfc9151.certificate_key_preferences->count > 0);
+            EXPECT_TRUE(non_local_rfc9151.certificate_signature_preferences->count > 0);
+
+            DEFER_CLEANUP(struct s2n_config *config = s2n_config_new(), s2n_config_ptr_free);
+            EXPECT_NOT_NULL(config);
+            EXPECT_SUCCESS(s2n_config_add_cert_chain_and_key_to_store(config, valid_cert));
+
+            /* Invalidate the domain map so that attempting to use it will trigger
+             * an error. We want to ensure that we DON'T use it.
+             * Iterating over a map requires that map to be immutable / complete.
+             */
+            EXPECT_OK(s2n_map_unlock(config->domain_name_to_cert_map));
+
+            /* Control case: if local validation needed, attempt to use invalid domain map */
+            non_local_rfc9151.certificate_preferences_apply_locally = true;
+            EXPECT_ERROR_WITH_ERRNO(
+                    s2n_config_validate_loaded_certificates(config, &non_local_rfc9151),
+                    S2N_ERR_MAP_MUTABLE);
+
+            /* Test case: if no local validation needed, do not use invalid domain map */
+            non_local_rfc9151.certificate_preferences_apply_locally = false;
+            EXPECT_OK(s2n_config_validate_loaded_certificates(config, &non_local_rfc9151));
+        };
     };
 
     /* Checks that servers don't use a config before the client hello callback is executed.

tls/s2n_config.c

@@ -574,6 +574,15 @@ S2N_RESULT s2n_config_validate_loaded_certificates(const struct s2n_config *conf
         return S2N_RESULT_OK;
     }
 
+    /* Duplicates a check in s2n_security_policy_validate_certificate_chain.
+     * If a large number of certificates are configured, even iterating
+     * over the chains to call s2n_security_policy_validate_certificate_chain
+     * could be prohibitively expensive.
+     */
+    if (!security_policy->certificate_preferences_apply_locally) {
+        return S2N_RESULT_OK;
+    }
+
     /* validate the default certs */
     for (int i = 0; i < S2N_CERT_TYPE_COUNT; i++) {
         struct s2n_cert_chain_and_key *cert = config->default_certs_by_type.certs[i];