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/*
* Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License").
* You may not use this file except in compliance with the License.
* A copy of the License is located at
*
* http://aws.amazon.com/apache2.0
*
* or in the "license" file accompanying this file. This file is distributed
* on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
* express or implied. See the License for the specific language governing
* permissions and limitations under the License.
*/
#include "tls/extensions/s2n_client_supported_versions.h"
#include <stdint.h>
#include <sys/param.h>
#include "tls/extensions/s2n_supported_versions.h"
#include "tls/s2n_alerts.h"
#include "tls/s2n_cipher_preferences.h"
#include "tls/s2n_tls.h"
#include "tls/s2n_tls_parameters.h"
#include "utils/s2n_safety.h"
/**
* Specified in https://tools.ietf.org/html/rfc8446#section-4.2.1
*
* "The "supported_versions" extension is used by the client to indicate
* which versions of TLS it supports and by the server to indicate which
* version it is using. The extension contains a list of supported
* versions in preference order, with the most preferred version first."
*
* Structure:
* Extension type (2 bytes)
* Extension size (2 bytes)
* Version list length (1 byte)
* Version list (number of versions * 2 bytes)
*
* Note: We assume in these functions that the supported version numbers
* are consecutive. This is true because S2N does not support SSLv2, and
* is already an assumption made in the old client hello version handling.
**/
static int s2n_client_supported_versions_send(struct s2n_connection *conn, struct s2n_stuffer *out);
static int s2n_client_supported_versions_recv(struct s2n_connection *conn, struct s2n_stuffer *in);
const s2n_extension_type s2n_client_supported_versions_extension = {
.iana_value = TLS_EXTENSION_SUPPORTED_VERSIONS,
.is_response = false,
.send = s2n_client_supported_versions_send,
.recv = s2n_client_supported_versions_recv,
.should_send = s2n_extension_send_if_tls13_connection,
.if_missing = s2n_extension_noop_if_missing,
};
static int s2n_client_supported_versions_send(struct s2n_connection *conn, struct s2n_stuffer *out)
{
uint8_t highest_supported_version = conn->client_protocol_version;
uint8_t minimum_supported_version = s2n_unknown_protocol_version;
POSIX_GUARD_RESULT(s2n_connection_get_minimum_supported_version(conn, &minimum_supported_version));
POSIX_ENSURE(highest_supported_version >= minimum_supported_version, S2N_ERR_PROTOCOL_VERSION_UNSUPPORTED);
uint8_t version_list_length = highest_supported_version - minimum_supported_version + 1;
POSIX_GUARD(s2n_stuffer_write_uint8(out, version_list_length * S2N_TLS_PROTOCOL_VERSION_LEN));
for (uint8_t i = highest_supported_version; i >= minimum_supported_version; i--) {
POSIX_GUARD(s2n_stuffer_write_uint8(out, i / 10));
POSIX_GUARD(s2n_stuffer_write_uint8(out, i % 10));
}
return S2N_SUCCESS;
}
int s2n_extensions_client_supported_versions_process(struct s2n_connection *conn, struct s2n_stuffer *extension,
uint8_t *client_protocol_version_out, uint8_t *actual_protocol_version_out)
{
uint8_t highest_supported_version = conn->server_protocol_version;
uint8_t minimum_supported_version = s2n_unknown_protocol_version;
POSIX_GUARD_RESULT(s2n_connection_get_minimum_supported_version(conn, &minimum_supported_version));
uint8_t size_of_version_list;
POSIX_GUARD(s2n_stuffer_read_uint8(extension, &size_of_version_list));
S2N_ERROR_IF(size_of_version_list != s2n_stuffer_data_available(extension), S2N_ERR_BAD_MESSAGE);
S2N_ERROR_IF(size_of_version_list % S2N_TLS_PROTOCOL_VERSION_LEN != 0, S2N_ERR_BAD_MESSAGE);
uint8_t client_protocol_version = s2n_unknown_protocol_version;
uint8_t actual_protocol_version = s2n_unknown_protocol_version;
for (int i = 0; i < size_of_version_list; i += S2N_TLS_PROTOCOL_VERSION_LEN) {
uint8_t client_version_parts[S2N_TLS_PROTOCOL_VERSION_LEN];
POSIX_GUARD(s2n_stuffer_read_bytes(extension, client_version_parts, S2N_TLS_PROTOCOL_VERSION_LEN));
/* If the client version is outside of our supported versions, then ignore the value.
* S2N does not support SSLv2 except for upgrading connections. Since this extension is
* a TLS1.3 extension, we will skip any SSLv2 values. */
if (client_version_parts[0] != 3 || client_version_parts[1] > 4) {
continue;
}
uint16_t client_version = (client_version_parts[0] * 10) + client_version_parts[1];
client_protocol_version = MAX(client_version, client_protocol_version);
if (client_version > highest_supported_version) {
continue;
}
if (client_version < minimum_supported_version) {
continue;
}
/* We ignore the client's preferred order and instead choose
* the highest version that both client and server support. */
actual_protocol_version = MAX(client_version, actual_protocol_version);
}
*client_protocol_version_out = client_protocol_version;
*actual_protocol_version_out = actual_protocol_version;
return S2N_SUCCESS;
}
static S2N_RESULT s2n_client_supported_versions_recv_impl(struct s2n_connection *conn, struct s2n_stuffer *extension)
{
RESULT_ENSURE_REF(conn);
RESULT_ENSURE_REF(extension);
RESULT_GUARD_POSIX(s2n_extensions_client_supported_versions_process(conn, extension, &conn->client_protocol_version,
&conn->actual_protocol_version));
RESULT_ENSURE(conn->client_protocol_version != s2n_unknown_protocol_version, S2N_ERR_UNKNOWN_PROTOCOL_VERSION);
RESULT_ENSURE(conn->actual_protocol_version != s2n_unknown_protocol_version, S2N_ERR_PROTOCOL_VERSION_UNSUPPORTED);
return S2N_RESULT_OK;
}
static int s2n_client_supported_versions_recv(struct s2n_connection *conn, struct s2n_stuffer *extension)
{
/* For backwards compatibility, the supported versions extension isn't used for protocol
* version selection if the server doesn't support TLS 1.3. This ensures that TLS 1.2 servers
* experience no behavior change due to processing the TLS 1.3 extension. See
* https://github.com/aws/s2n-tls/issues/4240.
*
*= https://www.rfc-editor.org/rfc/rfc8446#section-4.2.1
*= type=exception
*= reason=The client hello legacy version is used for version selection on TLS 1.2 servers for backwards compatibility
*# If this extension is present in the ClientHello, servers MUST NOT use
*# the ClientHello.legacy_version value for version negotiation and MUST
*# use only the "supported_versions" extension to determine client
*# preferences.
*/
if (s2n_connection_get_protocol_version(conn) < S2N_TLS13) {
return S2N_SUCCESS;
}
s2n_result result = s2n_client_supported_versions_recv_impl(conn, extension);
if (s2n_result_is_error(result)) {
conn->client_protocol_version = s2n_unknown_protocol_version;
conn->actual_protocol_version = s2n_unknown_protocol_version;
s2n_queue_reader_unsupported_protocol_version_alert(conn);
POSIX_ENSURE(s2n_errno != S2N_ERR_SAFETY, S2N_ERR_BAD_MESSAGE);
}
POSIX_GUARD_RESULT(result);
return S2N_SUCCESS;
}
/* Old-style extension functions -- remove after extensions refactor is complete */
int s2n_extensions_client_supported_versions_size(struct s2n_connection *conn)
{
uint8_t minimum_supported_version = s2n_unknown_protocol_version;
POSIX_GUARD_RESULT(s2n_connection_get_minimum_supported_version(conn, &minimum_supported_version));
uint8_t highest_supported_version = conn->client_protocol_version;
uint8_t version_list_length = highest_supported_version - minimum_supported_version + 1;
return version_list_length * S2N_TLS_PROTOCOL_VERSION_LEN + 5;
}
/* still used in fuzz test */
int s2n_extensions_client_supported_versions_recv(struct s2n_connection *conn, struct s2n_stuffer *extension)
{
return s2n_extension_recv(&s2n_client_supported_versions_extension, conn, extension);
}
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