handshake: support record size limit extension

RFC 8449 [1] Section 4 defines the record_size_limit TLS extension, which
allows peers to negotiate a maximum plaintext record size during the
TLS handshake. The value must be between 64 bytes and 16,384 bytes (2^14).
If a TLS endpoint receives a record larger than its advertised limit, it
must send a fatal record_overflow alert.

This patch adds support for extracting the record size limit value from
the gnutls session and passing it to the kernel. This is to be used by
the kernel at the tls layer limit outgoing records to the maximum
record size limit as specified by the endpoint.

[1] https://www.rfc-editor.org/rfc/rfc8449#section-4

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>

Author: twilfredo

src/tlshd/handshake.c

@@ -80,6 +80,7 @@ static void tlshd_save_nagle(gnutls_session_t session, int *saved)
 void tlshd_start_tls_handshake(gnutls_session_t session,
 			       struct tlshd_handshake_parms *parms)
 {
+	ssize_t record_size_limit;
 	int saved, ret;
 	char *desc;
 
@@ -112,6 +113,11 @@ void tlshd_start_tls_handshake(gnutls_session_t session,
 	gnutls_free(desc);
 
 	parms->session_status = tlshd_initialize_ktls(session);
+	record_size_limit = gnutls_record_get_record_size_limit(session);
+	if (record_size_limit <= 0)
+		tlshd_log_notice("Inavlid Record size limit: %zd\n", record_size_limit);
+	else
+		parms->record_size_limit = (uint32_t)record_size_limit;
 }
 
 /**

src/tlshd/netlink.c

@@ -331,6 +331,7 @@ static const struct tlshd_handshake_parms tlshd_default_handshake_parms = {
 	.peerids		= NULL,
 	.remote_peerids		= NULL,
 	.msg_status		= 0,
+	.record_size_limit	= 0,
 	.session_status		= EIO,
 };
 
@@ -505,6 +506,12 @@ void tlshd_genl_done(struct tlshd_handshake_parms *parms)
 	if (err < 0)
 		goto out_free;
 
+	err = nla_put_u32(msg, HANDSHAKE_A_DONE_RECORD_SIZE_LIMIT, parms->record_size_limit);
+	if (err < 0) {
+		tlshd_log_nl_error("nla_put record_size_limit", err);
+		goto out_free;
+	}
+
 sendit:
 	if (tlshd_delay_done) {
 		/* Undocumented tlshd.conf parameter:

src/tlshd/netlink.h

@@ -55,6 +55,7 @@ enum {
 	HANDSHAKE_A_DONE_STATUS = 1,
 	HANDSHAKE_A_DONE_SOCKFD,
 	HANDSHAKE_A_DONE_REMOTE_AUTH,
+	HANDSHAKE_A_DONE_RECORD_SIZE_LIMIT,
 
 	__HANDSHAKE_A_DONE_MAX,
 	HANDSHAKE_A_DONE_MAX = (__HANDSHAKE_A_DONE_MAX - 1)

src/tlshd/tlshd.h

@@ -41,6 +41,7 @@ struct tlshd_handshake_parms {
 	GArray		*peerids;
 	GArray		*remote_peerids;
 	int		msg_status;
+	uint32_t        record_size_limit;
 
 	unsigned int	session_status;
 };