net/tls: allow limiting maximum record size

During a handshake, an endpoint may specify a maximum record size limit.
Currently, the kernel defaults to TLS_MAX_PAYLOAD_SIZE (16KB) for the
maximum record size. Meaning that, the outgoing records from the kernel
can exceed a lower size negotiated during the handshake. In such a case,
the TLS endpoint must send a fatal "record_overflow" alert [1], and
thus the record is discarded.

This patch adds support for retrieving the negotiated record size limit
during a handshake, and enforcing it at the TLS layer such that outgoing
records are no larger than the size negotiated. This patch depends on
the respective userspace support in tlshd [2] and GnuTLS [3].

[1] https://www.rfc-editor.org/rfc/rfc8449
[2] oracle/ktls-utils#112
[3] https://gitlab.com/gnutls/gnutls/-/merge_requests/2005

Signed-off-by: Wilfred Mallawa <[email protected]>

Author: twilfredo

Documentation/netlink/specs/handshake.yaml

@@ -87,6 +87,9 @@ attribute-sets:
         name: remote-auth
         type: u32
         multi-attr: true
+      -
+          name: record-size-limit
+          type: u32
 
 operations:
   list:

include/net/tls.h

@@ -250,6 +250,8 @@ struct tls_context {
 			       */
 	unsigned long flags;
 
+	u32 tls_record_size_limit;
+
 	/* cache cold stuff */
 	struct proto *sk_proto;
 	struct sock *sk;

include/uapi/linux/handshake.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)

net/handshake/genl.c

@@ -16,10 +16,11 @@ static const struct nla_policy handshake_accept_nl_policy[HANDSHAKE_A_ACCEPT_HAN
 };
 
 /* HANDSHAKE_CMD_DONE - do */
-static const struct nla_policy handshake_done_nl_policy[HANDSHAKE_A_DONE_REMOTE_AUTH + 1] = {
+static const struct nla_policy handshake_done_nl_policy[HANDSHAKE_A_DONE_RECORD_SIZE_LIMIT + 1] = {
 	[HANDSHAKE_A_DONE_STATUS] = { .type = NLA_U32, },
 	[HANDSHAKE_A_DONE_SOCKFD] = { .type = NLA_S32, },
 	[HANDSHAKE_A_DONE_REMOTE_AUTH] = { .type = NLA_U32, },
+	[HANDSHAKE_A_DONE_RECORD_SIZE_LIMIT] = { .type = NLA_U32, },
 };
 
 /* Ops table for handshake */
@@ -35,7 +36,7 @@ static const struct genl_split_ops handshake_nl_ops[] = {
 		.cmd		= HANDSHAKE_CMD_DONE,
 		.doit		= handshake_nl_done_doit,
 		.policy		= handshake_done_nl_policy,
-		.maxattr	= HANDSHAKE_A_DONE_REMOTE_AUTH,
+		.maxattr	= HANDSHAKE_A_DONE_RECORD_SIZE_LIMIT,
 		.flags		= GENL_CMD_CAP_DO,
 	},
 };

net/handshake/tlshd.c

@@ -19,6 +19,7 @@
 #include <net/handshake.h>
 #include <net/genetlink.h>
 #include <net/tls_prot.h>
+#include <net/tls.h>
 
 #include <uapi/linux/keyctl.h>
 #include <uapi/linux/handshake.h>
@@ -37,6 +38,8 @@ struct tls_handshake_req {
 	key_serial_t		th_certificate;
 	key_serial_t		th_privkey;
 
+	struct socket		*th_sock;
+
 	unsigned int		th_num_peerids;
 	key_serial_t		th_peerid[5];
 };
@@ -52,6 +55,7 @@ tls_handshake_req_init(struct handshake_req *req,
 	treq->th_consumer_data = args->ta_data;
 	treq->th_peername = args->ta_peername;
 	treq->th_keyring = args->ta_keyring;
+	treq->th_sock = args->ta_sock;
 	treq->th_num_peerids = 0;
 	treq->th_certificate = TLS_NO_CERT;
 	treq->th_privkey = TLS_NO_PRIVKEY;
@@ -85,6 +89,27 @@ static void tls_handshake_remote_peerids(struct tls_handshake_req *treq,
 	}
 }
 
+static void tls_handshake_record_size(struct tls_handshake_req *treq,
+				      struct genl_info *info)
+{
+	struct tls_context *tls_ctx;
+	struct nlattr *head = nlmsg_attrdata(info->nlhdr, GENL_HDRLEN);
+	struct nlattr *nla;
+	u32 record_size_limit;
+	int rem, len = nlmsg_attrlen(info->nlhdr, GENL_HDRLEN);
+
+	nla_for_each_attr(nla, head, len, rem) {
+		if (nla_type(nla) == HANDSHAKE_A_DONE_RECORD_SIZE_LIMIT) {
+			record_size_limit = nla_get_u32(nla);
+			if (treq->th_sock) {
+				tls_ctx = tls_get_ctx(treq->th_sock->sk);
+				tls_ctx->tls_record_size_limit = record_size_limit;
+			}
+			break;
+		}
+	}
+}
+
 /**
  * tls_handshake_done - callback to handle a CMD_DONE request
  * @req: socket on which the handshake was performed
@@ -98,8 +123,10 @@ static void tls_handshake_done(struct handshake_req *req,
 	struct tls_handshake_req *treq = handshake_req_private(req);
 
 	treq->th_peerid[0] = TLS_NO_PEERID;
-	if (info)
+	if (info) {
 		tls_handshake_remote_peerids(treq, info);
+		tls_handshake_record_size(treq, info);
+	}
 
 	if (!status)
 		set_bit(HANDSHAKE_F_REQ_SESSION, &req->hr_flags);

net/tls/tls_sw.c

@@ -1037,6 +1037,7 @@ static int tls_sw_sendmsg_locked(struct sock *sk, struct msghdr *msg,
 	ssize_t copied = 0;
 	struct sk_msg *msg_pl, *msg_en;
 	struct tls_rec *rec;
+	u32 tls_record_size_limit;
 	int required_size;
 	int num_async = 0;
 	bool full_record;
@@ -1058,6 +1059,9 @@ static int tls_sw_sendmsg_locked(struct sock *sk, struct msghdr *msg,
 		}
 	}
 
+	tls_record_size_limit = min_not_zero(tls_ctx->tls_record_size_limit,
+					     TLS_MAX_PAYLOAD_SIZE);
+
 	while (msg_data_left(msg)) {
 		if (sk->sk_err) {
 			ret = -sk->sk_err;
@@ -1079,7 +1083,7 @@ static int tls_sw_sendmsg_locked(struct sock *sk, struct msghdr *msg,
 		orig_size = msg_pl->sg.size;
 		full_record = false;
 		try_to_copy = msg_data_left(msg);
-		record_room = TLS_MAX_PAYLOAD_SIZE - msg_pl->sg.size;
+		record_room = tls_record_size_limit - msg_pl->sg.size;
 		if (try_to_copy >= record_room) {
 			try_to_copy = record_room;
 			full_record = true;