--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+In-Kernel TLS Handshake
+=======================
+
+Overview
+========
+
+Transport Layer Security (TLS) is a Upper Layer Protocol (ULP) that runs
+over TCP. TLS provides end-to-end data integrity and confidentiality in
+addition to peer authentication.
+
+The kernel's kTLS implementation handles the TLS record subprotocol, but
+does not handle the TLS handshake subprotocol which is used to establish
+a TLS session. Kernel consumers can use the API described here to
+request TLS session establishment.
+
+There are several possible ways to provide a handshake service in the
+kernel. The API described here is designed to hide the details of those
+implementations so that in-kernel TLS consumers do not need to be
+aware of how the handshake gets done.
+
+
+User handshake agent
+====================
+
+As of this writing, there is no TLS handshake implementation in the
+Linux kernel. To provide a handshake service, a handshake agent
+(typically in user space) is started in each network namespace where a
+kernel consumer might require a TLS handshake. Handshake agents listen
+for events sent from the kernel that indicate a handshake request is
+waiting.
+
+An open socket is passed to a handshake agent via a netlink operation,
+which creates a socket descriptor in the agent's file descriptor table.
+If the handshake completes successfully, the handshake agent promotes
+the socket to use the TLS ULP and sets the session information using the
+SOL_TLS socket options. The handshake agent returns the socket to the
+kernel via a second netlink operation.
+
+
+Kernel Handshake API
+====================
+
+A kernel TLS consumer initiates a client-side TLS handshake on an open
+socket by invoking one of the tls_client_hello() functions. First, it
+fills in a structure that contains the parameters of the request:
+
+.. code-block:: c
+
+ struct tls_handshake_args {
+ struct socket *ta_sock;
+ tls_done_func_t ta_done;
+ void *ta_data;
+ unsigned int ta_timeout_ms;
+ key_serial_t ta_keyring;
+ key_serial_t ta_my_cert;
+ key_serial_t ta_my_privkey;
+ unsigned int ta_num_peerids;
+ key_serial_t ta_my_peerids[5];
+ };
+
+The @ta_sock field references an open and connected socket. The consumer
+must hold a reference on the socket to prevent it from being destroyed
+while the handshake is in progress. The consumer must also have
+instantiated a struct file in sock->file.
+
+
+@ta_done contains a callback function that is invoked when the handshake
+has completed. Further explanation of this function is in the "Handshake
+Completion" sesction below.
+
+The consumer can fill in the @ta_timeout_ms field to force the servicing
+handshake agent to exit after a number of milliseconds. This enables the
+socket to be fully closed once both the kernel and the handshake agent
+have closed their endpoints.
+
+Authentication material such as x.509 certificates, private certificate
+keys, and pre-shared keys are provided to the handshake agent in keys
+that are instantiated by the consumer before making the handshake
+request. The consumer can provide a private keyring that is linked into
+the handshake agent's process keyring in the @ta_keyring field to prevent
+access of those keys by other subsystems.
+
+To request an x.509-authenticated TLS session, the consumer fills in
+the @ta_my_cert and @ta_my_privkey fields with the serial numbers of
+keys containing an x.509 certificate and the private key for that
+certificate. Then, it invokes this function:
+
+.. code-block:: c
+
+ ret = tls_client_hello_x509(args, gfp_flags);
+
+The function returns zero when the handshake request is under way. A
+zero return guarantees the callback function @ta_done will be invoked
+for this socket. The function returns a negative errno if the handshake
+could not be started. A negative errno guarantees the callback function
+@ta_done will not be invoked on this socket.
+
+
+To initiate a client-side TLS handshake with a pre-shared key, use:
+
+.. code-block:: c
+
+ ret = tls_client_hello_psk(args, gfp_flags);
+
+However, in this case, the consumer fills in the @ta_my_peerids array
+with serial numbers of keys containing the peer identities it wishes
+to offer, and the @ta_num_peerids field with the number of array
+entries it has filled in. The other fields are filled in as above.
+
+
+To initiate an anonymous client-side TLS handshake use:
+
+.. code-block:: c
+
+ ret = tls_client_hello_anon(args, gfp_flags);
+
+The handshake agent presents no peer identity information to the remote
+during this type of handshake. Only server authentication (ie the client
+verifies the server's identity) is performed during the handshake. Thus
+the established session uses encryption only.
+
+
+Consumers that are in-kernel servers use:
+
+.. code-block:: c
+
+ ret = tls_server_hello_x509(args, gfp_flags);
+
+or
+
+.. code-block:: c
+
+ ret = tls_server_hello_psk(args, gfp_flags);
+
+The argument structure is filled in as above.
+
+
+If the consumer needs to cancel the handshake request, say, due to a ^C
+or other exigent event, the consumer can invoke:
+
+.. code-block:: c
+
+ bool tls_handshake_cancel(sock);
+
+This function returns true if the handshake request associated with
+@sock has been canceled. The consumer's handshake completion callback
+will not be invoked. If this function returns false, then the consumer's
+completion callback has already been invoked.
+
+
+Handshake Completion
+====================
+
+When the handshake agent has completed processing, it notifies the
+kernel that the socket may be used by the consumer again. At this point,
+the consumer's handshake completion callback, provided in the @ta_done
+field in the tls_handshake_args structure, is invoked.
+
+The synopsis of this function is:
+
+.. code-block:: c
+
+ typedef void (*tls_done_func_t)(void *data, int status,
+ key_serial_t peerid);
+
+The consumer provides a cookie in the @ta_data field of the
+tls_handshake_args structure that is returned in the @data parameter of
+this callback. The consumer uses the cookie to match the callback to the
+thread waiting for the handshake to complete.
+
+The success status of the handshake is returned via the @status
+parameter:
+
++------------+----------------------------------------------+
+| status | meaning |
++============+==============================================+
+| 0 | TLS session established successfully |
++------------+----------------------------------------------+
+| -EACCESS | Remote peer rejected the handshake or |
+| | authentication failed |
++------------+----------------------------------------------+
+| -ENOMEM | Temporary resource allocation failure |
++------------+----------------------------------------------+
+| -EINVAL | Consumer provided an invalid argument |
++------------+----------------------------------------------+
+| -ENOKEY | Missing authentication material |
++------------+----------------------------------------------+
+| -EIO | An unexpected fault occurred |
++------------+----------------------------------------------+
+
+The @peerid parameter contains the serial number of a key containing the
+remote peer's identity or the value TLS_NO_PEERID if the session is not
+authenticated.
+
+A best practice is to close and destroy the socket immediately if the
+handshake failed.
+
+
+Other considerations
+--------------------
+
+While a handshake is under way, the kernel consumer must alter the
+socket's sk_data_ready callback function to ignore all incoming data.
+Once the handshake completion callback function has been invoked, normal
+receive operation can be resumed.
+
+Once a TLS session is established, the consumer must provide a buffer
+for and then examine the control message (CMSG) that is part of every
+subsequent sock_recvmsg(). Each control message indicates whether the
+received message data is TLS record data or session metadata.
+
+See tls.rst for details on how a kTLS consumer recognizes incoming
+(decrypted) application data, alerts, and handshake packets once the
+socket has been promoted to use the TLS ULP.
--- /dev/null
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Establish a TLS session for a kernel socket consumer
+ * using the tlshd user space handler.
+ *
+ * Author: Chuck Lever <chuck.lever@oracle.com>
+ *
+ * Copyright (c) 2021-2023, Oracle and/or its affiliates.
+ */
+
+#include <linux/types.h>
+#include <linux/socket.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/key.h>
+
+#include <net/sock.h>
+#include <net/handshake.h>
+#include <net/genetlink.h>
+
+#include <uapi/linux/keyctl.h>
+#include <uapi/linux/handshake.h>
+#include "handshake.h"
+
+struct tls_handshake_req {
+ void (*th_consumer_done)(void *data, int status,
+ key_serial_t peerid);
+ void *th_consumer_data;
+
+ int th_type;
+ unsigned int th_timeout_ms;
+ int th_auth_mode;
+ key_serial_t th_keyring;
+ key_serial_t th_certificate;
+ key_serial_t th_privkey;
+
+ unsigned int th_num_peerids;
+ key_serial_t th_peerid[5];
+};
+
+static struct tls_handshake_req *
+tls_handshake_req_init(struct handshake_req *req,
+ const struct tls_handshake_args *args)
+{
+ struct tls_handshake_req *treq = handshake_req_private(req);
+
+ treq->th_timeout_ms = args->ta_timeout_ms;
+ treq->th_consumer_done = args->ta_done;
+ treq->th_consumer_data = args->ta_data;
+ treq->th_keyring = args->ta_keyring;
+ treq->th_num_peerids = 0;
+ treq->th_certificate = TLS_NO_CERT;
+ treq->th_privkey = TLS_NO_PRIVKEY;
+ return treq;
+}
+
+static void tls_handshake_remote_peerids(struct tls_handshake_req *treq,
+ struct genl_info *info)
+{
+ struct nlattr *head = nlmsg_attrdata(info->nlhdr, GENL_HDRLEN);
+ int rem, len = nlmsg_attrlen(info->nlhdr, GENL_HDRLEN);
+ struct nlattr *nla;
+ unsigned int i;
+
+ i = 0;
+ nla_for_each_attr(nla, head, len, rem) {
+ if (nla_type(nla) == HANDSHAKE_A_DONE_REMOTE_AUTH)
+ i++;
+ }
+ if (!i)
+ return;
+ treq->th_num_peerids = min_t(unsigned int, i,
+ ARRAY_SIZE(treq->th_peerid));
+
+ i = 0;
+ nla_for_each_attr(nla, head, len, rem) {
+ if (nla_type(nla) == HANDSHAKE_A_DONE_REMOTE_AUTH)
+ treq->th_peerid[i++] = nla_get_u32(nla);
+ if (i >= treq->th_num_peerids)
+ break;
+ }
+}
+
+/**
+ * tls_handshake_done - callback to handle a CMD_DONE request
+ * @req: socket on which the handshake was performed
+ * @status: session status code
+ * @info: full results of session establishment
+ *
+ */
+static void tls_handshake_done(struct handshake_req *req,
+ unsigned int status, struct genl_info *info)
+{
+ struct tls_handshake_req *treq = handshake_req_private(req);
+
+ treq->th_peerid[0] = TLS_NO_PEERID;
+ if (info)
+ tls_handshake_remote_peerids(treq, info);
+
+ treq->th_consumer_done(treq->th_consumer_data, -status,
+ treq->th_peerid[0]);
+}
+
+#if IS_ENABLED(CONFIG_KEYS)
+static int tls_handshake_private_keyring(struct tls_handshake_req *treq)
+{
+ key_ref_t process_keyring_ref, keyring_ref;
+ int ret;
+
+ if (treq->th_keyring == TLS_NO_KEYRING)
+ return 0;
+
+ process_keyring_ref = lookup_user_key(KEY_SPEC_PROCESS_KEYRING,
+ KEY_LOOKUP_CREATE,
+ KEY_NEED_WRITE);
+ if (IS_ERR(process_keyring_ref)) {
+ ret = PTR_ERR(process_keyring_ref);
+ goto out;
+ }
+
+ keyring_ref = lookup_user_key(treq->th_keyring, KEY_LOOKUP_CREATE,
+ KEY_NEED_LINK);
+ if (IS_ERR(keyring_ref)) {
+ ret = PTR_ERR(keyring_ref);
+ goto out_put_key;
+ }
+
+ ret = key_link(key_ref_to_ptr(process_keyring_ref),
+ key_ref_to_ptr(keyring_ref));
+
+ key_ref_put(keyring_ref);
+out_put_key:
+ key_ref_put(process_keyring_ref);
+out:
+ return ret;
+}
+#else
+static int tls_handshake_private_keyring(struct tls_handshake_req *treq)
+{
+ return 0;
+}
+#endif
+
+static int tls_handshake_put_peer_identity(struct sk_buff *msg,
+ struct tls_handshake_req *treq)
+{
+ unsigned int i;
+
+ for (i = 0; i < treq->th_num_peerids; i++)
+ if (nla_put_u32(msg, HANDSHAKE_A_ACCEPT_PEER_IDENTITY,
+ treq->th_peerid[i]) < 0)
+ return -EMSGSIZE;
+ return 0;
+}
+
+static int tls_handshake_put_certificate(struct sk_buff *msg,
+ struct tls_handshake_req *treq)
+{
+ struct nlattr *entry_attr;
+
+ if (treq->th_certificate == TLS_NO_CERT &&
+ treq->th_privkey == TLS_NO_PRIVKEY)
+ return 0;
+
+ entry_attr = nla_nest_start(msg, HANDSHAKE_A_ACCEPT_CERTIFICATE);
+ if (!entry_attr)
+ return -EMSGSIZE;
+
+ if (nla_put_u32(msg, HANDSHAKE_A_X509_CERT,
+ treq->th_certificate) ||
+ nla_put_u32(msg, HANDSHAKE_A_X509_PRIVKEY,
+ treq->th_privkey)) {
+ nla_nest_cancel(msg, entry_attr);
+ return -EMSGSIZE;
+ }
+
+ nla_nest_end(msg, entry_attr);
+ return 0;
+}
+
+/**
+ * tls_handshake_accept - callback to construct a CMD_ACCEPT response
+ * @req: handshake parameters to return
+ * @info: generic netlink message context
+ * @fd: file descriptor to be returned
+ *
+ * Returns zero on success, or a negative errno on failure.
+ */
+static int tls_handshake_accept(struct handshake_req *req,
+ struct genl_info *info, int fd)
+{
+ struct tls_handshake_req *treq = handshake_req_private(req);
+ struct nlmsghdr *hdr;
+ struct sk_buff *msg;
+ int ret;
+
+ ret = tls_handshake_private_keyring(treq);
+ if (ret < 0)
+ goto out;
+
+ ret = -ENOMEM;
+ msg = genlmsg_new(GENLMSG_DEFAULT_SIZE, GFP_KERNEL);
+ if (!msg)
+ goto out;
+ hdr = handshake_genl_put(msg, info);
+ if (!hdr)
+ goto out_cancel;
+
+ ret = -EMSGSIZE;
+ ret = nla_put_u32(msg, HANDSHAKE_A_ACCEPT_SOCKFD, fd);
+ if (ret < 0)
+ goto out_cancel;
+ ret = nla_put_u32(msg, HANDSHAKE_A_ACCEPT_MESSAGE_TYPE, treq->th_type);
+ if (ret < 0)
+ goto out_cancel;
+ if (treq->th_timeout_ms) {
+ ret = nla_put_u32(msg, HANDSHAKE_A_ACCEPT_TIMEOUT, treq->th_timeout_ms);
+ if (ret < 0)
+ goto out_cancel;
+ }
+
+ ret = nla_put_u32(msg, HANDSHAKE_A_ACCEPT_AUTH_MODE,
+ treq->th_auth_mode);
+ if (ret < 0)
+ goto out_cancel;
+ switch (treq->th_auth_mode) {
+ case HANDSHAKE_AUTH_PSK:
+ ret = tls_handshake_put_peer_identity(msg, treq);
+ if (ret < 0)
+ goto out_cancel;
+ break;
+ case HANDSHAKE_AUTH_X509:
+ ret = tls_handshake_put_certificate(msg, treq);
+ if (ret < 0)
+ goto out_cancel;
+ break;
+ }
+
+ genlmsg_end(msg, hdr);
+ return genlmsg_reply(msg, info);
+
+out_cancel:
+ genlmsg_cancel(msg, hdr);
+out:
+ return ret;
+}
+
+static const struct handshake_proto tls_handshake_proto = {
+ .hp_handler_class = HANDSHAKE_HANDLER_CLASS_TLSHD,
+ .hp_privsize = sizeof(struct tls_handshake_req),
+
+ .hp_accept = tls_handshake_accept,
+ .hp_done = tls_handshake_done,
+};
+
+/**
+ * tls_client_hello_anon - request an anonymous TLS handshake on a socket
+ * @args: socket and handshake parameters for this request
+ * @flags: memory allocation control flags
+ *
+ * Return values:
+ * %0: Handshake request enqueue; ->done will be called when complete
+ * %-ESRCH: No user agent is available
+ * %-ENOMEM: Memory allocation failed
+ */
+int tls_client_hello_anon(const struct tls_handshake_args *args, gfp_t flags)
+{
+ struct tls_handshake_req *treq;
+ struct handshake_req *req;
+
+ req = handshake_req_alloc(&tls_handshake_proto, flags);
+ if (!req)
+ return -ENOMEM;
+ treq = tls_handshake_req_init(req, args);
+ treq->th_type = HANDSHAKE_MSG_TYPE_CLIENTHELLO;
+ treq->th_auth_mode = HANDSHAKE_AUTH_UNAUTH;
+
+ return handshake_req_submit(args->ta_sock, req, flags);
+}
+EXPORT_SYMBOL(tls_client_hello_anon);
+
+/**
+ * tls_client_hello_x509 - request an x.509-based TLS handshake on a socket
+ * @args: socket and handshake parameters for this request
+ * @flags: memory allocation control flags
+ *
+ * Return values:
+ * %0: Handshake request enqueue; ->done will be called when complete
+ * %-ESRCH: No user agent is available
+ * %-ENOMEM: Memory allocation failed
+ */
+int tls_client_hello_x509(const struct tls_handshake_args *args, gfp_t flags)
+{
+ struct tls_handshake_req *treq;
+ struct handshake_req *req;
+
+ req = handshake_req_alloc(&tls_handshake_proto, flags);
+ if (!req)
+ return -ENOMEM;
+ treq = tls_handshake_req_init(req, args);
+ treq->th_type = HANDSHAKE_MSG_TYPE_CLIENTHELLO;
+ treq->th_auth_mode = HANDSHAKE_AUTH_X509;
+ treq->th_certificate = args->ta_my_cert;
+ treq->th_privkey = args->ta_my_privkey;
+
+ return handshake_req_submit(args->ta_sock, req, flags);
+}
+EXPORT_SYMBOL(tls_client_hello_x509);
+
+/**
+ * tls_client_hello_psk - request a PSK-based TLS handshake on a socket
+ * @args: socket and handshake parameters for this request
+ * @flags: memory allocation control flags
+ *
+ * Return values:
+ * %0: Handshake request enqueue; ->done will be called when complete
+ * %-EINVAL: Wrong number of local peer IDs
+ * %-ESRCH: No user agent is available
+ * %-ENOMEM: Memory allocation failed
+ */
+int tls_client_hello_psk(const struct tls_handshake_args *args, gfp_t flags)
+{
+ struct tls_handshake_req *treq;
+ struct handshake_req *req;
+ unsigned int i;
+
+ if (!args->ta_num_peerids ||
+ args->ta_num_peerids > ARRAY_SIZE(treq->th_peerid))
+ return -EINVAL;
+
+ req = handshake_req_alloc(&tls_handshake_proto, flags);
+ if (!req)
+ return -ENOMEM;
+ treq = tls_handshake_req_init(req, args);
+ treq->th_type = HANDSHAKE_MSG_TYPE_CLIENTHELLO;
+ treq->th_auth_mode = HANDSHAKE_AUTH_PSK;
+ treq->th_num_peerids = args->ta_num_peerids;
+ for (i = 0; i < args->ta_num_peerids; i++)
+ treq->th_peerid[i] = args->ta_my_peerids[i];
+
+ return handshake_req_submit(args->ta_sock, req, flags);
+}
+EXPORT_SYMBOL(tls_client_hello_psk);
+
+/**
+ * tls_server_hello_x509 - request a server TLS handshake on a socket
+ * @args: socket and handshake parameters for this request
+ * @flags: memory allocation control flags
+ *
+ * Return values:
+ * %0: Handshake request enqueue; ->done will be called when complete
+ * %-ESRCH: No user agent is available
+ * %-ENOMEM: Memory allocation failed
+ */
+int tls_server_hello_x509(const struct tls_handshake_args *args, gfp_t flags)
+{
+ struct tls_handshake_req *treq;
+ struct handshake_req *req;
+
+ req = handshake_req_alloc(&tls_handshake_proto, flags);
+ if (!req)
+ return -ENOMEM;
+ treq = tls_handshake_req_init(req, args);
+ treq->th_type = HANDSHAKE_MSG_TYPE_SERVERHELLO;
+ treq->th_auth_mode = HANDSHAKE_AUTH_X509;
+ treq->th_certificate = args->ta_my_cert;
+ treq->th_privkey = args->ta_my_privkey;
+
+ return handshake_req_submit(args->ta_sock, req, flags);
+}
+EXPORT_SYMBOL(tls_server_hello_x509);
+
+/**
+ * tls_server_hello_psk - request a server TLS handshake on a socket
+ * @args: socket and handshake parameters for this request
+ * @flags: memory allocation control flags
+ *
+ * Return values:
+ * %0: Handshake request enqueue; ->done will be called when complete
+ * %-ESRCH: No user agent is available
+ * %-ENOMEM: Memory allocation failed
+ */
+int tls_server_hello_psk(const struct tls_handshake_args *args, gfp_t flags)
+{
+ struct tls_handshake_req *treq;
+ struct handshake_req *req;
+
+ req = handshake_req_alloc(&tls_handshake_proto, flags);
+ if (!req)
+ return -ENOMEM;
+ treq = tls_handshake_req_init(req, args);
+ treq->th_type = HANDSHAKE_MSG_TYPE_SERVERHELLO;
+ treq->th_auth_mode = HANDSHAKE_AUTH_PSK;
+ treq->th_num_peerids = 1;
+ treq->th_peerid[0] = args->ta_my_peerids[0];
+
+ return handshake_req_submit(args->ta_sock, req, flags);
+}
+EXPORT_SYMBOL(tls_server_hello_psk);
+
+/**
+ * tls_handshake_cancel - cancel a pending handshake
+ * @sk: socket on which there is an ongoing handshake
+ *
+ * Request cancellation races with request completion. To determine
+ * who won, callers examine the return value from this function.
+ *
+ * Return values:
+ * %true - Uncompleted handshake request was canceled
+ * %false - Handshake request already completed or not found
+ */
+bool tls_handshake_cancel(struct sock *sk)
+{
+ return handshake_req_cancel(sk);
+}
+EXPORT_SYMBOL(tls_handshake_cancel);
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