net/handshake: Add a kernel API for requesting a TLSv1.3 handshake

    type: enum

    name: handler-class

    value-start: 0

    entries: [ none, max ]

    entries: [ none, tlshd, max ]

  -

    type: enum

    name: msg-type

  list:

    -

      name: none

    -

      name: tlshd

   scaling

   tls

   tls-offload

   tls-handshake

   nfc

   6lowpan

   6pack

.. 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.

L:	netdev@vger.kernel.org

S:	Maintained

F:	Documentation/netlink/specs/handshake.yaml

F:	Documentation/networking/tls-handshake.rst

F:	include/net/handshake.h

F:	include/trace/events/handshake.h

F:	net/handshake/

/* SPDX-License-Identifier: GPL-2.0-only */

/*

 * Generic netlink HANDSHAKE service.

 *

 * Author: Chuck Lever <chuck.lever@oracle.com>

 *

 * Copyright (c) 2023, Oracle and/or its affiliates.

 */

#ifndef _NET_HANDSHAKE_H

#define _NET_HANDSHAKE_H

enum {

	TLS_NO_KEYRING = 0,

	TLS_NO_PEERID = 0,

	TLS_NO_CERT = 0,

	TLS_NO_PRIVKEY = 0,

};

typedef void	(*tls_done_func_t)(void *data, int status,

				   key_serial_t peerid);

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];

};

int tls_client_hello_anon(const struct tls_handshake_args *args, gfp_t flags);

int tls_client_hello_x509(const struct tls_handshake_args *args, gfp_t flags);

int tls_client_hello_psk(const struct tls_handshake_args *args, gfp_t flags);

int tls_server_hello_x509(const struct tls_handshake_args *args, gfp_t flags);

int tls_server_hello_psk(const struct tls_handshake_args *args, gfp_t flags);

bool tls_handshake_cancel(struct sock *sk);

#endif /* _NET_HANDSHAKE_H */