src/kernel/linux/v4.19/Documentation/networking/tls.txt

Overview
========

Transport Layer Security (TLS) is a Upper Layer Protocol (ULP) that runs over
TCP. TLS provides end-to-end data integrity and confidentiality.

User interface
==============

Creating a TLS connection
-------------------------

First create a new TCP socket and set the TLS ULP.

  sock = socket(AF_INET, SOCK_STREAM, 0);
  setsockopt(sock, SOL_TCP, TCP_ULP, "tls", sizeof("tls"));

Setting the TLS ULP allows us to set/get TLS socket options. Currently
only the symmetric encryption is handled in the kernel.  After the TLS
handshake is complete, we have all the parameters required to move the
data-path to the kernel. There is a separate socket option for moving
the transmit and the receive into the kernel.

  /* From linux/tls.h */
  struct tls_crypto_info {
          unsigned short version;
          unsigned short cipher_type;
  };

  struct tls12_crypto_info_aes_gcm_128 {
          struct tls_crypto_info info;
          unsigned char iv[TLS_CIPHER_AES_GCM_128_IV_SIZE];
          unsigned char key[TLS_CIPHER_AES_GCM_128_KEY_SIZE];
          unsigned char salt[TLS_CIPHER_AES_GCM_128_SALT_SIZE];
          unsigned char rec_seq[TLS_CIPHER_AES_GCM_128_REC_SEQ_SIZE];
  };


  struct tls12_crypto_info_aes_gcm_128 crypto_info;

  crypto_info.info.version = TLS_1_2_VERSION;
  crypto_info.info.cipher_type = TLS_CIPHER_AES_GCM_128;
  memcpy(crypto_info.iv, iv_write, TLS_CIPHER_AES_GCM_128_IV_SIZE);
  memcpy(crypto_info.rec_seq, seq_number_write,
					TLS_CIPHER_AES_GCM_128_REC_SEQ_SIZE);
  memcpy(crypto_info.key, cipher_key_write, TLS_CIPHER_AES_GCM_128_KEY_SIZE);
  memcpy(crypto_info.salt, implicit_iv_write, TLS_CIPHER_AES_GCM_128_SALT_SIZE);

  setsockopt(sock, SOL_TLS, TLS_TX, &crypto_info, sizeof(crypto_info));

Transmit and receive are set separately, but the setup is the same, using either
TLS_TX or TLS_RX.

Sending TLS application data
----------------------------

After setting the TLS_TX socket option all application data sent over this
socket is encrypted using TLS and the parameters provided in the socket option.
For example, we can send an encrypted hello world record as follows:

  const char *msg = "hello world\n";
  send(sock, msg, strlen(msg));

send() data is directly encrypted from the userspace buffer provided
to the encrypted kernel send buffer if possible.

The sendfile system call will send the file's data over TLS records of maximum
length (2^14).

  file = open(filename, O_RDONLY);
  fstat(file, &stat);
  sendfile(sock, file, &offset, stat.st_size);

TLS records are created and sent after each send() call, unless
MSG_MORE is passed.  MSG_MORE will delay creation of a record until
MSG_MORE is not passed, or the maximum record size is reached.

The kernel will need to allocate a buffer for the encrypted data.
This buffer is allocated at the time send() is called, such that
either the entire send() call will return -ENOMEM (or block waiting
for memory), or the encryption will always succeed.  If send() returns
-ENOMEM and some data was left on the socket buffer from a previous
call using MSG_MORE, the MSG_MORE data is left on the socket buffer.

Receiving TLS application data
------------------------------

After setting the TLS_RX socket option, all recv family socket calls
are decrypted using TLS parameters provided.  A full TLS record must
be received before decryption can happen.

  char buffer[16384];
  recv(sock, buffer, 16384);

Received data is decrypted directly in to the user buffer if it is
large enough, and no additional allocations occur.  If the userspace
buffer is too small, data is decrypted in the kernel and copied to
userspace.

EINVAL is returned if the TLS version in the received message does not
match the version passed in setsockopt.

EMSGSIZE is returned if the received message is too big.

EBADMSG is returned if decryption failed for any other reason.

Send TLS control messages
-------------------------

Other than application data, TLS has control messages such as alert
messages (record type 21) and handshake messages (record type 22), etc.
These messages can be sent over the socket by providing the TLS record type
via a CMSG. For example the following function sends @data of @length bytes
using a record of type @record_type.

/* send TLS control message using record_type */
  static int klts_send_ctrl_message(int sock, unsigned char record_type,
                                  void *data, size_t length)
  {
        struct msghdr msg = {0};
        int cmsg_len = sizeof(record_type);
        struct cmsghdr *cmsg;
        char buf[CMSG_SPACE(cmsg_len)];
        struct iovec msg_iov;   /* Vector of data to send/receive into.  */

        msg.msg_control = buf;
        msg.msg_controllen = sizeof(buf);
        cmsg = CMSG_FIRSTHDR(&msg);
        cmsg->cmsg_level = SOL_TLS;
        cmsg->cmsg_type = TLS_SET_RECORD_TYPE;
        cmsg->cmsg_len = CMSG_LEN(cmsg_len);
        *CMSG_DATA(cmsg) = record_type;
        msg.msg_controllen = cmsg->cmsg_len;

        msg_iov.iov_base = data;
        msg_iov.iov_len = length;
        msg.msg_iov = &msg_iov;
        msg.msg_iovlen = 1;

        return sendmsg(sock, &msg, 0);
  }

Control message data should be provided unencrypted, and will be
encrypted by the kernel.

Receiving TLS control messages
------------------------------

TLS control messages are passed in the userspace buffer, with message
type passed via cmsg.  If no cmsg buffer is provided, an error is
returned if a control message is received.  Data messages may be
received without a cmsg buffer set.

  char buffer[16384];
  char cmsg[CMSG_SPACE(sizeof(unsigned char))];
  struct msghdr msg = {0};
  msg.msg_control = cmsg;
  msg.msg_controllen = sizeof(cmsg);

  struct iovec msg_iov;
  msg_iov.iov_base = buffer;
  msg_iov.iov_len = 16384;

  msg.msg_iov = &msg_iov;
  msg.msg_iovlen = 1;

  int ret = recvmsg(sock, &msg, 0 /* flags */);

  struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
  if (cmsg->cmsg_level == SOL_TLS &&
      cmsg->cmsg_type == TLS_GET_RECORD_TYPE) {
      int record_type = *((unsigned char *)CMSG_DATA(cmsg));
      // Do something with record_type, and control message data in
      // buffer.
      //
      // Note that record_type may be == to application data (23).
  } else {
      // Buffer contains application data.
  }

recv will never return data from mixed types of TLS records.

Integrating in to userspace TLS library
---------------------------------------

At a high level, the kernel TLS ULP is a replacement for the record
layer of a userspace TLS library.

A patchset to OpenSSL to use ktls as the record layer is here:

https://github.com/Mellanox/openssl/commits/tls_rx2

An example of calling send directly after a handshake using
gnutls.  Since it doesn't implement a full record layer, control
messages are not supported:

https://github.com/ktls/af_ktls-tool/commits/RX