.. _kernel_tls:

==========
Kernel TLS
==========

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 once the connection is established set the
TLS ULP.

.. code-block:: c

  sock = socket(AF_INET, SOCK_STREAM, 0);
  connect(sock, addr, addrlen);
  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.

.. code-block:: c

  /* 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:

.. code-block:: c

  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).

.. code-block:: c

  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.

.. code-block:: c

  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.

.. code-block:: c

  /* 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.

.. code-block:: c

  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.

TLS 1.3 Key Updates
-------------------

In TLS 1.3, KeyUpdate handshake messages signal that the sender is
updating its TX key. Any message sent after a KeyUpdate will be
encrypted using the new key. The userspace library can pass the new
key to the kernel using the TLS_TX and TLS_RX socket options, as for
the initial keys. TLS version and cipher cannot be changed.

To prevent attempting to decrypt incoming records using the wrong key,
decryption will be paused when a KeyUpdate message is received by the
kernel, until the new key has been provided using the TLS_RX socket
option. Any read occurring after the KeyUpdate has been read and
before the new key is provided will fail with EKEYEXPIRED. poll() will
not report any read events from the socket until the new key is
provided. There is no pausing on the transmit side.

Userspace should make sure that the crypto_info provided has been set
properly. In particular, the kernel will not check for key/nonce
reuse.

The number of successful and failed key updates is tracked in the
``TlsTxRekeyOk``, ``TlsRxRekeyOk``, ``TlsTxRekeyError``,
``TlsRxRekeyError`` statistics. The ``TlsRxRekeyReceived`` statistic
counts KeyUpdate handshake messages that have been received.

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 <https://github.com/ktls/af_ktls-tool/commits/RX>`_
of calling send directly after a handshake using gnutls.
Since it doesn't implement a full record layer, control
messages are not supported.

Optional optimizations
----------------------

There are certain condition-specific optimizations the TLS ULP can make,
if requested. Those optimizations are either not universally beneficial
or may impact correctness, hence they require an opt-in.
All options are set per-socket using setsockopt(), and their
state can be checked using getsockopt() and via socket diag (``ss``).

TLS_TX_ZEROCOPY_RO
~~~~~~~~~~~~~~~~~~

For device offload only. Allow sendfile() data to be transmitted directly
to the NIC without making an in-kernel copy. This allows true zero-copy
behavior when device offload is enabled.

The application must make sure that the data is not modified between being
submitted and transmission completing. In other words this is mostly
applicable if the data sent on a socket via sendfile() is read-only.

Modifying the data may result in different versions of the data being used
for the original TCP transmission and TCP retransmissions. To the receiver
this will look like TLS records had been tampered with and will result
in record authentication failures.

TLS_RX_EXPECT_NO_PAD
~~~~~~~~~~~~~~~~~~~~

TLS 1.3 only. Expect the sender to not pad records. This allows the data
to be decrypted directly into user space buffers with TLS 1.3.

This optimization is safe to enable only if the remote end is trusted,
otherwise it is an attack vector to doubling the TLS processing cost.

If the record decrypted turns out to had been padded or is not a data
record it will be decrypted again into a kernel buffer without zero copy.
Such events are counted in the ``TlsDecryptRetry`` statistic.

TLS_TX_MAX_PAYLOAD_LEN
~~~~~~~~~~~~~~~~~~~~~~

Specifies the maximum size of the plaintext payload for transmitted TLS records.

When this option is set, the kernel enforces the specified limit on all outgoing
TLS records. No plaintext fragment will exceed this size. This option can be used
to implement the TLS Record Size Limit extension [1].

* For TLS 1.2, the value corresponds directly to the record size limit.
* For TLS 1.3, the value should be set to record_size_limit - 1, since
  the record size limit includes one additional byte for the ContentType
  field.

The valid range for this option is 64 to 16384 bytes for TLS 1.2, and 63 to
16384 bytes for TLS 1.3. The lower minimum for TLS 1.3 accounts for the
extra byte used by the ContentType field.

[1] https://datatracker.ietf.org/doc/html/rfc8449

Statistics
==========

TLS implementation exposes the following per-namespace statistics
(``/proc/net/tls_stat``):

- ``TlsCurrTxSw``, ``TlsCurrRxSw`` -
  number of TX and RX sessions currently installed where host handles
  cryptography

- ``TlsCurrTxDevice``, ``TlsCurrRxDevice`` -
  number of TX and RX sessions currently installed where NIC handles
  cryptography

- ``TlsTxSw``, ``TlsRxSw`` -
  number of TX and RX sessions opened with host cryptography

- ``TlsTxDevice``, ``TlsRxDevice`` -
  number of TX and RX sessions opened with NIC cryptography

- ``TlsDecryptError`` -
  record decryption failed (e.g. due to incorrect authentication tag)

- ``TlsDeviceRxResync`` -
  number of RX resyncs sent to NICs handling cryptography

- ``TlsDecryptRetry`` -
  number of RX records which had to be re-decrypted due to
  ``TLS_RX_EXPECT_NO_PAD`` mis-prediction. Note that this counter will
  also increment for non-data records.

- ``TlsRxNoPadViolation`` -
  number of data RX records which had to be re-decrypted due to
  ``TLS_RX_EXPECT_NO_PAD`` mis-prediction.

- ``TlsTxRekeyOk``, ``TlsRxRekeyOk`` -
  number of successful rekeys on existing sessions for TX and RX

- ``TlsTxRekeyError``, ``TlsRxRekeyError`` -
  number of failed rekeys on existing sessions for TX and RX

- ``TlsRxRekeyReceived`` -
  number of received KeyUpdate handshake messages, requiring userspace
  to provide a new RX key
