Documentation: tls: RSTify the ktls documentation

Convert the TLS doc to RST.  Use C code blocks for the code
samples, and mark hyperlinks.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Dave Watson <davejwatson@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 213 insertions, 0 deletions

diff --git a/Documentation/networking/tls.rst b/Documentation/networking/tls.rst
new file mode 100644
index 000000000000..482bd73f18a2
--- /dev/null
+++ b/Documentation/networking/tls.rst
@@ -0,0 +1,213 @@
+==========
+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 set the TLS ULP.
+
+.. code-block:: c
+
+  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.
+
+.. 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.
+
+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.