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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));
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.
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.
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/tls-openssl
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/Mellanox/tls-af_ktls_tool
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