Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 Overview 2 ======== 3 4 Transport Layer Security (TLS) is a Upper Layer Protocol (ULP) that runs over 5 TCP. TLS provides end-to-end data integrity and confidentiality. 6 7 User interface 8 ============== 9 10 Creating a TLS connection 11 ------------------------- 12 13 First create a new TCP socket and set the TLS ULP. 14 15 sock = socket(AF_INET, SOCK_STREAM, 0); 16 setsockopt(sock, SOL_TCP, TCP_ULP, "tls", sizeof("tls")); 17 18 Setting the TLS ULP allows us to set/get TLS socket options. Currently 19 only the symmetric encryption is handled in the kernel. After the TLS 20 handshake is complete, we have all the parameters required to move the 21 data-path to the kernel. There is a separate socket option for moving 22 the transmit and the receive into the kernel. 23 24 /* From linux/tls.h */ 25 struct tls_crypto_info { 26 unsigned short version; 27 unsigned short cipher_type; 28 }; 29 30 struct tls12_crypto_info_aes_gcm_128 { 31 struct tls_crypto_info info; 32 unsigned char iv[TLS_CIPHER_AES_GCM_128_IV_SIZE]; 33 unsigned char key[TLS_CIPHER_AES_GCM_128_KEY_SIZE]; 34 unsigned char salt[TLS_CIPHER_AES_GCM_128_SALT_SIZE]; 35 unsigned char rec_seq[TLS_CIPHER_AES_GCM_128_REC_SEQ_SIZE]; 36 }; 37 38 39 struct tls12_crypto_info_aes_gcm_128 crypto_info; 40 41 crypto_info.info.version = TLS_1_2_VERSION; 42 crypto_info.info.cipher_type = TLS_CIPHER_AES_GCM_128; 43 memcpy(crypto_info.iv, iv_write, TLS_CIPHER_AES_GCM_128_IV_SIZE); 44 memcpy(crypto_info.rec_seq, seq_number_write, 45 TLS_CIPHER_AES_GCM_128_REC_SEQ_SIZE); 46 memcpy(crypto_info.key, cipher_key_write, TLS_CIPHER_AES_GCM_128_KEY_SIZE); 47 memcpy(crypto_info.salt, implicit_iv_write, TLS_CIPHER_AES_GCM_128_SALT_SIZE); 48 49 setsockopt(sock, SOL_TLS, TLS_TX, &crypto_info, sizeof(crypto_info)); 50 51 Sending TLS application data 52 ---------------------------- 53 54 After setting the TLS_TX socket option all application data sent over this 55 socket is encrypted using TLS and the parameters provided in the socket option. 56 For example, we can send an encrypted hello world record as follows: 57 58 const char *msg = "hello world\n"; 59 send(sock, msg, strlen(msg)); 60 61 send() data is directly encrypted from the userspace buffer provided 62 to the encrypted kernel send buffer if possible. 63 64 The sendfile system call will send the file's data over TLS records of maximum 65 length (2^14). 66 67 file = open(filename, O_RDONLY); 68 fstat(file, &stat); 69 sendfile(sock, file, &offset, stat.st_size); 70 71 TLS records are created and sent after each send() call, unless 72 MSG_MORE is passed. MSG_MORE will delay creation of a record until 73 MSG_MORE is not passed, or the maximum record size is reached. 74 75 The kernel will need to allocate a buffer for the encrypted data. 76 This buffer is allocated at the time send() is called, such that 77 either the entire send() call will return -ENOMEM (or block waiting 78 for memory), or the encryption will always succeed. If send() returns 79 -ENOMEM and some data was left on the socket buffer from a previous 80 call using MSG_MORE, the MSG_MORE data is left on the socket buffer. 81 82 Send TLS control messages 83 ------------------------- 84 85 Other than application data, TLS has control messages such as alert 86 messages (record type 21) and handshake messages (record type 22), etc. 87 These messages can be sent over the socket by providing the TLS record type 88 via a CMSG. For example the following function sends @data of @length bytes 89 using a record of type @record_type. 90 91 /* send TLS control message using record_type */ 92 static int klts_send_ctrl_message(int sock, unsigned char record_type, 93 void *data, size_t length) 94 { 95 struct msghdr msg = {0}; 96 int cmsg_len = sizeof(record_type); 97 struct cmsghdr *cmsg; 98 char buf[CMSG_SPACE(cmsg_len)]; 99 struct iovec msg_iov; /* Vector of data to send/receive into. */ 100 101 msg.msg_control = buf; 102 msg.msg_controllen = sizeof(buf); 103 cmsg = CMSG_FIRSTHDR(&msg); 104 cmsg->cmsg_level = SOL_TLS; 105 cmsg->cmsg_type = TLS_SET_RECORD_TYPE; 106 cmsg->cmsg_len = CMSG_LEN(cmsg_len); 107 *CMSG_DATA(cmsg) = record_type; 108 msg.msg_controllen = cmsg->cmsg_len; 109 110 msg_iov.iov_base = data; 111 msg_iov.iov_len = length; 112 msg.msg_iov = &msg_iov; 113 msg.msg_iovlen = 1; 114 115 return sendmsg(sock, &msg, 0); 116 } 117 118 Control message data should be provided unencrypted, and will be 119 encrypted by the kernel. 120 121 Integrating in to userspace TLS library 122 --------------------------------------- 123 124 At a high level, the kernel TLS ULP is a replacement for the record 125 layer of a userspace TLS library. 126 127 A patchset to OpenSSL to use ktls as the record layer is here: 128 129 https://github.com/Mellanox/tls-openssl 130 131 An example of calling send directly after a handshake using 132 gnutls. Since it doesn't implement a full record layer, control 133 messages are not supported: 134 135 https://github.com/Mellanox/tls-af_ktls_tool