1.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) 2 3================== 4Kernel TLS offload 5================== 6 7Kernel TLS operation 8==================== 9 10Linux kernel provides TLS connection offload infrastructure. Once a TCP 11connection is in ``ESTABLISHED`` state user space can enable the TLS Upper 12Layer Protocol (ULP) and install the cryptographic connection state. 13For details regarding the user-facing interface refer to the TLS 14documentation in :ref:`Documentation/networking/tls.rst <kernel_tls>`. 15 16``ktls`` can operate in three modes: 17 18 * Software crypto mode (``TLS_SW``) - CPU handles the cryptography. 19 In most basic cases only crypto operations synchronous with the CPU 20 can be used, but depending on calling context CPU may utilize 21 asynchronous crypto accelerators. The use of accelerators introduces extra 22 latency on socket reads (decryption only starts when a read syscall 23 is made) and additional I/O load on the system. 24 * Packet-based NIC offload mode (``TLS_HW``) - the NIC handles crypto 25 on a packet by packet basis, provided the packets arrive in order. 26 This mode integrates best with the kernel stack and is described in detail 27 in the remaining part of this document 28 (``ethtool`` flags ``tls-hw-tx-offload`` and ``tls-hw-rx-offload``). 29 * Full TCP NIC offload mode (``TLS_HW_RECORD``) - mode of operation where 30 NIC driver and firmware replace the kernel networking stack 31 with its own TCP handling, it is not usable in production environments 32 making use of the Linux networking stack for example any firewalling 33 abilities or QoS and packet scheduling (``ethtool`` flag ``tls-hw-record``). 34 35The operation mode is selected automatically based on device configuration, 36offload opt-in or opt-out on per-connection basis is not currently supported. 37 38TX 39-- 40 41At a high level user write requests are turned into a scatter list, the TLS ULP 42intercepts them, inserts record framing, performs encryption (in ``TLS_SW`` 43mode) and then hands the modified scatter list to the TCP layer. From this 44point on the TCP stack proceeds as normal. 45 46In ``TLS_HW`` mode the encryption is not performed in the TLS ULP. 47Instead packets reach a device driver, the driver will mark the packets 48for crypto offload based on the socket the packet is attached to, 49and send them to the device for encryption and transmission. 50 51RX 52-- 53 54On the receive side if the device handled decryption and authentication 55successfully, the driver will set the decrypted bit in the associated 56:c:type:`struct sk_buff <sk_buff>`. The packets reach the TCP stack and 57are handled normally. ``ktls`` is informed when data is queued to the socket 58and the ``strparser`` mechanism is used to delineate the records. Upon read 59request, records are retrieved from the socket and passed to decryption routine. 60If device decrypted all the segments of the record the decryption is skipped, 61otherwise software path handles decryption. 62 63.. kernel-figure:: tls-offload-layers.svg 64 :alt: TLS offload layers 65 :align: center 66 :figwidth: 28em 67 68 Layers of Kernel TLS stack 69 70Device configuration 71==================== 72 73During driver initialization device sets the ``NETIF_F_HW_TLS_RX`` and 74``NETIF_F_HW_TLS_TX`` features and installs its 75:c:type:`struct tlsdev_ops <tlsdev_ops>` 76pointer in the :c:member:`tlsdev_ops` member of the 77:c:type:`struct net_device <net_device>`. 78 79When TLS cryptographic connection state is installed on a ``ktls`` socket 80(note that it is done twice, once for RX and once for TX direction, 81and the two are completely independent), the kernel checks if the underlying 82network device is offload-capable and attempts the offload. In case offload 83fails the connection is handled entirely in software using the same mechanism 84as if the offload was never tried. 85 86Offload request is performed via the :c:member:`tls_dev_add` callback of 87:c:type:`struct tlsdev_ops <tlsdev_ops>`: 88 89.. code-block:: c 90 91 int (*tls_dev_add)(struct net_device *netdev, struct sock *sk, 92 enum tls_offload_ctx_dir direction, 93 struct tls_crypto_info *crypto_info, 94 u32 start_offload_tcp_sn); 95 96``direction`` indicates whether the cryptographic information is for 97the received or transmitted packets. Driver uses the ``sk`` parameter 98to retrieve the connection 5-tuple and socket family (IPv4 vs IPv6). 99Cryptographic information in ``crypto_info`` includes the key, iv, salt 100as well as TLS record sequence number. ``start_offload_tcp_sn`` indicates 101which TCP sequence number corresponds to the beginning of the record with 102sequence number from ``crypto_info``. The driver can add its state 103at the end of kernel structures (see :c:member:`driver_state` members 104in ``include/net/tls.h``) to avoid additional allocations and pointer 105dereferences. 106 107TX 108-- 109 110After TX state is installed, the stack guarantees that the first segment 111of the stream will start exactly at the ``start_offload_tcp_sn`` sequence 112number, simplifying TCP sequence number matching. 113 114TX offload being fully initialized does not imply that all segments passing 115through the driver and which belong to the offloaded socket will be after 116the expected sequence number and will have kernel record information. 117In particular, already encrypted data may have been queued to the socket 118before installing the connection state in the kernel. 119 120RX 121-- 122 123In RX direction local networking stack has little control over the segmentation, 124so the initial records' TCP sequence number may be anywhere inside the segment. 125 126Normal operation 127================ 128 129At the minimum the device maintains the following state for each connection, in 130each direction: 131 132 * crypto secrets (key, iv, salt) 133 * crypto processing state (partial blocks, partial authentication tag, etc.) 134 * record metadata (sequence number, processing offset and length) 135 * expected TCP sequence number 136 137There are no guarantees on record length or record segmentation. In particular 138segments may start at any point of a record and contain any number of records. 139Assuming segments are received in order, the device should be able to perform 140crypto operations and authentication regardless of segmentation. For this 141to be possible device has to keep small amount of segment-to-segment state. 142This includes at least: 143 144 * partial headers (if a segment carried only a part of the TLS header) 145 * partial data block 146 * partial authentication tag (all data had been seen but part of the 147 authentication tag has to be written or read from the subsequent segment) 148 149Record reassembly is not necessary for TLS offload. If the packets arrive 150in order the device should be able to handle them separately and make 151forward progress. 152 153TX 154-- 155 156The kernel stack performs record framing reserving space for the authentication 157tag and populating all other TLS header and tailer fields. 158 159Both the device and the driver maintain expected TCP sequence numbers 160due to the possibility of retransmissions and the lack of software fallback 161once the packet reaches the device. 162For segments passed in order, the driver marks the packets with 163a connection identifier (note that a 5-tuple lookup is insufficient to identify 164packets requiring HW offload, see the :ref:`5tuple_problems` section) 165and hands them to the device. The device identifies the packet as requiring 166TLS handling and confirms the sequence number matches its expectation. 167The device performs encryption and authentication of the record data. 168It replaces the authentication tag and TCP checksum with correct values. 169 170RX 171-- 172 173Before a packet is DMAed to the host (but after NIC's embedded switching 174and packet transformation functions) the device validates the Layer 4 175checksum and performs a 5-tuple lookup to find any TLS connection the packet 176may belong to (technically a 4-tuple 177lookup is sufficient - IP addresses and TCP port numbers, as the protocol 178is always TCP). If connection is matched device confirms if the TCP sequence 179number is the expected one and proceeds to TLS handling (record delineation, 180decryption, authentication for each record in the packet). The device leaves 181the record framing unmodified, the stack takes care of record decapsulation. 182Device indicates successful handling of TLS offload in the per-packet context 183(descriptor) passed to the host. 184 185Upon reception of a TLS offloaded packet, the driver sets 186the :c:member:`decrypted` mark in :c:type:`struct sk_buff <sk_buff>` 187corresponding to the segment. Networking stack makes sure decrypted 188and non-decrypted segments do not get coalesced (e.g. by GRO or socket layer) 189and takes care of partial decryption. 190 191Resync handling 192=============== 193 194In presence of packet drops or network packet reordering, the device may lose 195synchronization with the TLS stream, and require a resync with the kernel's 196TCP stack. 197 198Note that resync is only attempted for connections which were successfully 199added to the device table and are in TLS_HW mode. For example, 200if the table was full when cryptographic state was installed in the kernel, 201such connection will never get offloaded. Therefore the resync request 202does not carry any cryptographic connection state. 203 204TX 205-- 206 207Segments transmitted from an offloaded socket can get out of sync 208in similar ways to the receive side-retransmissions - local drops 209are possible, though network reorders are not. There are currently 210two mechanisms for dealing with out of order segments. 211 212Crypto state rebuilding 213~~~~~~~~~~~~~~~~~~~~~~~ 214 215Whenever an out of order segment is transmitted the driver provides 216the device with enough information to perform cryptographic operations. 217This means most likely that the part of the record preceding the current 218segment has to be passed to the device as part of the packet context, 219together with its TCP sequence number and TLS record number. The device 220can then initialize its crypto state, process and discard the preceding 221data (to be able to insert the authentication tag) and move onto handling 222the actual packet. 223 224In this mode depending on the implementation the driver can either ask 225for a continuation with the crypto state and the new sequence number 226(next expected segment is the one after the out of order one), or continue 227with the previous stream state - assuming that the out of order segment 228was just a retransmission. The former is simpler, and does not require 229retransmission detection therefore it is the recommended method until 230such time it is proven inefficient. 231 232Next record sync 233~~~~~~~~~~~~~~~~ 234 235Whenever an out of order segment is detected the driver requests 236that the ``ktls`` software fallback code encrypt it. If the segment's 237sequence number is lower than expected the driver assumes retransmission 238and doesn't change device state. If the segment is in the future, it 239may imply a local drop, the driver asks the stack to sync the device 240to the next record state and falls back to software. 241 242Resync request is indicated with: 243 244.. code-block:: c 245 246 void tls_offload_tx_resync_request(struct sock *sk, u32 got_seq, u32 exp_seq) 247 248Until resync is complete driver should not access its expected TCP 249sequence number (as it will be updated from a different context). 250Following helper should be used to test if resync is complete: 251 252.. code-block:: c 253 254 bool tls_offload_tx_resync_pending(struct sock *sk) 255 256Next time ``ktls`` pushes a record it will first send its TCP sequence number 257and TLS record number to the driver. Stack will also make sure that 258the new record will start on a segment boundary (like it does when 259the connection is initially added). 260 261RX 262-- 263 264A small amount of RX reorder events may not require a full resynchronization. 265In particular the device should not lose synchronization 266when record boundary can be recovered: 267 268.. kernel-figure:: tls-offload-reorder-good.svg 269 :alt: reorder of non-header segment 270 :align: center 271 272 Reorder of non-header segment 273 274Green segments are successfully decrypted, blue ones are passed 275as received on wire, red stripes mark start of new records. 276 277In above case segment 1 is received and decrypted successfully. 278Segment 2 was dropped so 3 arrives out of order. The device knows 279the next record starts inside 3, based on record length in segment 1. 280Segment 3 is passed untouched, because due to lack of data from segment 2 281the remainder of the previous record inside segment 3 cannot be handled. 282The device can, however, collect the authentication algorithm's state 283and partial block from the new record in segment 3 and when 4 and 5 284arrive continue decryption. Finally when 2 arrives it's completely outside 285of expected window of the device so it's passed as is without special 286handling. ``ktls`` software fallback handles the decryption of record 287spanning segments 1, 2 and 3. The device did not get out of sync, 288even though two segments did not get decrypted. 289 290Kernel synchronization may be necessary if the lost segment contained 291a record header and arrived after the next record header has already passed: 292 293.. kernel-figure:: tls-offload-reorder-bad.svg 294 :alt: reorder of header segment 295 :align: center 296 297 Reorder of segment with a TLS header 298 299In this example segment 2 gets dropped, and it contains a record header. 300Device can only detect that segment 4 also contains a TLS header 301if it knows the length of the previous record from segment 2. In this case 302the device will lose synchronization with the stream. 303 304Stream scan resynchronization 305~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 306 307When the device gets out of sync and the stream reaches TCP sequence 308numbers more than a max size record past the expected TCP sequence number, 309the device starts scanning for a known header pattern. For example 310for TLS 1.2 and TLS 1.3 subsequent bytes of value ``0x03 0x03`` occur 311in the SSL/TLS version field of the header. Once pattern is matched 312the device continues attempting parsing headers at expected locations 313(based on the length fields at guessed locations). 314Whenever the expected location does not contain a valid header the scan 315is restarted. 316 317When the header is matched the device sends a confirmation request 318to the kernel, asking if the guessed location is correct (if a TLS record 319really starts there), and which record sequence number the given header had. 320The kernel confirms the guessed location was correct and tells the device 321the record sequence number. Meanwhile, the device had been parsing 322and counting all records since the just-confirmed one, it adds the number 323of records it had seen to the record number provided by the kernel. 324At this point the device is in sync and can resume decryption at next 325segment boundary. 326 327In a pathological case the device may latch onto a sequence of matching 328headers and never hear back from the kernel (there is no negative 329confirmation from the kernel). The implementation may choose to periodically 330restart scan. Given how unlikely falsely-matching stream is, however, 331periodic restart is not deemed necessary. 332 333Special care has to be taken if the confirmation request is passed 334asynchronously to the packet stream and record may get processed 335by the kernel before the confirmation request. 336 337Stack-driven resynchronization 338~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 339 340The driver may also request the stack to perform resynchronization 341whenever it sees the records are no longer getting decrypted. 342If the connection is configured in this mode the stack automatically 343schedules resynchronization after it has received two completely encrypted 344records. 345 346The stack waits for the socket to drain and informs the device about 347the next expected record number and its TCP sequence number. If the 348records continue to be received fully encrypted stack retries the 349synchronization with an exponential back off (first after 2 encrypted 350records, then after 4 records, after 8, after 16... up until every 351128 records). 352 353Error handling 354============== 355 356TX 357-- 358 359Packets may be redirected or rerouted by the stack to a different 360device than the selected TLS offload device. The stack will handle 361such condition using the :c:func:`sk_validate_xmit_skb` helper 362(TLS offload code installs :c:func:`tls_validate_xmit_skb` at this hook). 363Offload maintains information about all records until the data is 364fully acknowledged, so if skbs reach the wrong device they can be handled 365by software fallback. 366 367Any device TLS offload handling error on the transmission side must result 368in the packet being dropped. For example if a packet got out of order 369due to a bug in the stack or the device, reached the device and can't 370be encrypted such packet must be dropped. 371 372RX 373-- 374 375If the device encounters any problems with TLS offload on the receive 376side it should pass the packet to the host's networking stack as it was 377received on the wire. 378 379For example authentication failure for any record in the segment should 380result in passing the unmodified packet to the software fallback. This means 381packets should not be modified "in place". Splitting segments to handle partial 382decryption is not advised. In other words either all records in the packet 383had been handled successfully and authenticated or the packet has to be passed 384to the host's stack as it was on the wire (recovering original packet in the 385driver if device provides precise error is sufficient). 386 387The Linux networking stack does not provide a way of reporting per-packet 388decryption and authentication errors, packets with errors must simply not 389have the :c:member:`decrypted` mark set. 390 391A packet should also not be handled by the TLS offload if it contains 392incorrect checksums. 393 394Performance metrics 395=================== 396 397TLS offload can be characterized by the following basic metrics: 398 399 * max connection count 400 * connection installation rate 401 * connection installation latency 402 * total cryptographic performance 403 404Note that each TCP connection requires a TLS session in both directions, 405the performance may be reported treating each direction separately. 406 407Max connection count 408-------------------- 409 410The number of connections device can support can be exposed via 411``devlink resource`` API. 412 413Total cryptographic performance 414------------------------------- 415 416Offload performance may depend on segment and record size. 417 418Overload of the cryptographic subsystem of the device should not have 419significant performance impact on non-offloaded streams. 420 421Statistics 422========== 423 424Following minimum set of TLS-related statistics should be reported 425by the driver: 426 427 * ``rx_tls_decrypted_packets`` - number of successfully decrypted RX packets 428 which were part of a TLS stream. 429 * ``rx_tls_decrypted_bytes`` - number of TLS payload bytes in RX packets 430 which were successfully decrypted. 431 * ``rx_tls_ctx`` - number of TLS RX HW offload contexts added to device for 432 decryption. 433 * ``rx_tls_del`` - number of TLS RX HW offload contexts deleted from device 434 (connection has finished). 435 * ``rx_tls_resync_req_pkt`` - number of received TLS packets with a resync 436 request. 437 * ``rx_tls_resync_req_start`` - number of times the TLS async resync request 438 was started. 439 * ``rx_tls_resync_req_end`` - number of times the TLS async resync request 440 properly ended with providing the HW tracked tcp-seq. 441 * ``rx_tls_resync_req_skip`` - number of times the TLS async resync request 442 procedure was started by not properly ended. 443 * ``rx_tls_resync_res_ok`` - number of times the TLS resync response call to 444 the driver was successfully handled. 445 * ``rx_tls_resync_res_skip`` - number of times the TLS resync response call to 446 the driver was terminated unsuccessfully. 447 * ``rx_tls_err`` - number of RX packets which were part of a TLS stream 448 but were not decrypted due to unexpected error in the state machine. 449 * ``tx_tls_encrypted_packets`` - number of TX packets passed to the device 450 for encryption of their TLS payload. 451 * ``tx_tls_encrypted_bytes`` - number of TLS payload bytes in TX packets 452 passed to the device for encryption. 453 * ``tx_tls_ctx`` - number of TLS TX HW offload contexts added to device for 454 encryption. 455 * ``tx_tls_ooo`` - number of TX packets which were part of a TLS stream 456 but did not arrive in the expected order. 457 * ``tx_tls_skip_no_sync_data`` - number of TX packets which were part of 458 a TLS stream and arrived out-of-order, but skipped the HW offload routine 459 and went to the regular transmit flow as they were retransmissions of the 460 connection handshake. 461 * ``tx_tls_drop_no_sync_data`` - number of TX packets which were part of 462 a TLS stream dropped, because they arrived out of order and associated 463 record could not be found. 464 * ``tx_tls_drop_bypass_req`` - number of TX packets which were part of a TLS 465 stream dropped, because they contain both data that has been encrypted by 466 software and data that expects hardware crypto offload. 467 468Notable corner cases, exceptions and additional requirements 469============================================================ 470 471.. _5tuple_problems: 472 4735-tuple matching limitations 474---------------------------- 475 476The device can only recognize received packets based on the 5-tuple 477of the socket. Current ``ktls`` implementation will not offload sockets 478routed through software interfaces such as those used for tunneling 479or virtual networking. However, many packet transformations performed 480by the networking stack (most notably any BPF logic) do not require 481any intermediate software device, therefore a 5-tuple match may 482consistently miss at the device level. In such cases the device 483should still be able to perform TX offload (encryption) and should 484fallback cleanly to software decryption (RX). 485 486Out of order 487------------ 488 489Introducing extra processing in NICs should not cause packets to be 490transmitted or received out of order, for example pure ACK packets 491should not be reordered with respect to data segments. 492 493Ingress reorder 494--------------- 495 496A device is permitted to perform packet reordering for consecutive 497TCP segments (i.e. placing packets in the correct order) but any form 498of additional buffering is disallowed. 499 500Coexistence with standard networking offload features 501----------------------------------------------------- 502 503Offloaded ``ktls`` sockets should support standard TCP stack features 504transparently. Enabling device TLS offload should not cause any difference 505in packets as seen on the wire. 506 507Transport layer transparency 508---------------------------- 509 510The device should not modify any packet headers for the purpose 511of the simplifying TLS offload. 512 513The device should not depend on any packet headers beyond what is strictly 514necessary for TLS offload. 515 516Segment drops 517------------- 518 519Dropping packets is acceptable only in the event of catastrophic 520system errors and should never be used as an error handling mechanism 521in cases arising from normal operation. In other words, reliance 522on TCP retransmissions to handle corner cases is not acceptable. 523 524TLS device features 525------------------- 526 527Drivers should ignore the changes to the TLS device feature flags. 528These flags will be acted upon accordingly by the core ``ktls`` code. 529TLS device feature flags only control adding of new TLS connection 530offloads, old connections will remain active after flags are cleared. 531 532TLS encryption cannot be offloaded to devices without checksum calculation 533offload. Hence, TLS TX device feature flag requires TX csum offload being set. 534Disabling the latter implies clearing the former. Disabling TX checksum offload 535should not affect old connections, and drivers should make sure checksum 536calculation does not break for them. 537Similarly, device-offloaded TLS decryption implies doing RXCSUM. If the user 538does not want to enable RX csum offload, TLS RX device feature is disabled 539as well. 540
