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 * ``tx_tls_encrypted_packets`` - number of TX packets passed to the device
432   for encryption of their TLS payload.
433 * ``tx_tls_encrypted_bytes`` - number of TLS payload bytes in TX packets
434   passed to the device for encryption.
435 * ``tx_tls_ctx`` - number of TLS TX HW offload contexts added to device for
436   encryption.
437 * ``tx_tls_ooo`` - number of TX packets which were part of a TLS stream
438   but did not arrive in the expected order.
439 * ``tx_tls_skip_no_sync_data`` - number of TX packets which were part of
440   a TLS stream and arrived out-of-order, but skipped the HW offload routine
441   and went to the regular transmit flow as they were retransmissions of the
442   connection handshake.
443 * ``tx_tls_drop_no_sync_data`` - number of TX packets which were part of
444   a TLS stream dropped, because they arrived out of order and associated
445   record could not be found.
446 * ``tx_tls_drop_bypass_req`` - number of TX packets which were part of a TLS
447   stream dropped, because they contain both data that has been encrypted by
448   software and data that expects hardware crypto offload.
449
450Notable corner cases, exceptions and additional requirements
451============================================================
452
453.. _5tuple_problems:
454
4555-tuple matching limitations
456----------------------------
457
458The device can only recognize received packets based on the 5-tuple
459of the socket. Current ``ktls`` implementation will not offload sockets
460routed through software interfaces such as those used for tunneling
461or virtual networking. However, many packet transformations performed
462by the networking stack (most notably any BPF logic) do not require
463any intermediate software device, therefore a 5-tuple match may
464consistently miss at the device level. In such cases the device
465should still be able to perform TX offload (encryption) and should
466fallback cleanly to software decryption (RX).
467
468Out of order
469------------
470
471Introducing extra processing in NICs should not cause packets to be
472transmitted or received out of order, for example pure ACK packets
473should not be reordered with respect to data segments.
474
475Ingress reorder
476---------------
477
478A device is permitted to perform packet reordering for consecutive
479TCP segments (i.e. placing packets in the correct order) but any form
480of additional buffering is disallowed.
481
482Coexistence with standard networking offload features
483-----------------------------------------------------
484
485Offloaded ``ktls`` sockets should support standard TCP stack features
486transparently. Enabling device TLS offload should not cause any difference
487in packets as seen on the wire.
488
489Transport layer transparency
490----------------------------
491
492The device should not modify any packet headers for the purpose
493of the simplifying TLS offload.
494
495The device should not depend on any packet headers beyond what is strictly
496necessary for TLS offload.
497
498Segment drops
499-------------
500
501Dropping packets is acceptable only in the event of catastrophic
502system errors and should never be used as an error handling mechanism
503in cases arising from normal operation. In other words, reliance
504on TCP retransmissions to handle corner cases is not acceptable.
505
506TLS device features
507-------------------
508
509Drivers should ignore the changes to TLS the device feature flags.
510These flags will be acted upon accordingly by the core ``ktls`` code.
511TLS device feature flags only control adding of new TLS connection
512offloads, old connections will remain active after flags are cleared.
513