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diff --git a/Documentation/networking/tls-offload.rst b/Documentation/networking/tls-offload.rst new file mode 100644 index 000000000000..048e5ca44824 --- /dev/null +++ b/Documentation/networking/tls-offload.rst @@ -0,0 +1,515 @@ +.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) + +================== +Kernel TLS offload +================== + +Kernel TLS operation +==================== + +Linux kernel provides TLS connection offload infrastructure. Once a TCP +connection is in ``ESTABLISHED`` state user space can enable the TLS Upper +Layer Protocol (ULP) and install the cryptographic connection state. +For details regarding the user-facing interface refer to the TLS +documentation in :ref:`Documentation/networking/tls.rst <kernel_tls>`. + +``ktls`` can operate in three modes: + + * Software crypto mode (``TLS_SW``) - CPU handles the cryptography. + In most basic cases only crypto operations synchronous with the CPU + can be used, but depending on calling context CPU may utilize + asynchronous crypto accelerators. The use of accelerators introduces extra + latency on socket reads (decryption only starts when a read syscall + is made) and additional I/O load on the system. + * Packet-based NIC offload mode (``TLS_HW``) - the NIC handles crypto + on a packet by packet basis, provided the packets arrive in order. + This mode integrates best with the kernel stack and is described in detail + in the remaining part of this document + (``ethtool`` flags ``tls-hw-tx-offload`` and ``tls-hw-rx-offload``). + * Full TCP NIC offload mode (``TLS_HW_RECORD``) - mode of operation where + NIC driver and firmware replace the kernel networking stack + with its own TCP handling, it is not usable in production environments + making use of the Linux networking stack for example any firewalling + abilities or QoS and packet scheduling (``ethtool`` flag ``tls-hw-record``). + +The operation mode is selected automatically based on device configuration, +offload opt-in or opt-out on per-connection basis is not currently supported. + +TX +-- + +At a high level user write requests are turned into a scatter list, the TLS ULP +intercepts them, inserts record framing, performs encryption (in ``TLS_SW`` +mode) and then hands the modified scatter list to the TCP layer. From this +point on the TCP stack proceeds as normal. + +In ``TLS_HW`` mode the encryption is not performed in the TLS ULP. +Instead packets reach a device driver, the driver will mark the packets +for crypto offload based on the socket the packet is attached to, +and send them to the device for encryption and transmission. + +RX +-- + +On the receive side if the device handled decryption and authentication +successfully, the driver will set the decrypted bit in the associated +:c:type:`struct sk_buff <sk_buff>`. The packets reach the TCP stack and +are handled normally. ``ktls`` is informed when data is queued to the socket +and the ``strparser`` mechanism is used to delineate the records. Upon read +request, records are retrieved from the socket and passed to decryption routine. +If device decrypted all the segments of the record the decryption is skipped, +otherwise software path handles decryption. + +.. kernel-figure:: tls-offload-layers.svg + :alt: TLS offload layers + :align: center + :figwidth: 28em + + Layers of Kernel TLS stack + +Device configuration +==================== + +During driver initialization device sets the ``NETIF_F_HW_TLS_RX`` and +``NETIF_F_HW_TLS_TX`` features and installs its +:c:type:`struct tlsdev_ops <tlsdev_ops>` +pointer in the :c:member:`tlsdev_ops` member of the +:c:type:`struct net_device <net_device>`. + +When TLS cryptographic connection state is installed on a ``ktls`` socket +(note that it is done twice, once for RX and once for TX direction, +and the two are completely independent), the kernel checks if the underlying +network device is offload-capable and attempts the offload. In case offload +fails the connection is handled entirely in software using the same mechanism +as if the offload was never tried. + +Offload request is performed via the :c:member:`tls_dev_add` callback of +:c:type:`struct tlsdev_ops <tlsdev_ops>`: + +.. code-block:: c + + int (*tls_dev_add)(struct net_device *netdev, struct sock *sk, + enum tls_offload_ctx_dir direction, + struct tls_crypto_info *crypto_info, + u32 start_offload_tcp_sn); + +``direction`` indicates whether the cryptographic information is for +the received or transmitted packets. Driver uses the ``sk`` parameter +to retrieve the connection 5-tuple and socket family (IPv4 vs IPv6). +Cryptographic information in ``crypto_info`` includes the key, iv, salt +as well as TLS record sequence number. ``start_offload_tcp_sn`` indicates +which TCP sequence number corresponds to the beginning of the record with +sequence number from ``crypto_info``. The driver can add its state +at the end of kernel structures (see :c:member:`driver_state` members +in ``include/net/tls.h``) to avoid additional allocations and pointer +dereferences. + +TX +-- + +After TX state is installed, the stack guarantees that the first segment +of the stream will start exactly at the ``start_offload_tcp_sn`` sequence +number, simplifying TCP sequence number matching. + +TX offload being fully initialized does not imply that all segments passing +through the driver and which belong to the offloaded socket will be after +the expected sequence number and will have kernel record information. +In particular, already encrypted data may have been queued to the socket +before installing the connection state in the kernel. + +RX +-- + +In RX direction local networking stack has little control over the segmentation, +so the initial records' TCP sequence number may be anywhere inside the segment. + +Normal operation +================ + +At the minimum the device maintains the following state for each connection, in +each direction: + + * crypto secrets (key, iv, salt) + * crypto processing state (partial blocks, partial authentication tag, etc.) + * record metadata (sequence number, processing offset and length) + * expected TCP sequence number + +There are no guarantees on record length or record segmentation. In particular +segments may start at any point of a record and contain any number of records. +Assuming segments are received in order, the device should be able to perform +crypto operations and authentication regardless of segmentation. For this +to be possible device has to keep small amount of segment-to-segment state. +This includes at least: + + * partial headers (if a segment carried only a part of the TLS header) + * partial data block + * partial authentication tag (all data had been seen but part of the + authentication tag has to be written or read from the subsequent segment) + +Record reassembly is not necessary for TLS offload. If the packets arrive +in order the device should be able to handle them separately and make +forward progress. + +TX +-- + +The kernel stack performs record framing reserving space for the authentication +tag and populating all other TLS header and tailer fields. + +Both the device and the driver maintain expected TCP sequence numbers +due to the possibility of retransmissions and the lack of software fallback +once the packet reaches the device. +For segments passed in order, the driver marks the packets with +a connection identifier (note that a 5-tuple lookup is insufficient to identify +packets requiring HW offload, see the :ref:`5tuple_problems` section) +and hands them to the device. The device identifies the packet as requiring +TLS handling and confirms the sequence number matches its expectation. +The device performs encryption and authentication of the record data. +It replaces the authentication tag and TCP checksum with correct values. + +RX +-- + +Before a packet is DMAed to the host (but after NIC's embedded switching +and packet transformation functions) the device validates the Layer 4 +checksum and performs a 5-tuple lookup to find any TLS connection the packet +may belong to (technically a 4-tuple +lookup is sufficient - IP addresses and TCP port numbers, as the protocol +is always TCP). If connection is matched device confirms if the TCP sequence +number is the expected one and proceeds to TLS handling (record delineation, +decryption, authentication for each record in the packet). The device leaves +the record framing unmodified, the stack takes care of record decapsulation. +Device indicates successful handling of TLS offload in the per-packet context +(descriptor) passed to the host. + +Upon reception of a TLS offloaded packet, the driver sets +the :c:member:`decrypted` mark in :c:type:`struct sk_buff <sk_buff>` +corresponding to the segment. Networking stack makes sure decrypted +and non-decrypted segments do not get coalesced (e.g. by GRO or socket layer) +and takes care of partial decryption. + +Resync handling +=============== + +In presence of packet drops or network packet reordering, the device may lose +synchronization with the TLS stream, and require a resync with the kernel's +TCP stack. + +Note that resync is only attempted for connections which were successfully +added to the device table and are in TLS_HW mode. For example, +if the table was full when cryptographic state was installed in the kernel, +such connection will never get offloaded. Therefore the resync request +does not carry any cryptographic connection state. + +TX +-- + +Segments transmitted from an offloaded socket can get out of sync +in similar ways to the receive side-retransmissions - local drops +are possible, though network reorders are not. There are currently +two mechanisms for dealing with out of order segments. + +Crypto state rebuilding +~~~~~~~~~~~~~~~~~~~~~~~ + +Whenever an out of order segment is transmitted the driver provides +the device with enough information to perform cryptographic operations. +This means most likely that the part of the record preceding the current +segment has to be passed to the device as part of the packet context, +together with its TCP sequence number and TLS record number. The device +can then initialize its crypto state, process and discard the preceding +data (to be able to insert the authentication tag) and move onto handling +the actual packet. + +In this mode depending on the implementation the driver can either ask +for a continuation with the crypto state and the new sequence number +(next expected segment is the one after the out of order one), or continue +with the previous stream state - assuming that the out of order segment +was just a retransmission. The former is simpler, and does not require +retransmission detection therefore it is the recommended method until +such time it is proven inefficient. + +Next record sync +~~~~~~~~~~~~~~~~ + +Whenever an out of order segment is detected the driver requests +that the ``ktls`` software fallback code encrypt it. If the segment's +sequence number is lower than expected the driver assumes retransmission +and doesn't change device state. If the segment is in the future, it +may imply a local drop, the driver asks the stack to sync the device +to the next record state and falls back to software. + +Resync request is indicated with: + +.. code-block:: c + + void tls_offload_tx_resync_request(struct sock *sk, u32 got_seq, u32 exp_seq) + +Until resync is complete driver should not access its expected TCP +sequence number (as it will be updated from a different context). +Following helper should be used to test if resync is complete: + +.. code-block:: c + + bool tls_offload_tx_resync_pending(struct sock *sk) + +Next time ``ktls`` pushes a record it will first send its TCP sequence number +and TLS record number to the driver. Stack will also make sure that +the new record will start on a segment boundary (like it does when +the connection is initially added). + +RX +-- + +A small amount of RX reorder events may not require a full resynchronization. +In particular the device should not lose synchronization +when record boundary can be recovered: + +.. kernel-figure:: tls-offload-reorder-good.svg + :alt: reorder of non-header segment + :align: center + + Reorder of non-header segment + +Green segments are successfully decrypted, blue ones are passed +as received on wire, red stripes mark start of new records. + +In above case segment 1 is received and decrypted successfully. +Segment 2 was dropped so 3 arrives out of order. The device knows +the next record starts inside 3, based on record length in segment 1. +Segment 3 is passed untouched, because due to lack of data from segment 2 +the remainder of the previous record inside segment 3 cannot be handled. +The device can, however, collect the authentication algorithm's state +and partial block from the new record in segment 3 and when 4 and 5 +arrive continue decryption. Finally when 2 arrives it's completely outside +of expected window of the device so it's passed as is without special +handling. ``ktls`` software fallback handles the decryption of record +spanning segments 1, 2 and 3. The device did not get out of sync, +even though two segments did not get decrypted. + +Kernel synchronization may be necessary if the lost segment contained +a record header and arrived after the next record header has already passed: + +.. kernel-figure:: tls-offload-reorder-bad.svg + :alt: reorder of header segment + :align: center + + Reorder of segment with a TLS header + +In this example segment 2 gets dropped, and it contains a record header. +Device can only detect that segment 4 also contains a TLS header +if it knows the length of the previous record from segment 2. In this case +the device will lose synchronization with the stream. + +Stream scan resynchronization +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +When the device gets out of sync and the stream reaches TCP sequence +numbers more than a max size record past the expected TCP sequence number, +the device starts scanning for a known header pattern. For example +for TLS 1.2 and TLS 1.3 subsequent bytes of value ``0x03 0x03`` occur +in the SSL/TLS version field of the header. Once pattern is matched +the device continues attempting parsing headers at expected locations +(based on the length fields at guessed locations). +Whenever the expected location does not contain a valid header the scan +is restarted. + +When the header is matched the device sends a confirmation request +to the kernel, asking if the guessed location is correct (if a TLS record +really starts there), and which record sequence number the given header had. +The kernel confirms the guessed location was correct and tells the device +the record sequence number. Meanwhile, the device had been parsing +and counting all records since the just-confirmed one, it adds the number +of records it had seen to the record number provided by the kernel. +At this point the device is in sync and can resume decryption at next +segment boundary. + +In a pathological case the device may latch onto a sequence of matching +headers and never hear back from the kernel (there is no negative +confirmation from the kernel). The implementation may choose to periodically +restart scan. Given how unlikely falsely-matching stream is, however, +periodic restart is not deemed necessary. + +Special care has to be taken if the confirmation request is passed +asynchronously to the packet stream and record may get processed +by the kernel before the confirmation request. + +Stack-driven resynchronization +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The driver may also request the stack to perform resynchronization +whenever it sees the records are no longer getting decrypted. +If the connection is configured in this mode the stack automatically +schedules resynchronization after it has received two completely encrypted +records. + +The stack waits for the socket to drain and informs the device about +the next expected record number and its TCP sequence number. If the +records continue to be received fully encrypted stack retries the +synchronization with an exponential back off (first after 2 encrypted +records, then after 4 records, after 8, after 16... up until every +128 records). + +Error handling +============== + +TX +-- + +Packets may be redirected or rerouted by the stack to a different +device than the selected TLS offload device. The stack will handle +such condition using the :c:func:`sk_validate_xmit_skb` helper +(TLS offload code installs :c:func:`tls_validate_xmit_skb` at this hook). +Offload maintains information about all records until the data is +fully acknowledged, so if skbs reach the wrong device they can be handled +by software fallback. + +Any device TLS offload handling error on the transmission side must result +in the packet being dropped. For example if a packet got out of order +due to a bug in the stack or the device, reached the device and can't +be encrypted such packet must be dropped. + +RX +-- + +If the device encounters any problems with TLS offload on the receive +side it should pass the packet to the host's networking stack as it was +received on the wire. + +For example authentication failure for any record in the segment should +result in passing the unmodified packet to the software fallback. This means +packets should not be modified "in place". Splitting segments to handle partial +decryption is not advised. In other words either all records in the packet +had been handled successfully and authenticated or the packet has to be passed +to the host's stack as it was on the wire (recovering original packet in the +driver if device provides precise error is sufficient). + +The Linux networking stack does not provide a way of reporting per-packet +decryption and authentication errors, packets with errors must simply not +have the :c:member:`decrypted` mark set. + +A packet should also not be handled by the TLS offload if it contains +incorrect checksums. + +Performance metrics +=================== + +TLS offload can be characterized by the following basic metrics: + + * max connection count + * connection installation rate + * connection installation latency + * total cryptographic performance + +Note that each TCP connection requires a TLS session in both directions, +the performance may be reported treating each direction separately. + +Max connection count +-------------------- + +The number of connections device can support can be exposed via +``devlink resource`` API. + +Total cryptographic performance +------------------------------- + +Offload performance may depend on segment and record size. + +Overload of the cryptographic subsystem of the device should not have +significant performance impact on non-offloaded streams. + +Statistics +========== + +Following minimum set of TLS-related statistics should be reported +by the driver: + + * ``rx_tls_decrypted`` - number of successfully decrypted TLS segments + * ``tx_tls_encrypted`` - number of in-order TLS segments passed to device + for encryption + * ``tx_tls_ooo`` - number of TX packets which were part of a TLS stream + but did not arrive in the expected order + * ``tx_tls_drop_no_sync_data`` - number of TX packets dropped because + they arrived out of order and associated record could not be found + +Notable corner cases, exceptions and additional requirements +============================================================ + +.. _5tuple_problems: + +5-tuple matching limitations +---------------------------- + +The device can only recognize received packets based on the 5-tuple +of the socket. Current ``ktls`` implementation will not offload sockets +routed through software interfaces such as those used for tunneling +or virtual networking. However, many packet transformations performed +by the networking stack (most notably any BPF logic) do not require +any intermediate software device, therefore a 5-tuple match may +consistently miss at the device level. In such cases the device +should still be able to perform TX offload (encryption) and should +fallback cleanly to software decryption (RX). + +Out of order +------------ + +Introducing extra processing in NICs should not cause packets to be +transmitted or received out of order, for example pure ACK packets +should not be reordered with respect to data segments. + +Ingress reorder +--------------- + +A device is permitted to perform packet reordering for consecutive +TCP segments (i.e. placing packets in the correct order) but any form +of additional buffering is disallowed. + +Coexistence with standard networking offload features +----------------------------------------------------- + +Offloaded ``ktls`` sockets should support standard TCP stack features +transparently. Enabling device TLS offload should not cause any difference +in packets as seen on the wire. + +Transport layer transparency +---------------------------- + +The device should not modify any packet headers for the purpose +of the simplifying TLS offload. + +The device should not depend on any packet headers beyond what is strictly +necessary for TLS offload. + +Segment drops +------------- + +Dropping packets is acceptable only in the event of catastrophic +system errors and should never be used as an error handling mechanism +in cases arising from normal operation. In other words, reliance +on TCP retransmissions to handle corner cases is not acceptable. + +TLS device features +------------------- + +Drivers should ignore the changes to TLS the device feature flags. +These flags will be acted upon accordingly by the core ``ktls`` code. +TLS device feature flags only control adding of new TLS connection +offloads, old connections will remain active after flags are cleared. + +Known bugs +========== + +skb_orphan() leaks clear text +----------------------------- + +Currently drivers depend on the :c:member:`sk` member of +:c:type:`struct sk_buff <sk_buff>` to identify segments requiring +encryption. Any operation which removes or does not preserve the socket +association such as :c:func:`skb_orphan` or :c:func:`skb_clone` +will cause the driver to miss the packets and lead to clear text leaks. + +Redirects leak clear text +------------------------- + +In the RX direction, if segment has already been decrypted by the device +and it gets redirected or mirrored - clear text will be transmitted out. |