[PATCH net-next v7 01/19] psp: add documentation

[Daniel Zahka <daniel.zahka@gmail.com>]

From: Jakub Kicinski <kuba@kernel.org>

Add documentation of things which belong in the docs rather
than commit messages.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
---

Notes:
    v3:
    - reword driver requirement about double rotating keys when the device
      supports requesting arbitrary spi key pairs.
    v2:
    - add note about MITM deletion attack, and expectation from userspace
    - add information about accepting clear text ACKs, RSTs, and FINs to
      `Securing Connections` section.
    v1:
    - https://lore.kernel.org/netdev/20240510030435.120935-2-kuba@kernel.org/

 Documentation/networking/index.rst |   1 +
 Documentation/networking/psp.rst   | 183 +++++++++++++++++++++++++++++
 2 files changed, 184 insertions(+)
 create mode 100644 Documentation/networking/psp.rst

diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index b7a4969e9bc9..c775cababc8c 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -101,6 +101,7 @@ Contents:
    ppp_generic
    proc_net_tcp
    pse-pd/index
+   psp
    radiotap-headers
    rds
    regulatory
diff --git a/Documentation/networking/psp.rst b/Documentation/networking/psp.rst
new file mode 100644
index 000000000000..4ac09e64e95a
--- /dev/null
+++ b/Documentation/networking/psp.rst
@@ -0,0 +1,183 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+=====================
+PSP Security Protocol
+=====================
+
+Protocol
+========
+
+PSP Security Protocol (PSP) was defined at Google and published in:
+
+https://raw.githubusercontent.com/google/psp/main/doc/PSP_Arch_Spec.pdf
+
+This section briefly covers protocol aspects crucial for understanding
+the kernel API. Refer to the protocol specification for further details.
+
+Note that the kernel implementation and documentation uses the term
+"device key" in place of "master key", it is both less confusing
+to an average developer and is less likely to run afoul any naming
+guidelines.
+
+Derived Rx keys
+---------------
+
+PSP borrows some terms and mechanisms from IPsec. PSP was designed
+with HW offloads in mind. The key feature of PSP is that Rx keys for every
+connection do not have to be stored by the receiver but can be derived
+from device key and information present in packet headers.
+This makes it possible to implement receivers which require a constant
+amount of memory regardless of the number of connections (``O(1)`` scaling).
+
+Tx keys have to be stored like with any other protocol, but Tx is much
+less latency sensitive than Rx, and delays in fetching keys from slow
+memory is less likely to cause packet drops. Preferably, the Tx keys
+should be provided with the packet (e.g. as part of the descriptors).
+
+Key rotation
+------------
+
+The device key known only to the receiver is fundamental to the design.
+Per specification this state cannot be directly accessible (it must be
+impossible to read it out of the hardware of the receiver NIC).
+Moreover, it has to be "rotated" periodically (usually daily). Rotation
+means that new device key gets generated (by a random number generator
+of the device), and used for all new connections. To avoid disrupting
+old connections the old device key remains in the NIC. A phase bit
+carried in the packet headers indicates which generation of device key
+the packet has been encrypted with.
+
+User facing API
+===============
+
+PSP is designed primarily for hardware offloads. There is currently
+no software fallback for systems which do not have PSP capable NICs.
+There is also no standard (or otherwise defined) way of establishing
+a PSP-secured connection or exchanging the symmetric keys.
+
+The expectation is that higher layer protocols will take care of
+protocol and key negotiation. For example one may use TLS key exchange,
+announce the PSP capability, and switch to PSP if both endpoints
+are PSP-capable.
+
+All configuration of PSP is performed via the PSP netlink family.
+
+Device discovery
+----------------
+
+The PSP netlink family defines operations to retrieve information
+about the PSP devices available on the system, configure them and
+access PSP related statistics.
+
+Securing a connection
+---------------------
+
+PSP encryption is currently only supported for TCP connections.
+Rx and Tx keys are allocated separately. First the ``rx-assoc``
+Netlink command needs to be issued, specifying a target TCP socket.
+Kernel will allocate a new PSP Rx key from the NIC and associate it
+with given socket. At this stage socket will accept both PSP-secured
+and plain text TCP packets.
+
+Tx keys are installed using the ``tx-assoc`` Netlink command.
+Once the Tx keys are installed, all data read from the socket will
+be PSP-secured. In other words act of installing Tx keys has a secondary
+effect on the Rx direction.
+
+There is an intermediate period after ``tx-assoc`` successfully
+returns and before the TCP socket encounters it's first PSP
+authenticated packet, where the TCP stack will allow certain nondata
+packets, i.e. ACKs, FINs, and RSTs, to enter TCP receive processing
+even if not PSP authenticated. During the ``tx-assoc`` call, the TCP
+socket's ``rcv_nxt`` field is recorded. At this point, ACKs and RSTs
+will be accepted with any sequence number, while FINs will only be
+accepted at the latched value of ``rcv_nxt``. Once the TCP stack
+encounters the first TCP packet containing PSP authenticated data, the
+other end of the connection must have executed the ``tx-assoc``
+command, so any TCP packet, including those without data, will be
+dropped before receive processing if it is not successfully
+authenticated. This is summarized in the table below. The
+aforementioned state of rejecting all non-PSP packets is labeled "PSP
+Full".
+
++----------------+------------+------------+-------------+-------------+
+| Event          | Normal TCP | Rx PSP     | Tx PSP      | PSP Full    |
++================+============+============+=============+=============+
+| Rx plain       | accept     | accept     | drop        | drop        |
+| (data)         |            |            |             |             |
++----------------+------------+------------+-------------+-------------+
+| Rx plain       | accept     | accept     | accept      | drop        |
+| (ACK|FIN|RST)  |            |            |             |             |
++----------------+------------+------------+-------------+-------------+
+| Rx PSP (good)  | drop       | accept     | accept      | accept      |
++----------------+------------+------------+-------------+-------------+
+| Rx PSP (bad    | drop       | drop       | drop        | drop        |
+| crypt, !=SPI)  |            |            |             |             |
++----------------+------------+------------+-------------+-------------+
+| Tx             | plain text | plain text | encrypted   | encrypted   |
+|                |            |            | (excl. rtx) | (excl. rtx) |
++----------------+------------+------------+-------------+-------------+
+
+To ensure that any data read from the socket after the ``tx-assoc``
+call returns success has been authenticated, the kernel will scan the
+receive and ofo queues of the socket at ``tx-assoc`` time. If any
+enqueued packet was received in clear text, the Tx association will
+fail, and the application should retry installing the Tx key after
+draining the socket (this should not be necessary if both endpoints
+are well behaved).
+
+Because TCP sequence numbers are not integrity protected prior to
+upgrading to PSP, it is possible that a MITM could offset sequence
+numbers in a way that deletes a prefix of the PSP protected part of
+the TCP stream. If userspace cares to mitigate this type of attack, a
+special "start of PSP" message should be exchanged after ``tx-assoc``.
+
+Rotation notifications
+----------------------
+
+The rotations of device key happen asynchronously and are usually
+performed by management daemons, not under application control.
+The PSP netlink family will generate a notification whenever keys
+are rotated. The applications are expected to re-establish connections
+before keys are rotated again.
+
+Kernel implementation
+=====================
+
+Driver notes
+------------
+
+Drivers are expected to start with no PSP enabled (``psp-versions-ena``
+in ``dev-get`` set to ``0``) whenever possible. The user space should
+not depend on this behavior, as future extension may necessitate creation
+of devices with PSP already enabled, nonetheless drivers should not enable
+PSP by default. Enabling PSP should be the responsibility of the system
+component which also takes care of key rotation.
+
+Note that ``psp-versions-ena`` is expected to be used only for enabling
+receive processing. The device is not expected to reject transmit requests
+after ``psp-versions-ena`` has been disabled. User may also disable
+``psp-versions-ena`` while there are active associations, which will
+break all PSP Rx processing.
+
+Drivers are expected to ensure that a device key is usable and secure
+upon init, without explicit key rotation by the user space. It must be
+possible to allocate working keys, and that no duplicate keys must be
+generated. If the device allows the host to request the key for an
+arbitrary SPI - driver should discard both device keys (rotate the
+device key twice), to avoid potentially using a SPI+key which previous
+OS instance already had access to.
+
+Drivers must use ``psp_skb_get_assoc_rcu()`` to check if PSP Tx offload
+was requested for given skb. On Rx drivers should allocate and populate
+the ``SKB_EXT_PSP`` skb extension, and set the skb->decrypted bit to 1.
+
+Kernel implementation notes
+---------------------------
+
+PSP implementation follows the TLS offload more closely than the IPsec
+offload, with per-socket state, and the use of skb->decrypted to prevent
+clear text leaks.
+
+PSP device is separate from netdev, to make it possible to "delegate"
+PSP offload capabilities to software devices (e.g. ``veth``).
-- 
2.47.3