--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Inline Encryption
+=================
+
+Background
+==========
+
+Inline encryption hardware sits logically between memory and the disk, and can
+en/decrypt data as it goes in/out of the disk. Inline encryption hardware has a
+fixed number of "keyslots" - slots into which encryption contexts (i.e. the
+encryption key, encryption algorithm, data unit size) can be programmed by the
+kernel at any time. Each request sent to the disk can be tagged with the index
+of a keyslot (and also a data unit number to act as an encryption tweak), and
+the inline encryption hardware will en/decrypt the data in the request with the
+encryption context programmed into that keyslot. This is very different from
+full disk encryption solutions like self encrypting drives/TCG OPAL/ATA
+Security standards, since with inline encryption, any block on disk could be
+encrypted with any encryption context the kernel chooses.
+
+
+Objective
+=========
+
+We want to support inline encryption (IE) in the kernel.
+To allow for testing, we also want a crypto API fallback when actual
+IE hardware is absent. We also want IE to work with layered devices
+like dm and loopback (i.e. we want to be able to use the IE hardware
+of the underlying devices if present, or else fall back to crypto API
+en/decryption).
+
+
+Constraints and notes
+=====================
+
+- IE hardware has a limited number of "keyslots" that can be programmed
+ with an encryption context (key, algorithm, data unit size, etc.) at any time.
+ One can specify a keyslot in a data request made to the device, and the
+ device will en/decrypt the data using the encryption context programmed into
+ that specified keyslot. When possible, we want to make multiple requests with
+ the same encryption context share the same keyslot.
+
+- We need a way for upper layers like filesystems to specify an encryption
+ context to use for en/decrypting a struct bio, and a device driver (like UFS)
+ needs to be able to use that encryption context when it processes the bio.
+
+- We need a way for device drivers to expose their inline encryption
+ capabilities in a unified way to the upper layers.
+
+
+Design
+======
+
+We add a :c:type:`struct bio_crypt_ctx` to :c:type:`struct bio` that can
+represent an encryption context, because we need to be able to pass this
+encryption context from the upper layers (like the fs layer) to the
+device driver to act upon.
+
+While IE hardware works on the notion of keyslots, the FS layer has no
+knowledge of keyslots - it simply wants to specify an encryption context to
+use while en/decrypting a bio.
+
+We introduce a keyslot manager (KSM) that handles the translation from
+encryption contexts specified by the FS to keyslots on the IE hardware.
+This KSM also serves as the way IE hardware can expose its capabilities to
+upper layers. The generic mode of operation is: each device driver that wants
+to support IE will construct a KSM and set it up in its struct request_queue.
+Upper layers that want to use IE on this device can then use this KSM in
+the device's struct request_queue to translate an encryption context into
+a keyslot. The presence of the KSM in the request queue shall be used to mean
+that the device supports IE.
+
+The KSM uses refcounts to track which keyslots are idle (either they have no
+encryption context programmed, or there are no in-flight struct bios
+referencing that keyslot). When a new encryption context needs a keyslot, it
+tries to find a keyslot that has already been programmed with the same
+encryption context, and if there is no such keyslot, it evicts the least
+recently used idle keyslot and programs the new encryption context into that
+one. If no idle keyslots are available, then the caller will sleep until there
+is at least one.
+
+
+blk-mq changes, other block layer changes and blk-crypto-fallback
+=================================================================
+
+We add a pointer to a ``bi_crypt_context`` and ``keyslot`` to
+:c:type:`struct request`. These will be referred to as the ``crypto fields``
+for the request. This ``keyslot`` is the keyslot into which the
+``bi_crypt_context`` has been programmed in the KSM of the ``request_queue``
+that this request is being sent to.
+
+We introduce ``block/blk-crypto-fallback.c``, which allows upper layers to remain
+blissfully unaware of whether or not real inline encryption hardware is present
+underneath. When a bio is submitted with a target ``request_queue`` that doesn't
+support the encryption context specified with the bio, the block layer will
+en/decrypt the bio with the blk-crypto-fallback.
+
+If the bio is a ``WRITE`` bio, a bounce bio is allocated, and the data in the bio
+is encrypted stored in the bounce bio - blk-mq will then proceed to process the
+bounce bio as if it were not encrypted at all (except when blk-integrity is
+concerned). ``blk-crypto-fallback`` sets the bounce bio's ``bi_end_io`` to an
+internal function that cleans up the bounce bio and ends the original bio.
+
+If the bio is a ``READ`` bio, the bio's ``bi_end_io`` (and also ``bi_private``)
+is saved and overwritten by ``blk-crypto-fallback`` to
+``bio_crypto_fallback_decrypt_bio``. The bio's ``bi_crypt_context`` is also
+overwritten with ``NULL``, so that to the rest of the stack, the bio looks
+as if it was a regular bio that never had an encryption context specified.
+``bio_crypto_fallback_decrypt_bio`` will decrypt the bio, restore the original
+``bi_end_io`` (and also ``bi_private``) and end the bio again.
+
+Regardless of whether real inline encryption hardware is used or the
+blk-crypto-fallback is used, the ciphertext written to disk (and hence the
+on-disk format of data) will be the same (assuming the hardware's implementation
+of the algorithm being used adheres to spec and functions correctly).
+
+If a ``request queue``'s inline encryption hardware claimed to support the
+encryption context specified with a bio, then it will not be handled by the
+``blk-crypto-fallback``. We will eventually reach a point in blk-mq when a
+:c:type:`struct request` needs to be allocated for that bio. At that point,
+blk-mq tries to program the encryption context into the ``request_queue``'s
+keyslot_manager, and obtain a keyslot, which it stores in its newly added
+``keyslot`` field. This keyslot is released when the request is completed.
+
+When the first bio is added to a request, ``blk_crypto_rq_bio_prep`` is called,
+which sets the request's ``crypt_ctx`` to a copy of the bio's
+``bi_crypt_context``. bio_crypt_do_front_merge is called whenever a subsequent
+bio is merged to the front of the request, which updates the ``crypt_ctx`` of
+the request so that it matches the newly merged bio's ``bi_crypt_context``. In particular, the request keeps a copy of the ``bi_crypt_context`` of the first
+bio in its bio-list (blk-mq needs to be careful to maintain this invariant
+during bio and request merges).
+
+To make it possible for inline encryption to work with request queue based
+layered devices, when a request is cloned, its ``crypto fields`` are cloned as
+well. When the cloned request is submitted, blk-mq programs the
+``bi_crypt_context`` of the request into the clone's request_queue's keyslot
+manager, and stores the returned keyslot in the clone's ``keyslot``.
+
+
+API presented to users of the block layer
+=========================================
+
+``struct blk_crypto_key`` represents a crypto key (the raw key, size of the
+key, the crypto algorithm to use, the data unit size to use, and the number of
+bytes required to represent data unit numbers that will be specified with the
+``bi_crypt_context``).
+
+``blk_crypto_init_key`` allows upper layers to initialize such a
+``blk_crypto_key``.
+
+``bio_crypt_set_ctx`` should be called on any bio that a user of
+the block layer wants en/decrypted via inline encryption (or the
+blk-crypto-fallback, if hardware support isn't available for the desired
+crypto configuration). This function takes the ``blk_crypto_key`` and the
+data unit number (DUN) to use when en/decrypting the bio.
+
+``blk_crypto_config_supported`` allows upper layers to query whether or not the
+an encryption context passed to request queue can be handled by blk-crypto
+(either by real inline encryption hardware, or by the blk-crypto-fallback).
+This is useful e.g. when blk-crypto-fallback is disabled, and the upper layer
+wants to use an algorithm that may not supported by hardware - this function
+lets the upper layer know ahead of time that the algorithm isn't supported,
+and the upper layer can fallback to something else if appropriate.
+
+``blk_crypto_start_using_key`` - Upper layers must call this function on
+``blk_crypto_key`` and a ``request_queue`` before using the key with any bio
+headed for that ``request_queue``. This function ensures that either the
+hardware supports the key's crypto settings, or the crypto API fallback has
+transforms for the needed mode allocated and ready to go. Note that this
+function may allocate an ``skcipher``, and must not be called from the data
+path, since allocating ``skciphers`` from the data path can deadlock.
+
+``blk_crypto_evict_key`` *must* be called by upper layers before a
+``blk_crypto_key`` is freed. Further, it *must* only be called only once
+there are no more in-flight requests that use that ``blk_crypto_key``.
+``blk_crypto_evict_key`` will ensure that a key is removed from any keyslots in
+inline encryption hardware that the key might have been programmed into (or the blk-crypto-fallback).
+
+API presented to device drivers
+===============================
+
+A :c:type:``struct blk_keyslot_manager`` should be set up by device drivers in
+the ``request_queue`` of the device. The device driver needs to call
+``blk_ksm_init`` on the ``blk_keyslot_manager``, which specifying the number of
+keyslots supported by the hardware.
+
+The device driver also needs to tell the KSM how to actually manipulate the
+IE hardware in the device to do things like programming the crypto key into
+the IE hardware into a particular keyslot. All this is achieved through the
+:c:type:`struct blk_ksm_ll_ops` field in the KSM that the device driver
+must fill up after initing the ``blk_keyslot_manager``.
+
+The KSM also handles runtime power management for the device when applicable
+(e.g. when it wants to program a crypto key into the IE hardware, the device
+must be runtime powered on) - so the device driver must also set the ``dev``
+field in the ksm to point to the `struct device` for the KSM to use for runtime
+power management.
+
+``blk_ksm_reprogram_all_keys`` can be called by device drivers if the device
+needs each and every of its keyslots to be reprogrammed with the key it
+"should have" at the point in time when the function is called. This is useful
+e.g. if a device loses all its keys on runtime power down/up.
+
+``blk_ksm_destroy`` should be called to free up all resources used by a keyslot
+manager upon ``blk_ksm_init``, once the ``blk_keyslot_manager`` is no longer
+needed.
+
+
+Layered Devices
+===============
+
+Request queue based layered devices like dm-rq that wish to support IE need to
+create their own keyslot manager for their request queue, and expose whatever
+functionality they choose. When a layered device wants to pass a clone of that
+request to another ``request_queue``, blk-crypto will initialize and prepare the
+clone as necessary - see ``blk_crypto_insert_cloned_request`` in
+``blk-crypto.c``.
+
+
+Future Optimizations for layered devices
+========================================
+
+Creating a keyslot manager for a layered device uses up memory for each
+keyslot, and in general, a layered device merely passes the request on to a
+"child" device, so the keyslots in the layered device itself are completely
+unused, and don't need any refcounting or keyslot programming. We can instead
+define a new type of KSM; the "passthrough KSM", that layered devices can use
+to advertise an unlimited number of keyslots, and support for any encryption
+algorithms they choose, while not actually using any memory for each keyslot.
+Another use case for the "passthrough KSM" is for IE devices that do not have a
+limited number of keyslots.
+
+
+Interaction between inline encryption and blk integrity
+=======================================================
+
+At the time of this patch, there is no real hardware that supports both these
+features. However, these features do interact with each other, and it's not
+completely trivial to make them both work together properly. In particular,
+when a WRITE bio wants to use inline encryption on a device that supports both
+features, the bio will have an encryption context specified, after which
+its integrity information is calculated (using the plaintext data, since
+the encryption will happen while data is being written), and the data and
+integrity info is sent to the device. Obviously, the integrity info must be
+verified before the data is encrypted. After the data is encrypted, the device
+must not store the integrity info that it received with the plaintext data
+since that might reveal information about the plaintext data. As such, it must
+re-generate the integrity info from the ciphertext data and store that on disk
+instead. Another issue with storing the integrity info of the plaintext data is
+that it changes the on disk format depending on whether hardware inline
+encryption support is present or the kernel crypto API fallback is used (since
+if the fallback is used, the device will receive the integrity info of the
+ciphertext, not that of the plaintext).
+
+Because there isn't any real hardware yet, it seems prudent to assume that
+hardware implementations might not implement both features together correctly,
+and disallow the combination for now. Whenever a device supports integrity, the
+kernel will pretend that the device does not support hardware inline encryption
+(by essentially setting the keyslot manager in the request_queue of the device
+to NULL). When the crypto API fallback is enabled, this means that all bios with
+and encryption context will use the fallback, and IO will complete as usual.
+When the fallback is disabled, a bio with an encryption context will be failed.
projects / platform / kernel / linux-starfive.git / commitdiff