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1.. SPDX-License-Identifier: GPL-2.02 3.. _inline_encryption:4 5=================6Inline Encryption7=================8 9Background10==========11 12Inline encryption hardware sits logically between memory and disk, and can13en/decrypt data as it goes in/out of the disk. For each I/O request, software14can control exactly how the inline encryption hardware will en/decrypt the data15in terms of key, algorithm, data unit size (the granularity of en/decryption),16and data unit number (a value that determines the initialization vector(s)).17 18Some inline encryption hardware accepts all encryption parameters including raw19keys directly in low-level I/O requests. However, most inline encryption20hardware instead has a fixed number of "keyslots" and requires that the key,21algorithm, and data unit size first be programmed into a keyslot. Each22low-level I/O request then just contains a keyslot index and data unit number.23 24Note that inline encryption hardware is very different from traditional crypto25accelerators, which are supported through the kernel crypto API. Traditional26crypto accelerators operate on memory regions, whereas inline encryption27hardware operates on I/O requests. Thus, inline encryption hardware needs to be28managed by the block layer, not the kernel crypto API.29 30Inline encryption hardware is also very different from "self-encrypting drives",31such as those based on the TCG Opal or ATA Security standards. Self-encrypting32drives don't provide fine-grained control of encryption and provide no way to33verify the correctness of the resulting ciphertext. Inline encryption hardware34provides fine-grained control of encryption, including the choice of key and35initialization vector for each sector, and can be tested for correctness.36 37Objective38=========39 40We want to support inline encryption in the kernel. To make testing easier, we41also want support for falling back to the kernel crypto API when actual inline42encryption hardware is absent. We also want inline encryption to work with43layered devices like device-mapper and loopback (i.e. we want to be able to use44the inline encryption hardware of the underlying devices if present, or else45fall back to crypto API en/decryption).46 47Constraints and notes48=====================49 50- We need a way for upper layers (e.g. filesystems) to specify an encryption51 context to use for en/decrypting a bio, and device drivers (e.g. UFSHCD) need52 to be able to use that encryption context when they process the request.53 Encryption contexts also introduce constraints on bio merging; the block layer54 needs to be aware of these constraints.55 56- Different inline encryption hardware has different supported algorithms,57 supported data unit sizes, maximum data unit numbers, etc. We call these58 properties the "crypto capabilities". We need a way for device drivers to59 advertise crypto capabilities to upper layers in a generic way.60 61- Inline encryption hardware usually (but not always) requires that keys be62 programmed into keyslots before being used. Since programming keyslots may be63 slow and there may not be very many keyslots, we shouldn't just program the64 key for every I/O request, but rather keep track of which keys are in the65 keyslots and reuse an already-programmed keyslot when possible.66 67- Upper layers typically define a specific end-of-life for crypto keys, e.g.68 when an encrypted directory is locked or when a crypto mapping is torn down.69 At these times, keys are wiped from memory. We must provide a way for upper70 layers to also evict keys from any keyslots they are present in.71 72- When possible, device-mapper devices must be able to pass through the inline73 encryption support of their underlying devices. However, it doesn't make74 sense for device-mapper devices to have keyslots themselves.75 76Basic design77============78 79We introduce ``struct blk_crypto_key`` to represent an inline encryption key and80how it will be used. This includes the actual bytes of the key; the size of the81key; the algorithm and data unit size the key will be used with; and the number82of bytes needed to represent the maximum data unit number the key will be used83with.84 85We introduce ``struct bio_crypt_ctx`` to represent an encryption context. It86contains a data unit number and a pointer to a blk_crypto_key. We add pointers87to a bio_crypt_ctx to ``struct bio`` and ``struct request``; this allows users88of the block layer (e.g. filesystems) to provide an encryption context when89creating a bio and have it be passed down the stack for processing by the block90layer and device drivers. Note that the encryption context doesn't explicitly91say whether to encrypt or decrypt, as that is implicit from the direction of the92bio; WRITE means encrypt, and READ means decrypt.93 94We also introduce ``struct blk_crypto_profile`` to contain all generic inline95encryption-related state for a particular inline encryption device. The96blk_crypto_profile serves as the way that drivers for inline encryption hardware97advertise their crypto capabilities and provide certain functions (e.g.,98functions to program and evict keys) to upper layers. Each device driver that99wants to support inline encryption will construct a blk_crypto_profile, then100associate it with the disk's request_queue.101 102The blk_crypto_profile also manages the hardware's keyslots, when applicable.103This happens in the block layer, so that users of the block layer can just104specify encryption contexts and don't need to know about keyslots at all, nor do105device drivers need to care about most details of keyslot management.106 107Specifically, for each keyslot, the block layer (via the blk_crypto_profile)108keeps track of which blk_crypto_key that keyslot contains (if any), and how many109in-flight I/O requests are using it. When the block layer creates a110``struct request`` for a bio that has an encryption context, it grabs a keyslot111that already contains the key if possible. Otherwise it waits for an idle112keyslot (a keyslot that isn't in-use by any I/O), then programs the key into the113least-recently-used idle keyslot using the function the device driver provided.114In both cases, the resulting keyslot is stored in the ``crypt_keyslot`` field of115the request, where it is then accessible to device drivers and is released after116the request completes.117 118``struct request`` also contains a pointer to the original bio_crypt_ctx.119Requests can be built from multiple bios, and the block layer must take the120encryption context into account when trying to merge bios and requests. For two121bios/requests to be merged, they must have compatible encryption contexts: both122unencrypted, or both encrypted with the same key and contiguous data unit123numbers. Only the encryption context for the first bio in a request is124retained, since the remaining bios have been verified to be merge-compatible125with the first bio.126 127To make it possible for inline encryption to work with request_queue based128layered devices, when a request is cloned, its encryption context is cloned as129well. When the cloned request is submitted, it is then processed as usual; this130includes getting a keyslot from the clone's target device if needed.131 132blk-crypto-fallback133===================134 135It is desirable for the inline encryption support of upper layers (e.g.136filesystems) to be testable without real inline encryption hardware, and137likewise for the block layer's keyslot management logic. It is also desirable138to allow upper layers to just always use inline encryption rather than have to139implement encryption in multiple ways.140 141Therefore, we also introduce *blk-crypto-fallback*, which is an implementation142of inline encryption using the kernel crypto API. blk-crypto-fallback is built143into the block layer, so it works on any block device without any special setup.144Essentially, when a bio with an encryption context is submitted to a145block_device that doesn't support that encryption context, the block layer will146handle en/decryption of the bio using blk-crypto-fallback.147 148For encryption, the data cannot be encrypted in-place, as callers usually rely149on it being unmodified. Instead, blk-crypto-fallback allocates bounce pages,150fills a new bio with those bounce pages, encrypts the data into those bounce151pages, and submits that "bounce" bio. When the bounce bio completes,152blk-crypto-fallback completes the original bio. If the original bio is too153large, multiple bounce bios may be required; see the code for details.154 155For decryption, blk-crypto-fallback "wraps" the bio's completion callback156(``bi_complete``) and private data (``bi_private``) with its own, unsets the157bio's encryption context, then submits the bio. If the read completes158successfully, blk-crypto-fallback restores the bio's original completion159callback and private data, then decrypts the bio's data in-place using the160kernel crypto API. Decryption happens from a workqueue, as it may sleep.161Afterwards, blk-crypto-fallback completes the bio.162 163In both cases, the bios that blk-crypto-fallback submits no longer have an164encryption context. Therefore, lower layers only see standard unencrypted I/O.165 166blk-crypto-fallback also defines its own blk_crypto_profile and has its own167"keyslots"; its keyslots contain ``struct crypto_skcipher`` objects. The reason168for this is twofold. First, it allows the keyslot management logic to be tested169without actual inline encryption hardware. Second, similar to actual inline170encryption hardware, the crypto API doesn't accept keys directly in requests but171rather requires that keys be set ahead of time, and setting keys can be172expensive; moreover, allocating a crypto_skcipher can't happen on the I/O path173at all due to the locks it takes. Therefore, the concept of keyslots still174makes sense for blk-crypto-fallback.175 176Note that regardless of whether real inline encryption hardware or177blk-crypto-fallback is used, the ciphertext written to disk (and hence the178on-disk format of data) will be the same (assuming that both the inline179encryption hardware's implementation and the kernel crypto API's implementation180of the algorithm being used adhere to spec and function correctly).181 182blk-crypto-fallback is optional and is controlled by the183``CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK`` kernel configuration option.184 185API presented to users of the block layer186=========================================187 188``blk_crypto_config_supported()`` allows users to check ahead of time whether189inline encryption with particular crypto settings will work on a particular190block_device -- either via hardware or via blk-crypto-fallback. This function191takes in a ``struct blk_crypto_config`` which is like blk_crypto_key, but omits192the actual bytes of the key and instead just contains the algorithm, data unit193size, etc. This function can be useful if blk-crypto-fallback is disabled.194 195``blk_crypto_init_key()`` allows users to initialize a blk_crypto_key.196 197Users must call ``blk_crypto_start_using_key()`` before actually starting to use198a blk_crypto_key on a block_device (even if ``blk_crypto_config_supported()``199was called earlier). This is needed to initialize blk-crypto-fallback if it200will be needed. This must not be called from the data path, as this may have to201allocate resources, which may deadlock in that case.202 203Next, to attach an encryption context to a bio, users should call204``bio_crypt_set_ctx()``. This function allocates a bio_crypt_ctx and attaches205it to a bio, given the blk_crypto_key and the data unit number that will be used206for en/decryption. Users don't need to worry about freeing the bio_crypt_ctx207later, as that happens automatically when the bio is freed or reset.208 209Finally, when done using inline encryption with a blk_crypto_key on a210block_device, users must call ``blk_crypto_evict_key()``. This ensures that211the key is evicted from all keyslots it may be programmed into and unlinked from212any kernel data structures it may be linked into.213 214In summary, for users of the block layer, the lifecycle of a blk_crypto_key is215as follows:216 2171. ``blk_crypto_config_supported()`` (optional)2182. ``blk_crypto_init_key()``2193. ``blk_crypto_start_using_key()``2204. ``bio_crypt_set_ctx()`` (potentially many times)2215. ``blk_crypto_evict_key()`` (after all I/O has completed)2226. Zeroize the blk_crypto_key (this has no dedicated function)223 224If a blk_crypto_key is being used on multiple block_devices, then225``blk_crypto_config_supported()`` (if used), ``blk_crypto_start_using_key()``,226and ``blk_crypto_evict_key()`` must be called on each block_device.227 228API presented to device drivers229===============================230 231A device driver that wants to support inline encryption must set up a232blk_crypto_profile in the request_queue of its device. To do this, it first233must call ``blk_crypto_profile_init()`` (or its resource-managed variant234``devm_blk_crypto_profile_init()``), providing the number of keyslots.235 236Next, it must advertise its crypto capabilities by setting fields in the237blk_crypto_profile, e.g. ``modes_supported`` and ``max_dun_bytes_supported``.238 239It then must set function pointers in the ``ll_ops`` field of the240blk_crypto_profile to tell upper layers how to control the inline encryption241hardware, e.g. how to program and evict keyslots. Most drivers will need to242implement ``keyslot_program`` and ``keyslot_evict``. For details, see the243comments for ``struct blk_crypto_ll_ops``.244 245Once the driver registers a blk_crypto_profile with a request_queue, I/O246requests the driver receives via that queue may have an encryption context. All247encryption contexts will be compatible with the crypto capabilities declared in248the blk_crypto_profile, so drivers don't need to worry about handling249unsupported requests. Also, if a nonzero number of keyslots was declared in the250blk_crypto_profile, then all I/O requests that have an encryption context will251also have a keyslot which was already programmed with the appropriate key.252 253If the driver implements runtime suspend and its blk_crypto_ll_ops don't work254while the device is runtime-suspended, then the driver must also set the ``dev``255field of the blk_crypto_profile to point to the ``struct device`` that will be256resumed before any of the low-level operations are called.257 258If there are situations where the inline encryption hardware loses the contents259of its keyslots, e.g. device resets, the driver must handle reprogramming the260keyslots. To do this, the driver may call ``blk_crypto_reprogram_all_keys()``.261 262Finally, if the driver used ``blk_crypto_profile_init()`` instead of263``devm_blk_crypto_profile_init()``, then it is responsible for calling264``blk_crypto_profile_destroy()`` when the crypto profile is no longer needed.265 266Layered Devices267===============268 269Request queue based layered devices like dm-rq that wish to support inline270encryption need to create their own blk_crypto_profile for their request_queue,271and expose whatever functionality they choose. When a layered device wants to272pass a clone of that request to another request_queue, blk-crypto will273initialize and prepare the clone as necessary.274 275Interaction between inline encryption and blk integrity276=======================================================277 278At the time of this patch, there is no real hardware that supports both these279features. However, these features do interact with each other, and it's not280completely trivial to make them both work together properly. In particular,281when a WRITE bio wants to use inline encryption on a device that supports both282features, the bio will have an encryption context specified, after which283its integrity information is calculated (using the plaintext data, since284the encryption will happen while data is being written), and the data and285integrity info is sent to the device. Obviously, the integrity info must be286verified before the data is encrypted. After the data is encrypted, the device287must not store the integrity info that it received with the plaintext data288since that might reveal information about the plaintext data. As such, it must289re-generate the integrity info from the ciphertext data and store that on disk290instead. Another issue with storing the integrity info of the plaintext data is291that it changes the on disk format depending on whether hardware inline292encryption support is present or the kernel crypto API fallback is used (since293if the fallback is used, the device will receive the integrity info of the294ciphertext, not that of the plaintext).295 296Because there isn't any real hardware yet, it seems prudent to assume that297hardware implementations might not implement both features together correctly,298and disallow the combination for now. Whenever a device supports integrity, the299kernel will pretend that the device does not support hardware inline encryption300(by setting the blk_crypto_profile in the request_queue of the device to NULL).301When the crypto API fallback is enabled, this means that all bios with and302encryption context will use the fallback, and IO will complete as usual. When303the fallback is disabled, a bio with an encryption context will be failed.304