1 .. SPDX-License-Identifier: GPL-2.0 2 3 .. _inline_encryption: 4 5 ================= 6 Inline Encryption 7 ================= 8 9 Background 10 ========== 11 12 Inline encryption hardware sits logically between memory and disk, and can 13 en/decrypt data as it goes in/out of the disk. For each I/O request, software 14 can control exactly how the inline encryption hardware will en/decrypt the data 15 in terms of key, algorithm, data unit size (the granularity of en/decryption), 16 and data unit number (a value that determines the initialization vector(s)). 17 18 Some inline encryption hardware accepts all encryption parameters including raw 19 keys directly in low-level I/O requests. However, most inline encryption 20 hardware instead has a fixed number of "keyslots" and requires that the key, 21 algorithm, and data unit size first be programmed into a keyslot. Each 22 low-level I/O request then just contains a keyslot index and data unit number. 23 24 Note that inline encryption hardware is very different from traditional crypto 25 accelerators, which are supported through the kernel crypto API. Traditional 26 crypto accelerators operate on memory regions, whereas inline encryption 27 hardware operates on I/O requests. Thus, inline encryption hardware needs to be 28 managed by the block layer, not the kernel crypto API. 29 30 Inline encryption hardware is also very different from "self-encrypting drives", 31 such as those based on the TCG Opal or ATA Security standards. Self-encrypting 32 drives don't provide fine-grained control of encryption and provide no way to 33 verify the correctness of the resulting ciphertext. Inline encryption hardware 34 provides fine-grained control of encryption, including the choice of key and 35 initialization vector for each sector, and can be tested for correctness. 36 37 Objective 38 ========= 39 40 We want to support inline encryption in the kernel. To make testing easier, we 41 also want support for falling back to the kernel crypto API when actual inline 42 encryption hardware is absent. We also want inline encryption to work with 43 layered devices like device-mapper and loopback (i.e. we want to be able to use 44 the inline encryption hardware of the underlying devices if present, or else 45 fall back to crypto API en/decryption). 46 47 Constraints and notes 48 ===================== 49 50 - We need a way for upper layers (e.g. filesystems) to specify an encryption 51 context to use for en/decrypting a bio, and device drivers (e.g. UFSHCD) need 52 to be able to use that encryption context when they process the request. 53 Encryption contexts also introduce constraints on bio merging; the block layer 54 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 these 58 properties the "crypto capabilities". We need a way for device drivers to 59 advertise crypto capabilities to upper layers in a generic way. 60 61 - Inline encryption hardware usually (but not always) requires that keys be 62 programmed into keyslots before being used. Since programming keyslots may be 63 slow and there may not be very many keyslots, we shouldn't just program the 64 key for every I/O request, but rather keep track of which keys are in the 65 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 upper 70 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 inline 73 encryption support of their underlying devices. However, it doesn't make 74 sense for device-mapper devices to have keyslots themselves. 75 76 Basic design 77 ============ 78 79 We introduce ``struct blk_crypto_key`` to represent an inline encryption key and 80 how it will be used. This includes the actual bytes of the key; the size of the 81 key; the algorithm and data unit size the key will be used with; and the number 82 of bytes needed to represent the maximum data unit number the key will be used 83 with. 84 85 We introduce ``struct bio_crypt_ctx`` to represent an encryption context. It 86 contains a data unit number and a pointer to a blk_crypto_key. We add pointers 87 to a bio_crypt_ctx to ``struct bio`` and ``struct request``; this allows users 88 of the block layer (e.g. filesystems) to provide an encryption context when 89 creating a bio and have it be passed down the stack for processing by the block 90 layer and device drivers. Note that the encryption context doesn't explicitly 91 say whether to encrypt or decrypt, as that is implicit from the direction of the 92 bio; WRITE means encrypt, and READ means decrypt. 93 94 We also introduce ``struct blk_crypto_profile`` to contain all generic inline 95 encryption-related state for a particular inline encryption device. The 96 blk_crypto_profile serves as the way that drivers for inline encryption hardware 97 advertise their crypto capabilities and provide certain functions (e.g., 98 functions to program and evict keys) to upper layers. Each device driver that 99 wants to support inline encryption will construct a blk_crypto_profile, then 100 associate it with the disk's request_queue. 101 102 The blk_crypto_profile also manages the hardware's keyslots, when applicable. 103 This happens in the block layer, so that users of the block layer can just 104 specify encryption contexts and don't need to know about keyslots at all, nor do 105 device drivers need to care about most details of keyslot management. 106 107 Specifically, for each keyslot, the block layer (via the blk_crypto_profile) 108 keeps track of which blk_crypto_key that keyslot contains (if any), and how many 109 in-flight I/O requests are using it. When the block layer creates a 110 ``struct request`` for a bio that has an encryption context, it grabs a keyslot 111 that already contains the key if possible. Otherwise it waits for an idle 112 keyslot (a keyslot that isn't in-use by any I/O), then programs the key into the 113 least-recently-used idle keyslot using the function the device driver provided. 114 In both cases, the resulting keyslot is stored in the ``crypt_keyslot`` field of 115 the request, where it is then accessible to device drivers and is released after 116 the request completes. 117 118 ``struct request`` also contains a pointer to the original bio_crypt_ctx. 119 Requests can be built from multiple bios, and the block layer must take the 120 encryption context into account when trying to merge bios and requests. For two 121 bios/requests to be merged, they must have compatible encryption contexts: both 122 unencrypted, or both encrypted with the same key and contiguous data unit 123 numbers. Only the encryption context for the first bio in a request is 124 retained, since the remaining bios have been verified to be merge-compatible 125 with the first bio. 126 127 To make it possible for inline encryption to work with request_queue based 128 layered devices, when a request is cloned, its encryption context is cloned as 129 well. When the cloned request is submitted, it is then processed as usual; this 130 includes getting a keyslot from the clone's target device if needed. 131 132 blk-crypto-fallback 133 =================== 134 135 It is desirable for the inline encryption support of upper layers (e.g. 136 filesystems) to be testable without real inline encryption hardware, and 137 likewise for the block layer's keyslot management logic. It is also desirable 138 to allow upper layers to just always use inline encryption rather than have to 139 implement encryption in multiple ways. 140 141 Therefore, we also introduce *blk-crypto-fallback*, which is an implementation 142 of inline encryption using the kernel crypto API. blk-crypto-fallback is built 143 into the block layer, so it works on any block device without any special setup. 144 Essentially, when a bio with an encryption context is submitted to a 145 block_device that doesn't support that encryption context, the block layer will 146 handle en/decryption of the bio using blk-crypto-fallback. 147 148 For encryption, the data cannot be encrypted in-place, as callers usually rely 149 on it being unmodified. Instead, blk-crypto-fallback allocates bounce pages, 150 fills a new bio with those bounce pages, encrypts the data into those bounce 151 pages, and submits that "bounce" bio. When the bounce bio completes, 152 blk-crypto-fallback completes the original bio. If the original bio is too 153 large, multiple bounce bios may be required; see the code for details. 154 155 For decryption, blk-crypto-fallback "wraps" the bio's completion callback 156 (``bi_complete``) and private data (``bi_private``) with its own, unsets the 157 bio's encryption context, then submits the bio. If the read completes 158 successfully, blk-crypto-fallback restores the bio's original completion 159 callback and private data, then decrypts the bio's data in-place using the 160 kernel crypto API. Decryption happens from a workqueue, as it may sleep. 161 Afterwards, blk-crypto-fallback completes the bio. 162 163 In both cases, the bios that blk-crypto-fallback submits no longer have an 164 encryption context. Therefore, lower layers only see standard unencrypted I/O. 165 166 blk-crypto-fallback also defines its own blk_crypto_profile and has its own 167 "keyslots"; its keyslots contain ``struct crypto_skcipher`` objects. The reason 168 for this is twofold. First, it allows the keyslot management logic to be tested 169 without actual inline encryption hardware. Second, similar to actual inline 170 encryption hardware, the crypto API doesn't accept keys directly in requests but 171 rather requires that keys be set ahead of time, and setting keys can be 172 expensive; moreover, allocating a crypto_skcipher can't happen on the I/O path 173 at all due to the locks it takes. Therefore, the concept of keyslots still 174 makes sense for blk-crypto-fallback. 175 176 Note that regardless of whether real inline encryption hardware or 177 blk-crypto-fallback is used, the ciphertext written to disk (and hence the 178 on-disk format of data) will be the same (assuming that both the inline 179 encryption hardware's implementation and the kernel crypto API's implementation 180 of the algorithm being used adhere to spec and function correctly). 181 182 blk-crypto-fallback is optional and is controlled by the 183 ``CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK`` kernel configuration option. 184 185 API presented to users of the block layer 186 ========================================= 187 188 ``blk_crypto_config_supported()`` allows users to check ahead of time whether 189 inline encryption with particular crypto settings will work on a particular 190 block_device -- either via hardware or via blk-crypto-fallback. This function 191 takes in a ``struct blk_crypto_config`` which is like blk_crypto_key, but omits 192 the actual bytes of the key and instead just contains the algorithm, data unit 193 size, 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. 196 197 Users must call ``blk_crypto_start_using_key()`` before actually starting to use 198 a blk_crypto_key on a block_device (even if ``blk_crypto_config_supported()`` 199 was called earlier). This is needed to initialize blk-crypto-fallback if it 200 will be needed. This must not be called from the data path, as this may have to 201 allocate resources, which may deadlock in that case. 202 203 Next, to attach an encryption context to a bio, users should call 204 ``bio_crypt_set_ctx()``. This function allocates a bio_crypt_ctx and attaches 205 it to a bio, given the blk_crypto_key and the data unit number that will be used 206 for en/decryption. Users don't need to worry about freeing the bio_crypt_ctx 207 later, as that happens automatically when the bio is freed or reset. 208 209 Finally, when done using inline encryption with a blk_crypto_key on a 210 block_device, users must call ``blk_crypto_evict_key()``. This ensures that 211 the key is evicted from all keyslots it may be programmed into and unlinked from 212 any kernel data structures it may be linked into. 213 214 In summary, for users of the block layer, the lifecycle of a blk_crypto_key is 215 as follows: 216 217 1. ``blk_crypto_config_supported()`` (optional) 218 2. ``blk_crypto_init_key()`` 219 3. ``blk_crypto_start_using_key()`` 220 4. ``bio_crypt_set_ctx()`` (potentially many times) 221 5. ``blk_crypto_evict_key()`` (after all I/O has completed) 222 6. Zeroize the blk_crypto_key (this has no dedicated function) 223 224 If a blk_crypto_key is being used on multiple block_devices, then 225 ``blk_crypto_config_supported()`` (if used), ``blk_crypto_start_using_key()``, 226 and ``blk_crypto_evict_key()`` must be called on each block_device. 227 228 API presented to device drivers 229 =============================== 230 231 A device driver that wants to support inline encryption must set up a 232 blk_crypto_profile in the request_queue of its device. To do this, it first 233 must call ``blk_crypto_profile_init()`` (or its resource-managed variant 234 ``devm_blk_crypto_profile_init()``), providing the number of keyslots. 235 236 Next, it must advertise its crypto capabilities by setting fields in the 237 blk_crypto_profile, e.g. ``modes_supported`` and ``max_dun_bytes_supported``. 238 239 It then must set function pointers in the ``ll_ops`` field of the 240 blk_crypto_profile to tell upper layers how to control the inline encryption 241 hardware, e.g. how to program and evict keyslots. Most drivers will need to 242 implement ``keyslot_program`` and ``keyslot_evict``. For details, see the 243 comments for ``struct blk_crypto_ll_ops``. 244 245 Once the driver registers a blk_crypto_profile with a request_queue, I/O 246 requests the driver receives via that queue may have an encryption context. All 247 encryption contexts will be compatible with the crypto capabilities declared in 248 the blk_crypto_profile, so drivers don't need to worry about handling 249 unsupported requests. Also, if a nonzero number of keyslots was declared in the 250 blk_crypto_profile, then all I/O requests that have an encryption context will 251 also have a keyslot which was already programmed with the appropriate key. 252 253 If the driver implements runtime suspend and its blk_crypto_ll_ops don't work 254 while the device is runtime-suspended, then the driver must also set the ``dev`` 255 field of the blk_crypto_profile to point to the ``struct device`` that will be 256 resumed before any of the low-level operations are called. 257 258 If there are situations where the inline encryption hardware loses the contents 259 of its keyslots, e.g. device resets, the driver must handle reprogramming the 260 keyslots. To do this, the driver may call ``blk_crypto_reprogram_all_keys()``. 261 262 Finally, if the driver used ``blk_crypto_profile_init()`` instead of 263 ``devm_blk_crypto_profile_init()``, then it is responsible for calling 264 ``blk_crypto_profile_destroy()`` when the crypto profile is no longer needed. 265 266 Layered Devices 267 =============== 268 269 Request queue based layered devices like dm-rq that wish to support inline 270 encryption need to create their own blk_crypto_profile for their request_queue, 271 and expose whatever functionality they choose. When a layered device wants to 272 pass a clone of that request to another request_queue, blk-crypto will 273 initialize and prepare the clone as necessary. 274 275 Interaction between inline encryption and blk integrity 276 ======================================================= 277 278 At the time of this patch, there is no real hardware that supports both these 279 features. However, these features do interact with each other, and it's not 280 completely trivial to make them both work together properly. In particular, 281 when a WRITE bio wants to use inline encryption on a device that supports both 282 features, the bio will have an encryption context specified, after which 283 its integrity information is calculated (using the plaintext data, since 284 the encryption will happen while data is being written), and the data and 285 integrity info is sent to the device. Obviously, the integrity info must be 286 verified before the data is encrypted. After the data is encrypted, the device 287 must not store the integrity info that it received with the plaintext data 288 since that might reveal information about the plaintext data. As such, it must 289 re-generate the integrity info from the ciphertext data and store that on disk 290 instead. Another issue with storing the integrity info of the plaintext data is 291 that it changes the on disk format depending on whether hardware inline 292 encryption support is present or the kernel crypto API fallback is used (since 293 if the fallback is used, the device will receive the integrity info of the 294 ciphertext, not that of the plaintext). 295 296 Because there isn't any real hardware yet, it seems prudent to assume that 297 hardware implementations might not implement both features together correctly, 298 and disallow the combination for now. Whenever a device supports integrity, the 299 kernel will pretend that the device does not support hardware inline encryption 300 (by setting the blk_crypto_profile in the request_queue of the device to NULL). 301 When the crypto API fallback is enabled, this means that all bios with and 302 encryption context will use the fallback, and IO will complete as usual. When 303 the fallback is disabled, a bio with an encryption context will be failed.
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