1============ 2dm-integrity 3============ 4 5The dm-integrity target emulates a block device that has additional 6per-sector tags that can be used for storing integrity information. 7 8A general problem with storing integrity tags with every sector is that 9writing the sector and the integrity tag must be atomic - i.e. in case of 10crash, either both sector and integrity tag or none of them is written. 11 12To guarantee write atomicity, the dm-integrity target uses journal, it 13writes sector data and integrity tags into a journal, commits the journal 14and then copies the data and integrity tags to their respective location. 15 16The dm-integrity target can be used with the dm-crypt target - in this 17situation the dm-crypt target creates the integrity data and passes them 18to the dm-integrity target via bio_integrity_payload attached to the bio. 19In this mode, the dm-crypt and dm-integrity targets provide authenticated 20disk encryption - if the attacker modifies the encrypted device, an I/O 21error is returned instead of random data. 22 23The dm-integrity target can also be used as a standalone target, in this 24mode it calculates and verifies the integrity tag internally. In this 25mode, the dm-integrity target can be used to detect silent data 26corruption on the disk or in the I/O path. 27 28There's an alternate mode of operation where dm-integrity uses a bitmap 29instead of a journal. If a bit in the bitmap is 1, the corresponding 30region's data and integrity tags are not synchronized - if the machine 31crashes, the unsynchronized regions will be recalculated. The bitmap mode 32is faster than the journal mode, because we don't have to write the data 33twice, but it is also less reliable, because if data corruption happens 34when the machine crashes, it may not be detected. 35 36When loading the target for the first time, the kernel driver will format 37the device. But it will only format the device if the superblock contains 38zeroes. If the superblock is neither valid nor zeroed, the dm-integrity 39target can't be loaded. 40 41Accesses to the on-disk metadata area containing checksums (aka tags) are 42buffered using dm-bufio. When an access to any given metadata area 43occurs, each unique metadata area gets its own buffer(s). The buffer size 44is capped at the size of the metadata area, but may be smaller, thereby 45requiring multiple buffers to represent the full metadata area. A smaller 46buffer size will produce a smaller resulting read/write operation to the 47metadata area for small reads/writes. The metadata is still read even in 48a full write to the data covered by a single buffer. 49 50To use the target for the first time: 51 521. overwrite the superblock with zeroes 532. load the dm-integrity target with one-sector size, the kernel driver 54 will format the device 553. unload the dm-integrity target 564. read the "provided_data_sectors" value from the superblock 575. load the dm-integrity target with the target size 58 "provided_data_sectors" 596. if you want to use dm-integrity with dm-crypt, load the dm-crypt target 60 with the size "provided_data_sectors" 61 62 63Target arguments: 64 651. the underlying block device 66 672. the number of reserved sector at the beginning of the device - the 68 dm-integrity won't read of write these sectors 69 703. the size of the integrity tag (if "-" is used, the size is taken from 71 the internal-hash algorithm) 72 734. mode: 74 75 D - direct writes (without journal) 76 in this mode, journaling is 77 not used and data sectors and integrity tags are written 78 separately. In case of crash, it is possible that the data 79 and integrity tag doesn't match. 80 J - journaled writes 81 data and integrity tags are written to the 82 journal and atomicity is guaranteed. In case of crash, 83 either both data and tag or none of them are written. The 84 journaled mode degrades write throughput twice because the 85 data have to be written twice. 86 B - bitmap mode - data and metadata are written without any 87 synchronization, the driver maintains a bitmap of dirty 88 regions where data and metadata don't match. This mode can 89 only be used with internal hash. 90 R - recovery mode - in this mode, journal is not replayed, 91 checksums are not checked and writes to the device are not 92 allowed. This mode is useful for data recovery if the 93 device cannot be activated in any of the other standard 94 modes. 95 I - inline mode - in this mode, dm-integrity will store integrity 96 data directly in the underlying device sectors. 97 The underlying device must have an integrity profile that 98 allows storing user integrity data and provides enough 99 space for the selected integrity tag. 100 1015. the number of additional arguments 102 103Additional arguments: 104 105journal_sectors:number 106 The size of journal, this argument is used only if formatting the 107 device. If the device is already formatted, the value from the 108 superblock is used. 109 110interleave_sectors:number (default 32768) 111 The number of interleaved sectors. This values is rounded down to 112 a power of two. If the device is already formatted, the value from 113 the superblock is used. 114 115meta_device:device 116 Don't interleave the data and metadata on the device. Use a 117 separate device for metadata. 118 119buffer_sectors:number (default 128) 120 The number of sectors in one metadata buffer. The value is rounded 121 down to a power of two. 122 123journal_watermark:number (default 50) 124 The journal watermark in percents. When the size of the journal 125 exceeds this watermark, the thread that flushes the journal will 126 be started. 127 128commit_time:number (default 10000) 129 Commit time in milliseconds. When this time passes, the journal is 130 written. The journal is also written immediately if the FLUSH 131 request is received. 132 133internal_hash:algorithm(:key) (the key is optional) 134 Use internal hash or crc. 135 When this argument is used, the dm-integrity target won't accept 136 integrity tags from the upper target, but it will automatically 137 generate and verify the integrity tags. 138 139 You can use a crc algorithm (such as crc32), then integrity target 140 will protect the data against accidental corruption. 141 You can also use a hmac algorithm (for example 142 "hmac(sha256):0123456789abcdef"), in this mode it will provide 143 cryptographic authentication of the data without encryption. 144 145 When this argument is not used, the integrity tags are accepted 146 from an upper layer target, such as dm-crypt. The upper layer 147 target should check the validity of the integrity tags. 148 149recalculate 150 Recalculate the integrity tags automatically. It is only valid 151 when using internal hash. 152 153journal_crypt:algorithm(:key) (the key is optional) 154 Encrypt the journal using given algorithm to make sure that the 155 attacker can't read the journal. You can use a block cipher here 156 (such as "cbc(aes)") or a stream cipher (for example "chacha20" 157 or "ctr(aes)"). 158 159 The journal contains history of last writes to the block device, 160 an attacker reading the journal could see the last sector numbers 161 that were written. From the sector numbers, the attacker can infer 162 the size of files that were written. To protect against this 163 situation, you can encrypt the journal. 164 165journal_mac:algorithm(:key) (the key is optional) 166 Protect sector numbers in the journal from accidental or malicious 167 modification. To protect against accidental modification, use a 168 crc algorithm, to protect against malicious modification, use a 169 hmac algorithm with a key. 170 171 This option is not needed when using internal-hash because in this 172 mode, the integrity of journal entries is checked when replaying 173 the journal. Thus, modified sector number would be detected at 174 this stage. 175 176block_size:number (default 512) 177 The size of a data block in bytes. The larger the block size the 178 less overhead there is for per-block integrity metadata. 179 Supported values are 512, 1024, 2048 and 4096 bytes. 180 181sectors_per_bit:number 182 In the bitmap mode, this parameter specifies the number of 183 512-byte sectors that corresponds to one bitmap bit. 184 185bitmap_flush_interval:number 186 The bitmap flush interval in milliseconds. The metadata buffers 187 are synchronized when this interval expires. 188 189allow_discards 190 Allow block discard requests (a.k.a. TRIM) for the integrity device. 191 Discards are only allowed to devices using internal hash. 192 193 A discarded block is marked with a constant filler tag that anyone 194 with raw write access to the backing device can forge without the 195 key. Use allow_discards_keyed instead on new volumes. 196 197allow_discards_keyed 198 Like allow_discards, but marks a discarded block with a keyed 199 checksum of the sector number, HMAC_key(salt || sector), instead of 200 the constant filler tag, so it can't be forged without the 201 integrity key. 202 203 Not compatible with volumes that already have discarded blocks 204 marked the old way; only use on a freshly formatted volume. 205 206fix_padding 207 Use a smaller padding of the tag area that is more 208 space-efficient. If this option is not present, large padding is 209 used - that is for compatibility with older kernels. 210 211fix_hmac 212 Improve security of internal_hash and journal_mac: 213 214 - the section number is mixed to the mac, so that an attacker can't 215 copy sectors from one journal section to another journal section 216 - the superblock is protected by journal_mac 217 - a 16-byte salt stored in the superblock is mixed to the mac, so 218 that the attacker can't detect that two disks have the same hmac 219 key and also to disallow the attacker to move sectors from one 220 disk to another 221 222legacy_recalculate 223 Allow recalculating of volumes with HMAC keys. This is disabled by 224 default for security reasons - an attacker could modify the volume, 225 set recalc_sector to zero, and the kernel would not detect the 226 modification. 227 228The journal mode (D/J), buffer_sectors, journal_watermark, commit_time and 229allow_discards can be changed when reloading the target (load an inactive 230table and swap the tables with suspend and resume). The other arguments 231should not be changed when reloading the target because the layout of disk 232data depend on them and the reloaded target would be non-functional. 233 234For example, on a device using the default interleave_sectors of 32768, a 235block_size of 512, and an internal_hash of crc32c with a tag size of 4 236bytes, it will take 128 KiB of tags to track a full data area, requiring 237256 sectors of metadata per data area. With the default buffer_sectors of 238128, that means there will be 2 buffers per metadata area, or 2 buffers 239per 16 MiB of data. 240 241Status line: 242 2431. the number of integrity mismatches 2442. provided data sectors - that is the number of sectors that the user 245 could use 2463. the current recalculating position (or '-' if we didn't recalculate) 247 248 249The layout of the formatted block device: 250 251* reserved sectors 252 (they are not used by this target, they can be used for 253 storing LUKS metadata or for other purpose), the size of the reserved 254 area is specified in the target arguments 255 256* superblock (4kiB) 257 * magic string - identifies that the device was formatted 258 * version 259 * log2(interleave sectors) 260 * integrity tag size 261 * the number of journal sections 262 * provided data sectors - the number of sectors that this target 263 provides (i.e. the size of the device minus the size of all 264 metadata and padding). The user of this target should not send 265 bios that access data beyond the "provided data sectors" limit. 266 * flags 267 SB_FLAG_HAVE_JOURNAL_MAC 268 - a flag is set if journal_mac is used 269 SB_FLAG_RECALCULATING 270 - recalculating is in progress 271 SB_FLAG_DIRTY_BITMAP 272 - journal area contains the bitmap of dirty 273 blocks 274 * log2(sectors per block) 275 * a position where recalculating finished 276* journal 277 The journal is divided into sections, each section contains: 278 279 * metadata area (4kiB), it contains journal entries 280 281 - every journal entry contains: 282 283 * logical sector (specifies where the data and tag should 284 be written) 285 * last 8 bytes of data 286 * integrity tag (the size is specified in the superblock) 287 288 - every metadata sector ends with 289 290 * mac (8-bytes), all the macs in 8 metadata sectors form a 291 64-byte value. It is used to store hmac of sector 292 numbers in the journal section, to protect against a 293 possibility that the attacker tampers with sector 294 numbers in the journal. 295 * commit id 296 297 * data area (the size is variable; it depends on how many journal 298 entries fit into the metadata area) 299 300 - every sector in the data area contains: 301 302 * data (504 bytes of data, the last 8 bytes are stored in 303 the journal entry) 304 * commit id 305 306 To test if the whole journal section was written correctly, every 307 512-byte sector of the journal ends with 8-byte commit id. If the 308 commit id matches on all sectors in a journal section, then it is 309 assumed that the section was written correctly. If the commit id 310 doesn't match, the section was written partially and it should not 311 be replayed. 312 313* one or more runs of interleaved tags and data. 314 Each run contains: 315 316 * tag area - it contains integrity tags. There is one tag for each 317 sector in the data area. The size of this area is always 4KiB or 318 greater. 319 * data area - it contains data sectors. The number of data sectors 320 in one run must be a power of two. log2 of this value is stored 321 in the superblock. 322