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 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 41To use the target for the first time: 42 431. overwrite the superblock with zeroes 442. load the dm-integrity target with one-sector size, the kernel driver 45 will format the device 463. unload the dm-integrity target 474. read the "provided_data_sectors" value from the superblock 485. load the dm-integrity target with the the target size 49 "provided_data_sectors" 506. if you want to use dm-integrity with dm-crypt, load the dm-crypt target 51 with the size "provided_data_sectors" 52 53 54Target arguments: 55 561. the underlying block device 57 582. the number of reserved sector at the beginning of the device - the 59 dm-integrity won't read of write these sectors 60 613. the size of the integrity tag (if "-" is used, the size is taken from 62 the internal-hash algorithm) 63 644. mode: 65 66 D - direct writes (without journal) 67 in this mode, journaling is 68 not used and data sectors and integrity tags are written 69 separately. In case of crash, it is possible that the data 70 and integrity tag doesn't match. 71 J - journaled writes 72 data and integrity tags are written to the 73 journal and atomicity is guaranteed. In case of crash, 74 either both data and tag or none of them are written. The 75 journaled mode degrades write throughput twice because the 76 data have to be written twice. 77 B - bitmap mode - data and metadata are written without any 78 synchronization, the driver maintains a bitmap of dirty 79 regions where data and metadata don't match. This mode can 80 only be used with internal hash. 81 R - recovery mode - in this mode, journal is not replayed, 82 checksums are not checked and writes to the device are not 83 allowed. This mode is useful for data recovery if the 84 device cannot be activated in any of the other standard 85 modes. 86 875. the number of additional arguments 88 89Additional arguments: 90 91journal_sectors:number 92 The size of journal, this argument is used only if formatting the 93 device. If the device is already formatted, the value from the 94 superblock is used. 95 96interleave_sectors:number 97 The number of interleaved sectors. This values is rounded down to 98 a power of two. If the device is already formatted, the value from 99 the superblock is used. 100 101meta_device:device 102 Don't interleave the data and metadata on on device. Use a 103 separate device for metadata. 104 105buffer_sectors:number 106 The number of sectors in one buffer. The value is rounded down to 107 a power of two. 108 109 The tag area is accessed using buffers, the buffer size is 110 configurable. The large buffer size means that the I/O size will 111 be larger, but there could be less I/Os issued. 112 113journal_watermark:number 114 The journal watermark in percents. When the size of the journal 115 exceeds this watermark, the thread that flushes the journal will 116 be started. 117 118commit_time:number 119 Commit time in milliseconds. When this time passes, the journal is 120 written. The journal is also written immediatelly if the FLUSH 121 request is received. 122 123internal_hash:algorithm(:key) (the key is optional) 124 Use internal hash or crc. 125 When this argument is used, the dm-integrity target won't accept 126 integrity tags from the upper target, but it will automatically 127 generate and verify the integrity tags. 128 129 You can use a crc algorithm (such as crc32), then integrity target 130 will protect the data against accidental corruption. 131 You can also use a hmac algorithm (for example 132 "hmac(sha256):0123456789abcdef"), in this mode it will provide 133 cryptographic authentication of the data without encryption. 134 135 When this argument is not used, the integrity tags are accepted 136 from an upper layer target, such as dm-crypt. The upper layer 137 target should check the validity of the integrity tags. 138 139recalculate 140 Recalculate the integrity tags automatically. It is only valid 141 when using internal hash. 142 143journal_crypt:algorithm(:key) (the key is optional) 144 Encrypt the journal using given algorithm to make sure that the 145 attacker can't read the journal. You can use a block cipher here 146 (such as "cbc(aes)") or a stream cipher (for example "chacha20", 147 "salsa20", "ctr(aes)" or "ecb(arc4)"). 148 149 The journal contains history of last writes to the block device, 150 an attacker reading the journal could see the last sector nubmers 151 that were written. From the sector numbers, the attacker can infer 152 the size of files that were written. To protect against this 153 situation, you can encrypt the journal. 154 155journal_mac:algorithm(:key) (the key is optional) 156 Protect sector numbers in the journal from accidental or malicious 157 modification. To protect against accidental modification, use a 158 crc algorithm, to protect against malicious modification, use a 159 hmac algorithm with a key. 160 161 This option is not needed when using internal-hash because in this 162 mode, the integrity of journal entries is checked when replaying 163 the journal. Thus, modified sector number would be detected at 164 this stage. 165 166block_size:number 167 The size of a data block in bytes. The larger the block size the 168 less overhead there is for per-block integrity metadata. 169 Supported values are 512, 1024, 2048 and 4096 bytes. If not 170 specified the default block size is 512 bytes. 171 172sectors_per_bit:number 173 In the bitmap mode, this parameter specifies the number of 174 512-byte sectors that corresponds to one bitmap bit. 175 176bitmap_flush_interval:number 177 The bitmap flush interval in milliseconds. The metadata buffers 178 are synchronized when this interval expires. 179 180 181The journal mode (D/J), buffer_sectors, journal_watermark, commit_time can 182be changed when reloading the target (load an inactive table and swap the 183tables with suspend and resume). The other arguments should not be changed 184when reloading the target because the layout of disk data depend on them 185and the reloaded target would be non-functional. 186 187 188The layout of the formatted block device: 189 190* reserved sectors 191 (they are not used by this target, they can be used for 192 storing LUKS metadata or for other purpose), the size of the reserved 193 area is specified in the target arguments 194 195* superblock (4kiB) 196 * magic string - identifies that the device was formatted 197 * version 198 * log2(interleave sectors) 199 * integrity tag size 200 * the number of journal sections 201 * provided data sectors - the number of sectors that this target 202 provides (i.e. the size of the device minus the size of all 203 metadata and padding). The user of this target should not send 204 bios that access data beyond the "provided data sectors" limit. 205 * flags 206 SB_FLAG_HAVE_JOURNAL_MAC 207 - a flag is set if journal_mac is used 208 SB_FLAG_RECALCULATING 209 - recalculating is in progress 210 SB_FLAG_DIRTY_BITMAP 211 - journal area contains the bitmap of dirty 212 blocks 213 * log2(sectors per block) 214 * a position where recalculating finished 215* journal 216 The journal is divided into sections, each section contains: 217 218 * metadata area (4kiB), it contains journal entries 219 220 - every journal entry contains: 221 222 * logical sector (specifies where the data and tag should 223 be written) 224 * last 8 bytes of data 225 * integrity tag (the size is specified in the superblock) 226 227 - every metadata sector ends with 228 229 * mac (8-bytes), all the macs in 8 metadata sectors form a 230 64-byte value. It is used to store hmac of sector 231 numbers in the journal section, to protect against a 232 possibility that the attacker tampers with sector 233 numbers in the journal. 234 * commit id 235 236 * data area (the size is variable; it depends on how many journal 237 entries fit into the metadata area) 238 239 - every sector in the data area contains: 240 241 * data (504 bytes of data, the last 8 bytes are stored in 242 the journal entry) 243 * commit id 244 245 To test if the whole journal section was written correctly, every 246 512-byte sector of the journal ends with 8-byte commit id. If the 247 commit id matches on all sectors in a journal section, then it is 248 assumed that the section was written correctly. If the commit id 249 doesn't match, the section was written partially and it should not 250 be replayed. 251 252* one or more runs of interleaved tags and data. 253 Each run contains: 254 255 * tag area - it contains integrity tags. There is one tag for each 256 sector in the data area 257 * data area - it contains data sectors. The number of data sectors 258 in one run must be a power of two. log2 of this value is stored 259 in the superblock. 260
