1The dm-integrity target emulates a block device that has additional 2per-sector tags that can be used for storing integrity information. 3 4A general problem with storing integrity tags with every sector is that 5writing the sector and the integrity tag must be atomic - i.e. in case of 6crash, either both sector and integrity tag or none of them is written. 7 8To guarantee write atomicity, the dm-integrity target uses journal, it 9writes sector data and integrity tags into a journal, commits the journal 10and then copies the data and integrity tags to their respective location. 11 12The dm-integrity target can be used with the dm-crypt target - in this 13situation the dm-crypt target creates the integrity data and passes them 14to the dm-integrity target via bio_integrity_payload attached to the bio. 15In this mode, the dm-crypt and dm-integrity targets provide authenticated 16disk encryption - if the attacker modifies the encrypted device, an I/O 17error is returned instead of random data. 18 19The dm-integrity target can also be used as a standalone target, in this 20mode it calculates and verifies the integrity tag internally. In this 21mode, the dm-integrity target can be used to detect silent data 22corruption on the disk or in the I/O path. 23 24 25When loading the target for the first time, the kernel driver will format 26the device. But it will only format the device if the superblock contains 27zeroes. If the superblock is neither valid nor zeroed, the dm-integrity 28target can't be loaded. 29 30To use the target for the first time: 311. overwrite the superblock with zeroes 322. load the dm-integrity target with one-sector size, the kernel driver 33 will format the device 343. unload the dm-integrity target 354. read the "provided_data_sectors" value from the superblock 365. load the dm-integrity target with the the target size 37 "provided_data_sectors" 386. if you want to use dm-integrity with dm-crypt, load the dm-crypt target 39 with the size "provided_data_sectors" 40 41 42Target arguments: 43 441. the underlying block device 45 462. the number of reserved sector at the beginning of the device - the 47 dm-integrity won't read of write these sectors 48 493. the size of the integrity tag (if "-" is used, the size is taken from 50 the internal-hash algorithm) 51 524. mode: 53 D - direct writes (without journal) - in this mode, journaling is 54 not used and data sectors and integrity tags are written 55 separately. In case of crash, it is possible that the data 56 and integrity tag doesn't match. 57 J - journaled writes - data and integrity tags are written to the 58 journal and atomicity is guaranteed. In case of crash, 59 either both data and tag or none of them are written. The 60 journaled mode degrades write throughput twice because the 61 data have to be written twice. 62 R - recovery mode - in this mode, journal is not replayed, 63 checksums are not checked and writes to the device are not 64 allowed. This mode is useful for data recovery if the 65 device cannot be activated in any of the other standard 66 modes. 67 685. the number of additional arguments 69 70Additional arguments: 71 72journal_sectors:number 73 The size of journal, this argument is used only if formatting the 74 device. If the device is already formatted, the value from the 75 superblock is used. 76 77interleave_sectors:number 78 The number of interleaved sectors. This values is rounded down to 79 a power of two. If the device is already formatted, the value from 80 the superblock is used. 81 82buffer_sectors:number 83 The number of sectors in one buffer. The value is rounded down to 84 a power of two. 85 86 The tag area is accessed using buffers, the buffer size is 87 configurable. The large buffer size means that the I/O size will 88 be larger, but there could be less I/Os issued. 89 90journal_watermark:number 91 The journal watermark in percents. When the size of the journal 92 exceeds this watermark, the thread that flushes the journal will 93 be started. 94 95commit_time:number 96 Commit time in milliseconds. When this time passes, the journal is 97 written. The journal is also written immediatelly if the FLUSH 98 request is received. 99 100internal_hash:algorithm(:key) (the key is optional) 101 Use internal hash or crc. 102 When this argument is used, the dm-integrity target won't accept 103 integrity tags from the upper target, but it will automatically 104 generate and verify the integrity tags. 105 106 You can use a crc algorithm (such as crc32), then integrity target 107 will protect the data against accidental corruption. 108 You can also use a hmac algorithm (for example 109 "hmac(sha256):0123456789abcdef"), in this mode it will provide 110 cryptographic authentication of the data without encryption. 111 112 When this argument is not used, the integrity tags are accepted 113 from an upper layer target, such as dm-crypt. The upper layer 114 target should check the validity of the integrity tags. 115 116recalculate 117 Recalculate the integrity tags automatically. It is only valid 118 when using internal hash. 119 120journal_crypt:algorithm(:key) (the key is optional) 121 Encrypt the journal using given algorithm to make sure that the 122 attacker can't read the journal. You can use a block cipher here 123 (such as "cbc(aes)") or a stream cipher (for example "chacha20", 124 "salsa20", "ctr(aes)" or "ecb(arc4)"). 125 126 The journal contains history of last writes to the block device, 127 an attacker reading the journal could see the last sector nubmers 128 that were written. From the sector numbers, the attacker can infer 129 the size of files that were written. To protect against this 130 situation, you can encrypt the journal. 131 132journal_mac:algorithm(:key) (the key is optional) 133 Protect sector numbers in the journal from accidental or malicious 134 modification. To protect against accidental modification, use a 135 crc algorithm, to protect against malicious modification, use a 136 hmac algorithm with a key. 137 138 This option is not needed when using internal-hash because in this 139 mode, the integrity of journal entries is checked when replaying 140 the journal. Thus, modified sector number would be detected at 141 this stage. 142 143block_size:number 144 The size of a data block in bytes. The larger the block size the 145 less overhead there is for per-block integrity metadata. 146 Supported values are 512, 1024, 2048 and 4096 bytes. If not 147 specified the default block size is 512 bytes. 148 149The journal mode (D/J), buffer_sectors, journal_watermark, commit_time can 150be changed when reloading the target (load an inactive table and swap the 151tables with suspend and resume). The other arguments should not be changed 152when reloading the target because the layout of disk data depend on them 153and the reloaded target would be non-functional. 154 155 156The layout of the formatted block device: 157* reserved sectors (they are not used by this target, they can be used for 158 storing LUKS metadata or for other purpose), the size of the reserved 159 area is specified in the target arguments 160* superblock (4kiB) 161 * magic string - identifies that the device was formatted 162 * version 163 * log2(interleave sectors) 164 * integrity tag size 165 * the number of journal sections 166 * provided data sectors - the number of sectors that this target 167 provides (i.e. the size of the device minus the size of all 168 metadata and padding). The user of this target should not send 169 bios that access data beyond the "provided data sectors" limit. 170 * flags - a flag is set if journal_mac is used 171* journal 172 The journal is divided into sections, each section contains: 173 * metadata area (4kiB), it contains journal entries 174 every journal entry contains: 175 * logical sector (specifies where the data and tag should 176 be written) 177 * last 8 bytes of data 178 * integrity tag (the size is specified in the superblock) 179 every metadata sector ends with 180 * mac (8-bytes), all the macs in 8 metadata sectors form a 181 64-byte value. It is used to store hmac of sector 182 numbers in the journal section, to protect against a 183 possibility that the attacker tampers with sector 184 numbers in the journal. 185 * commit id 186 * data area (the size is variable; it depends on how many journal 187 entries fit into the metadata area) 188 every sector in the data area contains: 189 * data (504 bytes of data, the last 8 bytes are stored in 190 the journal entry) 191 * commit id 192 To test if the whole journal section was written correctly, every 193 512-byte sector of the journal ends with 8-byte commit id. If the 194 commit id matches on all sectors in a journal section, then it is 195 assumed that the section was written correctly. If the commit id 196 doesn't match, the section was written partially and it should not 197 be replayed. 198* one or more runs of interleaved tags and data. Each run contains: 199 * tag area - it contains integrity tags. There is one tag for each 200 sector in the data area 201 * data area - it contains data sectors. The number of data sectors 202 in one run must be a power of two. log2 of this value is stored 203 in the superblock. 204