Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 The dm-integrity target emulates a block device that has additional 2 per-sector tags that can be used for storing integrity information. 3 4 A general problem with storing integrity tags with every sector is that 5 writing the sector and the integrity tag must be atomic - i.e. in case of 6 crash, either both sector and integrity tag or none of them is written. 7 8 To guarantee write atomicity, the dm-integrity target uses journal, it 9 writes sector data and integrity tags into a journal, commits the journal 10 and then copies the data and integrity tags to their respective location. 11 12 The dm-integrity target can be used with the dm-crypt target - in this 13 situation the dm-crypt target creates the integrity data and passes them 14 to the dm-integrity target via bio_integrity_payload attached to the bio. 15 In this mode, the dm-crypt and dm-integrity targets provide authenticated 16 disk encryption - if the attacker modifies the encrypted device, an I/O 17 error is returned instead of random data. 18 19 The dm-integrity target can also be used as a standalone target, in this 20 mode it calculates and verifies the integrity tag internally. In this 21 mode, the dm-integrity target can be used to detect silent data 22 corruption on the disk or in the I/O path. 23 24 25 When loading the target for the first time, the kernel driver will format 26 the device. But it will only format the device if the superblock contains 27 zeroes. If the superblock is neither valid nor zeroed, the dm-integrity 28 target can't be loaded. 29 30 To use the target for the first time: 31 1. overwrite the superblock with zeroes 32 2. load the dm-integrity target with one-sector size, the kernel driver 33 will format the device 34 3. unload the dm-integrity target 35 4. read the "provided_data_sectors" value from the superblock 36 5. load the dm-integrity target with the the target size 37 "provided_data_sectors" 38 6. if you want to use dm-integrity with dm-crypt, load the dm-crypt target 39 with the size "provided_data_sectors" 40 41 42 Target arguments: 43 44 1. the underlying block device 45 46 2. the number of reserved sector at the beginning of the device - the 47 dm-integrity won't read of write these sectors 48 49 3. the size of the integrity tag (if "-" is used, the size is taken from 50 the internal-hash algorithm) 51 52 4. 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 68 5. the number of additional arguments 69 70 Additional arguments: 71 72 journal_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 77 interleave_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 82 buffer_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 90 journal_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 95 commit_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. 99 100 internal_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. 115 116 journal_crypt:algorithm(:key) (the key is optional) 117 Encrypt the journal using given algorithm to make sure that the 118 attacker can't read the journal. You can use a block cipher here 119 (such as "cbc(aes)") or a stream cipher (for example "chacha20", 120 "salsa20", "ctr(aes)" or "ecb(arc4)"). 121 122 The journal contains history of last writes to the block device, 123 an attacker reading the journal could see the last sector nubmers 124 that were written. From the sector numbers, the attacker can infer 125 the size of files that were written. To protect against this 126 situation, you can encrypt the journal. 127 128 journal_mac:algorithm(:key) (the key is optional) 129 Protect sector numbers in the journal from accidental or malicious 130 modification. To protect against accidental modification, use a 131 crc algorithm, to protect against malicious modification, use a 132 hmac algorithm with a key. 133 134 This option is not needed when using internal-hash because in this 135 mode, the integrity of journal entries is checked when replaying 136 the journal. Thus, modified sector number would be detected at 137 this stage. 138 139 block_size:number 140 The size of a data block in bytes. The larger the block size the 141 less overhead there is for per-block integrity metadata. 142 Supported values are 512, 1024, 2048 and 4096 bytes. If not 143 specified the default block size is 512 bytes. 144 145 The journal mode (D/J), buffer_sectors, journal_watermark, commit_time can 146 be changed when reloading the target (load an inactive table and swap the 147 tables with suspend and resume). The other arguments should not be changed 148 when reloading the target because the layout of disk data depend on them 149 and the reloaded target would be non-functional. 150 151 152 The layout of the formatted block device: 153 * reserved sectors (they are not used by this target, they can be used for 154 storing LUKS metadata or for other purpose), the size of the reserved 155 area is specified in the target arguments 156 * superblock (4kiB) 157 * magic string - identifies that the device was formatted 158 * version 159 * log2(interleave sectors) 160 * integrity tag size 161 * the number of journal sections 162 * provided data sectors - the number of sectors that this target 163 provides (i.e. the size of the device minus the size of all 164 metadata and padding). The user of this target should not send 165 bios that access data beyond the "provided data sectors" limit. 166 * flags - a flag is set if journal_mac is used 167 * journal 168 The journal is divided into sections, each section contains: 169 * metadata area (4kiB), it contains journal entries 170 every journal entry contains: 171 * logical sector (specifies where the data and tag should 172 be written) 173 * last 8 bytes of data 174 * integrity tag (the size is specified in the superblock) 175 every metadata sector ends with 176 * mac (8-bytes), all the macs in 8 metadata sectors form a 177 64-byte value. It is used to store hmac of sector 178 numbers in the journal section, to protect against a 179 possibility that the attacker tampers with sector 180 numbers in the journal. 181 * commit id 182 * data area (the size is variable; it depends on how many journal 183 entries fit into the metadata area) 184 every sector in the data area contains: 185 * data (504 bytes of data, the last 8 bytes are stored in 186 the journal entry) 187 * commit id 188 To test if the whole journal section was written correctly, every 189 512-byte sector of the journal ends with 8-byte commit id. If the 190 commit id matches on all sectors in a journal section, then it is 191 assumed that the section was written correctly. If the commit id 192 doesn't match, the section was written partially and it should not 193 be replayed. 194 * one or more runs of interleaved tags and data. Each run contains: 195 * tag area - it contains integrity tags. There is one tag for each 196 sector in the data area 197 * data area - it contains data sectors. The number of data sectors 198 in one run must be a power of two. log2 of this value is stored 199 in the superblock.