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Diffstat (limited to 'Documentation/device-mapper/verity.txt')
| -rw-r--r-- | Documentation/device-mapper/verity.txt | 131 |
1 files changed, 46 insertions, 85 deletions
diff --git a/Documentation/device-mapper/verity.txt b/Documentation/device-mapper/verity.txt index 32e4879..9884681 100644 --- a/Documentation/device-mapper/verity.txt +++ b/Documentation/device-mapper/verity.txt @@ -7,39 +7,39 @@ This target is read-only. Construction Parameters ======================= - <version> <dev> <hash_dev> <hash_start> + <version> <dev> <hash_dev> <data_block_size> <hash_block_size> <num_data_blocks> <hash_start_block> <algorithm> <digest> <salt> <version> - This is the version number of the on-disk format. + This is the type of the on-disk hash format. 0 is the original format used in the Chromium OS. - The salt is appended when hashing, digests are stored continuously and - the rest of the block is padded with zeros. + The salt is appended when hashing, digests are stored continuously and + the rest of the block is padded with zeros. 1 is the current format that should be used for new devices. - The salt is prepended when hashing and each digest is - padded with zeros to the power of two. + The salt is prepended when hashing and each digest is + padded with zeros to the power of two. <dev> - This is the device containing the data the integrity of which needs to be + This is the device containing data, the integrity of which needs to be checked. It may be specified as a path, like /dev/sdaX, or a device number, <major>:<minor>. <hash_dev> - This is the device that that supplies the hash tree data. It may be + This is the device that supplies the hash tree data. It may be specified similarly to the device path and may be the same device. If the - same device is used, the hash_start should be outside of the dm-verity - configured device size. + same device is used, the hash_start should be outside the configured + dm-verity device. <data_block_size> - The block size on a data device. Each block corresponds to one digest on - the hash device. + The block size on a data device in bytes. + Each block corresponds to one digest on the hash device. <hash_block_size> - The size of a hash block. + The size of a hash block in bytes. <num_data_blocks> The number of data blocks on the data device. Additional blocks are @@ -65,7 +65,7 @@ Construction Parameters Theory of operation =================== -dm-verity is meant to be setup as part of a verified boot path. This +dm-verity is meant to be set up as part of a verified boot path. This may be anything ranging from a boot using tboot or trustedgrub to just booting from a known-good device (like a USB drive or CD). @@ -73,20 +73,20 @@ When a dm-verity device is configured, it is expected that the caller has been authenticated in some way (cryptographic signatures, etc). After instantiation, all hashes will be verified on-demand during disk access. If they cannot be verified up to the root node of the -tree, the root hash, then the I/O will fail. This should identify +tree, the root hash, then the I/O will fail. This should detect tampering with any data on the device and the hash data. Cryptographic hashes are used to assert the integrity of the device on a -per-block basis. This allows for a lightweight hash computation on first read -into the page cache. Block hashes are stored linearly-aligned to the nearest -block the size of a page. +per-block basis. This allows for a lightweight hash computation on first read +into the page cache. Block hashes are stored linearly, aligned to the nearest +block size. Hash Tree --------- Each node in the tree is a cryptographic hash. If it is a leaf node, the hash -is of some block data on disk. If it is an intermediary node, then the hash is -of a number of child nodes. +of some data block on disk is calculated. If it is an intermediary node, +the hash of a number of child nodes is calculated. Each entry in the tree is a collection of neighboring nodes that fit in one block. The number is determined based on block_size and the size of the @@ -110,63 +110,23 @@ alg = sha256, num_blocks = 32768, block_size = 4096 On-disk format ============== -Below is the recommended on-disk format. The verity kernel code does not -read the on-disk header. It only reads the hash blocks which directly -follow the header. It is expected that a user-space tool will verify the -integrity of the verity_header and then call dmsetup with the correct -parameters. Alternatively, the header can be omitted and the dmsetup -parameters can be passed via the kernel command-line in a rooted chain -of trust where the command-line is verified. +The verity kernel code does not read the verity metadata on-disk header. +It only reads the hash blocks which directly follow the header. +It is expected that a user-space tool will verify the integrity of the +verity header. -The on-disk format is especially useful in cases where the hash blocks -are on a separate partition. The magic number allows easy identification -of the partition contents. Alternatively, the hash blocks can be stored -in the same partition as the data to be verified. In such a configuration -the filesystem on the partition would be sized a little smaller than -the full-partition, leaving room for the hash blocks. - -struct superblock { - uint8_t signature[8] - "verity\0\0"; - - uint8_t version; - 1 - current format - - uint8_t data_block_bits; - log2(data block size) - - uint8_t hash_block_bits; - log2(hash block size) - - uint8_t pad1[1]; - zero padding - - uint16_t salt_size; - big-endian salt size - - uint8_t pad2[2]; - zero padding - - uint32_t data_blocks_hi; - big-endian high 32 bits of the 64-bit number of data blocks - - uint32_t data_blocks_lo; - big-endian low 32 bits of the 64-bit number of data blocks - - uint8_t algorithm[16]; - cryptographic algorithm - - uint8_t salt[384]; - salt (the salt size is specified above) - - uint8_t pad3[88]; - zero padding to 512-byte boundary -} +Alternatively, the header can be omitted and the dmsetup parameters can +be passed via the kernel command-line in a rooted chain of trust where +the command-line is verified. Directly following the header (and with sector number padded to the next hash block boundary) are the hash blocks which are stored a depth at a time (starting from the root), sorted in order of increasing index. +The full specification of kernel parameters and on-disk metadata format +is available at the cryptsetup project's wiki page + http://code.google.com/p/cryptsetup/wiki/DMVerity + Status ====== V (for Valid) is returned if every check performed so far was valid. @@ -174,21 +134,22 @@ If any check failed, C (for Corruption) is returned. Example ======= - -Setup a device: - dmsetup create vroot --table \ - "0 2097152 "\ - "verity 1 /dev/sda1 /dev/sda2 4096 4096 2097152 1 "\ +Set up a device: + # dmsetup create vroot --readonly --table \ + "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ "1234000000000000000000000000000000000000000000000000000000000000" A command line tool veritysetup is available to compute or verify -the hash tree or activate the kernel driver. This is available from -the LVM2 upstream repository and may be supplied as a package called -device-mapper-verity-tools: - git://sources.redhat.com/git/lvm2 - http://sourceware.org/git/?p=lvm2.git - http://sourceware.org/cgi-bin/cvsweb.cgi/LVM2/verity?cvsroot=lvm2 - -veritysetup -a vroot /dev/sda1 /dev/sda2 \ - 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 +the hash tree or activate the kernel device. This is available from +the cryptsetup upstream repository http://code.google.com/p/cryptsetup/ +(as a libcryptsetup extension). + +Create hash on the device: + # veritysetup format /dev/sda1 /dev/sda2 + ... + Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 + +Activate the device: + # veritysetup create vroot /dev/sda1 /dev/sda2 \ + 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 |