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diff --git a/Documentation/device-mapper/verity.txt b/Documentation/device-mapper/verity.txt new file mode 100644 index 00000000000..32e48797a14 --- /dev/null +++ b/Documentation/device-mapper/verity.txt @@ -0,0 +1,194 @@ +dm-verity +========== + +Device-Mapper's "verity" target provides transparent integrity checking of +block devices using a cryptographic digest provided by the kernel crypto API. +This target is read-only. + +Construction Parameters +======================= + <version> <dev> <hash_dev> <hash_start> + <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. + + 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. + + 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. + +<dev> + This is the device containing the 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 + 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. + +<data_block_size> + The block size on a data device. Each block corresponds to one digest on + the hash device. + +<hash_block_size> + The size of a hash block. + +<num_data_blocks> + The number of data blocks on the data device. Additional blocks are + inaccessible. You can place hashes to the same partition as data, in this + case hashes are placed after <num_data_blocks>. + +<hash_start_block> + This is the offset, in <hash_block_size>-blocks, from the start of hash_dev + to the root block of the hash tree. + +<algorithm> + The cryptographic hash algorithm used for this device. This should + be the name of the algorithm, like "sha1". + +<digest> + The hexadecimal encoding of the cryptographic hash of the root hash block + and the salt. This hash should be trusted as there is no other authenticity + beyond this point. + +<salt> + The hexadecimal encoding of the salt value. + +Theory of operation +=================== + +dm-verity is meant to be setup 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). + +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 +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. + +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. + +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 +selected cryptographic digest algorithm. The hashes are linearly-ordered in +this entry and any unaligned trailing space is ignored but included when +calculating the parent node. + +The tree looks something like: + +alg = sha256, num_blocks = 32768, block_size = 4096 + + [ root ] + / . . . \ + [entry_0] [entry_1] + / . . . \ . . . \ + [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] + / ... \ / . . . \ / \ + blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 + + +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 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 +} + +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. + +Status +====== +V (for Valid) is returned if every check performed so far was valid. +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 "\ + "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 |