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This allows platforms that are CPU/memory contrained to verify data blocks only the first time they are read from the data device, rather than every time. As such, it provides a reduced level of security because only offline tampering of the data device's content will be detected, not online tampering. Hash blocks are still verified each time they are read from the hash device, since verification of hash blocks is less performance critical than data blocks, and a hash block will not be verified any more after all the data blocks it covers have been verified anyway. This option introduces a bitset that is used to check if a block has been validated before or not. A block can be validated more than once as there is no thread protection for the bitset. These changes were developed and tested on entry-level Android Go devices. Signed-off-by: Patrik Torstensson <totte@google.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
220 lines
8.5 KiB
Plaintext
220 lines
8.5 KiB
Plaintext
dm-verity
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==========
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Device-Mapper's "verity" target provides transparent integrity checking of
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block devices using a cryptographic digest provided by the kernel crypto API.
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This target is read-only.
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Construction Parameters
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=======================
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<version> <dev> <hash_dev>
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<data_block_size> <hash_block_size>
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<num_data_blocks> <hash_start_block>
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<algorithm> <digest> <salt>
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[<#opt_params> <opt_params>]
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<version>
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This is the type of the on-disk hash format.
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0 is the original format used in the Chromium OS.
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The salt is appended when hashing, digests are stored continuously and
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the rest of the block is padded with zeroes.
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1 is the current format that should be used for new devices.
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The salt is prepended when hashing and each digest is
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padded with zeroes to the power of two.
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<dev>
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This is the device containing data, the integrity of which needs to be
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checked. It may be specified as a path, like /dev/sdaX, or a device number,
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<major>:<minor>.
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<hash_dev>
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This is the device that supplies the hash tree data. It may be
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specified similarly to the device path and may be the same device. If the
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same device is used, the hash_start should be outside the configured
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dm-verity device.
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<data_block_size>
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The block size on a data device in bytes.
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Each block corresponds to one digest on the hash device.
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<hash_block_size>
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The size of a hash block in bytes.
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<num_data_blocks>
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The number of data blocks on the data device. Additional blocks are
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inaccessible. You can place hashes to the same partition as data, in this
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case hashes are placed after <num_data_blocks>.
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<hash_start_block>
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This is the offset, in <hash_block_size>-blocks, from the start of hash_dev
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to the root block of the hash tree.
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<algorithm>
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The cryptographic hash algorithm used for this device. This should
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be the name of the algorithm, like "sha1".
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<digest>
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The hexadecimal encoding of the cryptographic hash of the root hash block
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and the salt. This hash should be trusted as there is no other authenticity
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beyond this point.
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<salt>
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The hexadecimal encoding of the salt value.
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<#opt_params>
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Number of optional parameters. If there are no optional parameters,
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the optional paramaters section can be skipped or #opt_params can be zero.
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Otherwise #opt_params is the number of following arguments.
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Example of optional parameters section:
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1 ignore_corruption
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ignore_corruption
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Log corrupted blocks, but allow read operations to proceed normally.
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restart_on_corruption
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Restart the system when a corrupted block is discovered. This option is
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not compatible with ignore_corruption and requires user space support to
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avoid restart loops.
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ignore_zero_blocks
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Do not verify blocks that are expected to contain zeroes and always return
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zeroes instead. This may be useful if the partition contains unused blocks
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that are not guaranteed to contain zeroes.
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use_fec_from_device <fec_dev>
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Use forward error correction (FEC) to recover from corruption if hash
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verification fails. Use encoding data from the specified device. This
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may be the same device where data and hash blocks reside, in which case
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fec_start must be outside data and hash areas.
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If the encoding data covers additional metadata, it must be accessible
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on the hash device after the hash blocks.
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Note: block sizes for data and hash devices must match. Also, if the
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verity <dev> is encrypted the <fec_dev> should be too.
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fec_roots <num>
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Number of generator roots. This equals to the number of parity bytes in
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the encoding data. For example, in RS(M, N) encoding, the number of roots
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is M-N.
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fec_blocks <num>
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The number of encoding data blocks on the FEC device. The block size for
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the FEC device is <data_block_size>.
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fec_start <offset>
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This is the offset, in <data_block_size> blocks, from the start of the
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FEC device to the beginning of the encoding data.
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check_at_most_once
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Verify data blocks only the first time they are read from the data device,
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rather than every time. This reduces the overhead of dm-verity so that it
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can be used on systems that are memory and/or CPU constrained. However, it
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provides a reduced level of security because only offline tampering of the
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data device's content will be detected, not online tampering.
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Hash blocks are still verified each time they are read from the hash device,
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since verification of hash blocks is less performance critical than data
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blocks, and a hash block will not be verified any more after all the data
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blocks it covers have been verified anyway.
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Theory of operation
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===================
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dm-verity is meant to be set up as part of a verified boot path. This
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may be anything ranging from a boot using tboot or trustedgrub to just
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booting from a known-good device (like a USB drive or CD).
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When a dm-verity device is configured, it is expected that the caller
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has been authenticated in some way (cryptographic signatures, etc).
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After instantiation, all hashes will be verified on-demand during
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disk access. If they cannot be verified up to the root node of the
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tree, the root hash, then the I/O will fail. This should detect
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tampering with any data on the device and the hash data.
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Cryptographic hashes are used to assert the integrity of the device on a
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per-block basis. This allows for a lightweight hash computation on first read
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into the page cache. Block hashes are stored linearly, aligned to the nearest
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block size.
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If forward error correction (FEC) support is enabled any recovery of
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corrupted data will be verified using the cryptographic hash of the
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corresponding data. This is why combining error correction with
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integrity checking is essential.
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Hash Tree
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---------
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Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
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of some data block on disk is calculated. If it is an intermediary node,
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the hash of a number of child nodes is calculated.
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Each entry in the tree is a collection of neighboring nodes that fit in one
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block. The number is determined based on block_size and the size of the
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selected cryptographic digest algorithm. The hashes are linearly-ordered in
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this entry and any unaligned trailing space is ignored but included when
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calculating the parent node.
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The tree looks something like:
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alg = sha256, num_blocks = 32768, block_size = 4096
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[ root ]
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/ . . . \
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[entry_0] [entry_1]
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/ . . . \ . . . \
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[entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
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/ ... \ / . . . \ / \
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blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
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On-disk format
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==============
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The verity kernel code does not read the verity metadata on-disk header.
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It only reads the hash blocks which directly follow the header.
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It is expected that a user-space tool will verify the integrity of the
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verity header.
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Alternatively, the header can be omitted and the dmsetup parameters can
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be passed via the kernel command-line in a rooted chain of trust where
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the command-line is verified.
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Directly following the header (and with sector number padded to the next hash
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block boundary) are the hash blocks which are stored a depth at a time
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(starting from the root), sorted in order of increasing index.
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The full specification of kernel parameters and on-disk metadata format
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is available at the cryptsetup project's wiki page
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https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
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Status
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======
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V (for Valid) is returned if every check performed so far was valid.
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If any check failed, C (for Corruption) is returned.
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Example
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=======
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Set up a device:
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# dmsetup create vroot --readonly --table \
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"0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
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"4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
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"1234000000000000000000000000000000000000000000000000000000000000"
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A command line tool veritysetup is available to compute or verify
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the hash tree or activate the kernel device. This is available from
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the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
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(as a libcryptsetup extension).
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Create hash on the device:
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# veritysetup format /dev/sda1 /dev/sda2
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...
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Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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Activate the device:
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# veritysetup create vroot /dev/sda1 /dev/sda2 \
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4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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