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DMVerity

Last edited by Milan Broz
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dm-verity: device-mapper block integrity checking target

About dm-verity

Device-mapper is infrastructure in the Linux kernel that provides a generic way to create virtual layers of block devices.

Device-mapper verity target provides read-only transparent integrity checking of block devices using kernel crypto API.

Basic documentation of dm-verity mapping table comes with kernel source and the latest version is available in git repository.

The dm-verity was designed and developed by Chrome OS authors for verified boot implementation.

To enable dm-verity support, enable CONFIG_DM_VERITY in Device Drivers/Multi-device support (RAID and LVM) configuration option.

To configure you need userspace components: device mapper library (part of LVM2 package) and veritysetup.

Device mapper verity table mapping line specification

Mapping table in device mapper is defined like <start_sector> <size> <target name> <target mapping table>

  • start_sector is 0 (for tables using only one mapped segment, iow table contains only one line)
  • size is size of device in sectors
  • target name is name of mapping target, here "verity" for dm-verity

Sectors are always 512B sectors (even if device has bigger hw sector like 4k).

Table fields are separated by space.

dm-verity target version

Every device-mapper target has internal version which is increased when some new feature is added.

To check which version you have installed, load the dm target module (dm-verity.ko for dm-verity) and use "dmsetup targets" to check version.

Mapping table for verity target

The basic syntax is common for all 1.x.y dm-verity target versions. If some extension was added later, it is mentioned in the description.

<ver> <data_dev> <hash_dev> <data_blk_size> <hash_blk_size> <#blocks> <hash_start> <alg> <digest> <salt> [<#opt_params> <opt_params>]

  • ver: on-disk hash version

    • 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 zeroes.

    • 1 is the current format that should be used for new devices.
      The salt is prepended when hashing and each digest is
      padded with zeroes to the power of two.

  • data_dev: 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: device that that supplies the hash tree data.
    It may be specified similarly to the data 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_blk_size: The block size in bytes on a data device. Each block corresponds to one digest on the hash device.

  • hash_blk_size: The size of a hash block in bytes.

  • #blocks: The number of data blocks on the data device. Additional blocks are inaccessible.

  • hash_start: offset, in hash_blk_size blocks, from the start of hash_device to the root block of the hash tree.

  • alg: The cryptographic hash algorithm used for this device. This should be the name of the algorithm, like "sha256".

  • digest: The hexadecimal encoding of the cryptographic hash of the root hash block and the salt.
    This hash should be trusted.

  • salt: The hexadecimal encoding of the salt value.

  • #opt_params: Number of optional parameters.
    If there are no optional parameters, the optional parameters section can be skipped or it can be zero.
    Otherwise it is the number of following arguments.
    Available since: 1.2.0 (no proper version set) (kernel 4.1)

Optional parameters

  • ignore_corruption: Log corrupted blocks, but allow read operations to proceed normally.
    Available since: 1.2.0 (no proper version set) (kernel 4.1)

  • restart_on_corruption: Restart the system when a corrupted block is discovered.
    This option is not compatible with ignore_corruption and requires user space support to avoid restart loops.
    Available since: 1.2.0 (no proper version set) (kernel 4.1)

  • ignore_zero_blocks: Do not verify blocks that are expected to contain zeroes and always return zeroes instead.
    This may be useful if the partition contains unused blocks that are not guaranteed to contain zeroes.
    Available since: 1.3.0 (kernel 4.5)

  • check_at_most_once: Verify data blocks only the first time they are read from the data device, rather than every time.
    It provides a reduced level of security because only offline tampering of the data device's content will be detected, not online tampering.
    Available since: 1.4.0 (kernel 4.17)

  • root_hash_sig_key_desc <key_description>: The PKCS7 signature to validate the root hash during the creation of the device.
    The key must be in trusted kernel keyring.
    Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG being set.
    Available since: 1.5.0 (kernel 5.4)

Optional FEC (Forward-Error-Correction) parameters

  • use_fec_from_device <fec_dev>: Use forward error correction (FEC) to recover from corruption if hash verification fails.
    Use encoding data from the specified device. This may be the same device where data and hash blocks reside,
    in which case fec_start must be outside data and hash areas.
    If the encoding data covers additional metadata, it must be accessible on the hash device after the hash blocks.
    Note: block sizes for data and hash devices must match. Also, if the verity <dev> is encrypted the <fec_dev> should be too.
    Available since: 1.3.0 (kernel 4.5)

  • fec_roots <num>: Number of generator roots. This equals to the number of parity bytes in the encoding data.
    For example, in RS(M, N) encoding, the number of roots is M-N.
    M is 255 and M-N is between 2 and 24 (including).
    RS decoder can correct up to <num>/2 bytes that contains error and <num> errasures in each codeword.
    Erasures are identified by hash information on hash_device.
    Available since: 1.3.0 (kernel 4.5)

  • fec_blocks <num>: The number of encoding data blocks on the FEC device.
    The block size for the FEC device is <data_block_size>.
    FEC is computed over: data blocks | hash blocks | metadata (optional).
    We allow metadata after hash blocks to support a use case where all data is stored on the same device and FEC covers the entire area.
    Available since: 1.3.0 (kernel 4.5)

  • fec_start <offset>: This is the offset, in <data_block_size> blocks, from the start of the FEC device to the beginning of the encoding data.
    Available since: 1.3.0 (kernel 4.5)

Example of full mapping table

0 417792 verity 1 /dev/sdb /dev/sdc 4096 4096 52224 1 sha256 2aa4f7b7b6...f4952060e8 762307f4bc8...d2a6b7595d8..
|    |     |    |     |     |        |    |    |    |    |              |                        |  
start|     |    |  data_dev |  data_block | #blocks | hash_alg      root_digest                salt  
     size  |  version    hash_dev         |     hash_offset  
         target                       hash_block

Configuration with dmsetup tool

Dmsetup is used to create and remove devices, get information about
devices or reload tables (that means changing the mapping while the device is in use).

Usually this tool is only used for low-level access to dm device,
example here is mentioned just to show how the low level parameters works.
Always prefer using veritysetup if possible.

To create device and specify mapping table, use dmsetup create <name> --table " ..." command.

The example above is then created using (after hash device was populated with hashes)

SIZE=$(blockdev --getsz /dev/sdb)  
dmsetup create name -r --table "0 $SIZE verity 1 /dev/sdb /dev/sdc 4096 4096 52224 1 sha256 <root_hash> <salt>"

You can check the full mapping table using dmsetup table.

Note that for all device-mapper operations is required root privilege (CAP_SYSADMIN).

The newly created device then appears as /dev/mapper/name.

Configuration using veritysetup

Preparing hash device (formatting)
veritysetup format /dev/sdb /dev/sdc

Activation of verity data device
veritysetup create name /dev/sdb /dev/sdc <root_digest>

Theory of operation

Dm-verity is meant to be setup as part of a verified boot path.

When a dm-verity device is configured, it is expected that the caller has been authenticated in some way. 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 size.

Hash Tree

Each node in the tree is a cryptographic hash. If it is a leaf node, the hash is calculated of some data block on disk. If it is an intermediary node, the hash is calculated 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

Verity superblock format

While the kernel dm-verity doesn't read any on-disk metadata stored information (except hash blocks itself), it is useful to store hash device metadata information permanently to disk.

The only trusted information is then root hash which is stored outside of data and hash verification device pair.

Verity uses superblock by default (real hash area is then starting on next hash block offset after verity superblock). You can always disable use of verity suberblock by use of --no-superblock option.

The current on-disk dm-verity specification (size of superblock is always 512 bytes):

  struct verity_sb {
  uint8_t  signature[8];    /* "verity\0\0" */
  uint32_t version;         /* superblock version, 1 */
  uint32_t hash_type;       /* 0 - Chrome OS, 1 - normal */
  uint8_t  uuid[16];        /* UUID of hash device */
  uint8_t  algorithm[32];   /* hash algorithm name */
  uint32_t data_block_size; /* data block in bytes */
  uint32_t hash_block_size; /* hash block in bytes */
  uint64_t data_blocks;     /* number of data blocks */
  uint16_t salt_size;       /* salt size */
  uint8_t  _pad1[6];
  uint8_t  salt[256];       /* salt */
  uint8_t  _pad2[168];
} __attribute__((packed));

The DMVerity page is written and maintained by Milan Broz (with help of other project users and developers).

Please use New issue report if there is some bug or problem on this page, thank you.