dm: add verity target

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

       ---help---

         A target that intermittently fails I/O for debugging purposes.

config DM_VERITY

	tristate "Verity target support (EXPERIMENTAL)"

	depends on BLK_DEV_DM && EXPERIMENTAL

	select CRYPTO

	select CRYPTO_HASH

	select DM_BUFIO

	---help---

	  This device-mapper target creates a read-only device that

	  transparently validates the data on one underlying device against

	  a pre-generated tree of cryptographic checksums stored on a second

	  device.

	  You'll need to activate the digests you're going to use in the

	  cryptoapi configuration.

	  To compile this code as a module, choose M here: the module will

	  be called dm-verity.

	  If unsure, say N.

endif # MD

obj-$(CONFIG_DM_ZERO)		+= dm-zero.o

obj-$(CONFIG_DM_RAID)	+= dm-raid.o

obj-$(CONFIG_DM_THIN_PROVISIONING)	+= dm-thin-pool.o

obj-$(CONFIG_DM_VERITY)		+= dm-verity.o

ifeq ($(CONFIG_DM_UEVENT),y)

dm-mod-objs			+= dm-uevent.o