![Mikulas Patocka](/assets/img/avatar_default.png)
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. Two checksum device formats are supported: version 0 which is already shipping in Chromium OS and version 1 which incorporates some improvements. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Mandeep Singh Baines <msb@chromium.org> Signed-off-by: Will Drewry <wad@chromium.org> Signed-off-by: Elly Jones <ellyjones@chromium.org> Cc: Milan Broz <mbroz@redhat.com> Cc: Olof Johansson <olofj@chromium.org> Cc: Steffen Klassert <steffen.klassert@secunet.com> Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
195 lines
6.7 KiB
Plaintext
195 lines
6.7 KiB
Plaintext
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
|