Commit 18068bdd authored by Milan Broz's avatar Milan Broz Committed by Alasdair G Kergon

dm: verity fix documentation

Veritysetup is now part of cryptsetup package.
Remove on-disk header description (which is not parsed in kernel)
and point users to cryptsetup where it the format is documented.
Mention units for block size paramaters.
Fix target line specification and dmsetup parameters.
Signed-off-by: default avatarMilan Broz <mbroz@redhat.com>
Cc: stable@kernel.org
Signed-off-by: default avatarAlasdair G Kergon <agk@redhat.com>
parent b0239faa
...@@ -7,39 +7,39 @@ This target is read-only. ...@@ -7,39 +7,39 @@ This target is read-only.
Construction Parameters Construction Parameters
======================= =======================
<version> <dev> <hash_dev> <hash_start> <version> <dev> <hash_dev>
<data_block_size> <hash_block_size> <data_block_size> <hash_block_size>
<num_data_blocks> <hash_start_block> <num_data_blocks> <hash_start_block>
<algorithm> <digest> <salt> <algorithm> <digest> <salt>
<version> <version>
This is the version number of the on-disk format. This is the type of the on-disk hash format.
0 is the original format used in the Chromium OS. 0 is the original format used in the Chromium OS.
The salt is appended when hashing, digests are stored continuously and The salt is appended when hashing, digests are stored continuously and
the rest of the block is padded with zeros. the rest of the block is padded with zeros.
1 is the current format that should be used for new devices. 1 is the current format that should be used for new devices.
The salt is prepended when hashing and each digest is The salt is prepended when hashing and each digest is
padded with zeros to the power of two. padded with zeros to the power of two.
<dev> <dev>
This is the device containing the data the integrity of which needs to be This is the device containing data, the integrity of which needs to be
checked. It may be specified as a path, like /dev/sdaX, or a device number, checked. It may be specified as a path, like /dev/sdaX, or a device number,
<major>:<minor>. <major>:<minor>.
<hash_dev> <hash_dev>
This is the device that that supplies the hash tree data. It may be This is the device that supplies the hash tree data. It may be
specified similarly to the device path and may be the same device. If the 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 same device is used, the hash_start should be outside the configured
configured device size. dm-verity device.
<data_block_size> <data_block_size>
The block size on a data device. Each block corresponds to one digest on The block size on a data device in bytes.
the hash device. Each block corresponds to one digest on the hash device.
<hash_block_size> <hash_block_size>
The size of a hash block. The size of a hash block in bytes.
<num_data_blocks> <num_data_blocks>
The number of data blocks on the data device. Additional blocks are The number of data blocks on the data device. Additional blocks are
...@@ -65,7 +65,7 @@ Construction Parameters ...@@ -65,7 +65,7 @@ Construction Parameters
Theory of operation Theory of operation
=================== ===================
dm-verity is meant to be setup as part of a verified boot path. This dm-verity is meant to be set up as part of a verified boot path. This
may be anything ranging from a boot using tboot or trustedgrub to just 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). booting from a known-good device (like a USB drive or CD).
...@@ -73,20 +73,20 @@ When a dm-verity device is configured, it is expected that the caller ...@@ -73,20 +73,20 @@ When a dm-verity device is configured, it is expected that the caller
has been authenticated in some way (cryptographic signatures, etc). has been authenticated in some way (cryptographic signatures, etc).
After instantiation, all hashes will be verified on-demand during After instantiation, all hashes will be verified on-demand during
disk access. If they cannot be verified up to the root node of the 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 tree, the root hash, then the I/O will fail. This should detect
tampering with any data on the device and the hash data. tampering with any data on the device and the hash data.
Cryptographic hashes are used to assert the integrity of the device on a 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 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 into the page cache. Block hashes are stored linearly, aligned to the nearest
block the size of a page. block size.
Hash Tree Hash Tree
--------- ---------
Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 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 some data block on disk is calculated. If it is an intermediary node,
of a number of child nodes. the hash of a number of child nodes is calculated.
Each entry in the tree is a collection of neighboring nodes that fit in one 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 block. The number is determined based on block_size and the size of the
...@@ -110,63 +110,23 @@ alg = sha256, num_blocks = 32768, block_size = 4096 ...@@ -110,63 +110,23 @@ alg = sha256, num_blocks = 32768, block_size = 4096
On-disk format On-disk format
============== ==============
Below is the recommended on-disk format. The verity kernel code does not The verity kernel code does not read the verity metadata on-disk header.
read the on-disk header. It only reads the hash blocks which directly It only reads the hash blocks which directly follow the header.
follow the header. It is expected that a user-space tool will verify the It is expected that a user-space tool will verify the integrity of the
integrity of the verity_header and then call dmsetup with the correct verity header.
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 Alternatively, the header can be omitted and the dmsetup parameters can
are on a separate partition. The magic number allows easy identification be passed via the kernel command-line in a rooted chain of trust where
of the partition contents. Alternatively, the hash blocks can be stored the command-line is verified.
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 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 block boundary) are the hash blocks which are stored a depth at a time
(starting from the root), sorted in order of increasing index. (starting from the root), sorted in order of increasing index.
The full specification of kernel parameters and on-disk metadata format
is available at the cryptsetup project's wiki page
http://code.google.com/p/cryptsetup/wiki/DMVerity
Status Status
====== ======
V (for Valid) is returned if every check performed so far was valid. V (for Valid) is returned if every check performed so far was valid.
...@@ -174,21 +134,22 @@ If any check failed, C (for Corruption) is returned. ...@@ -174,21 +134,22 @@ If any check failed, C (for Corruption) is returned.
Example Example
======= =======
Set up a device:
Setup a device: # dmsetup create vroot --readonly --table \
dmsetup create vroot --table \ "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
"0 2097152 "\
"verity 1 /dev/sda1 /dev/sda2 4096 4096 2097152 1 "\
"4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
"1234000000000000000000000000000000000000000000000000000000000000" "1234000000000000000000000000000000000000000000000000000000000000"
A command line tool veritysetup is available to compute or verify A command line tool veritysetup is available to compute or verify
the hash tree or activate the kernel driver. This is available from the hash tree or activate the kernel device. This is available from
the LVM2 upstream repository and may be supplied as a package called the cryptsetup upstream repository http://code.google.com/p/cryptsetup/
device-mapper-verity-tools: (as a libcryptsetup extension).
git://sources.redhat.com/git/lvm2
http://sourceware.org/git/?p=lvm2.git Create hash on the device:
http://sourceware.org/cgi-bin/cvsweb.cgi/LVM2/verity?cvsroot=lvm2 # veritysetup format /dev/sda1 /dev/sda2
...
veritysetup -a vroot /dev/sda1 /dev/sda2 \ Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
Activate the device:
# veritysetup create vroot /dev/sda1 /dev/sda2 \
4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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