Construction Parameters
=======================
- <version> <dev> <hash_dev> <hash_start>
+ <version> <dev> <hash_dev>
<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.
+ This is the type of the on-disk hash 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.
+ 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.
+ 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
+ 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,
<major>:<minor>.
<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
- same device is used, the hash_start should be outside of the dm-verity
- configured device size.
+ same device is used, the hash_start should be outside the configured
+ dm-verity device.
<data_block_size>
- The block size on a data device. Each block corresponds to one digest on
- the hash device.
+ The block size on a data device in bytes.
+ Each block corresponds to one digest on the hash device.
<hash_block_size>
- The size of a hash block.
+ The size of a hash block in bytes.
<num_data_blocks>
The number of data blocks on the data device. Additional blocks are
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
booting from a known-good device (like a USB drive or CD).
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
+tree, the root hash, then the I/O will fail. This should detect
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.
+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 of some block data on disk. If it is an intermediary node, then the hash is
-of a number of child nodes.
+of some data block on disk is calculated. If it is an intermediary node,
+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
block. The number is determined based on block_size and the size of the
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 verity kernel code does not read the verity metadata 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.
-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
-}
+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.
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.
+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
======
V (for Valid) is returned if every check performed so far was valid.
Example
=======
-
-Setup a device:
- dmsetup create vroot --table \
- "0 2097152 "\
- "verity 1 /dev/sda1 /dev/sda2 4096 4096 2097152 1 "\
+Set up a device:
+ # dmsetup create vroot --readonly --table \
+ "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
"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
+the hash tree or activate the kernel device. This is available from
+the cryptsetup upstream repository http://code.google.com/p/cryptsetup/
+(as a libcryptsetup extension).
+
+Create hash on the device:
+ # veritysetup format /dev/sda1 /dev/sda2
+ ...
+ Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
+
+Activate the device:
+ # veritysetup create vroot /dev/sda1 /dev/sda2 \
+ 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
projects / firefly-linux-kernel-4.4.55.git / blobdiff