Ubuntu Manpage: fsverity - userspace utility for fs-verity

NAME

       fsverity - userspace utility for fs-verity

SYNOPSIS

       fsverity digest [OPTION...]  FILE...
       fsverity dump_metadata [OPTION...]  TYPE FILE
       fsverity enable [OPTION...]  FILE
       fsverity measure FILE...
       fsverity sign [OPTION...]  FILE OUT_SIGFILE

DESCRIPTION

       fsverity  is  a  userspace  utility for fs-verity.  fs-verity is a Linux kernel filesystem
       feature that does transparent on-demand verification of the contents  of  read-only  files
       using Merkle trees.

       fsverity  can enable fs-verity on files, retrieve the digests of fs-verity files, and sign
       files for use with fs-verity (among other things).  fsverity’s  functionality  is  divided
       among various subcommands.

       This  manual  page  focuses  on  documenting  all  fsverity  subcommands and options.  For
       examples and more information about the fs-verity kernel feature, see  the  references  at
       the end of this page.

OPTIONS

       fsverity always accepts the following options:

       --help Show the help, for either one subcommand or for all subcommands.

       --version
              Show the version of fsverity-utils.

SUBCOMMANDS

fsverity digest [OPTION...] FILE...
Compute the fs-verity digest of the given file(s). This is mainly intended to used in
preparation for signing the digest. In some cases fsverity sign can be used instead to
digest and sign the file in one step.

Options accepted by fsverity digest:

--block-size=BLOCK_SIZE
The Merkle tree block size (in bytes) to use. This must be a power of 2 and at
least twice the size of the hash values.

Note that the Linux kernel implementations of fs-verity place further restrictions
on the Merkle tree block size. Linux v6.2 and earlier require that the Merkle tree
block size be equal to both the system page size and the filesystem block size.
These values are often 4096. Linux v6.3 and later are more flexible; they require
that the Merkle tree block size be a power of 2 that is greater than or equal to
1024 and less than or equal to the system page size and the filesystem block size.
The default value of this option is 4096.

--compact
When printing the file digest, only print the actual digest hex string; don’t print
the algorithm name and filename.

--for-builtin-sig
Format the file digest in a way that is compatible with the Linux kernel’s fs-
verity built-in signature verification support. This means formatting it as a
struct fsverity_formatted_digest. Use this option if you are using built-in
signatures but are not using fsverity sign to do the signing.

--hash-alg=HASH_ALG
The hash algorithm to use to build the Merkle tree. Valid options are sha256 and
sha512. Default is sha256.

--out-merkle-tree=FILE
Write the computed Merkle tree to the given file. The Merkle tree layout will be
the same as that used by the Linux kernel’s FS_IOC_READ_VERITY_METADATA ioctl.

Normally this option isn’t useful, but it can be needed in cases where the fs-
verity metadata needs to be consumed by something other than one of the native
Linux kernel implementations of fs-verity. This is not needed for file signing.

--out-descriptor=FILE
Write the computed fs-verity descriptor to the given file.

--salt=SALT
The salt to use in the Merkle tree, as a hex string. The salt is a value that is
prepended to every hashed block; it can be used to personalize the hashing for a
particular file or device. The default is no salt.

fsverity dump_metadata [OPTION...] TYPE FILE
Dump the fs-verity metadata of the given file. The file must have fs-verity enabled, and
the filesystem must support the FS_IOC_READ_VERITY_METADATA ioctl (it was added in Linux
v5.12). This subcommand normally isn’t useful, but it can be useful in cases where a
userspace server program is serving a verity file to a client which implements fs-verity
compatible verification.

TYPE may be “merkle_tree”, “descriptor”, or “signature”, indicating the type of metadata
to dump. “signature” refers to the built-in signature, if present; userspace-managed
signatures will not be included.

Options accepted by fsverity dump_metadata:

--length=LENGTH
Length in bytes to dump from the specified metadata item. Only accepted in
combination with --offset.

--offset=offset
Offset in bytes into the specified metadata item at which to start dumping. Only
accepted in combination with --length.

fsverity enable [OPTION...] FILE
Enable fs-verity on the specified file. This will only work if the filesystem supports
fs-verity.

Options accepted by fsverity enable:

--block-size=BLOCK_SIZE
Same as for fsverity digest.

--hash-alg=HASH_ALG
Same as for fsverity digest.

--salt=SALT
Same as for fsverity digest.

--signature=SIGFILE
Specifies the built-in signature to apply to the file. SIGFILE must be a file that
contains the signature in PKCS#7 DER format, e.g. as produced by the fsverity sign
command.

Note that this option is only needed if the Linux kernel’s fs-verity built-in
signature verification support is being used. It is not needed if the signatures
will be verified in userspace, as in that case the signatures should be stored
separately.

fsverity measure FILE...
Display the fs-verity digest of the given file(s). The files must have fs-verity enabled.
The output will be the same as fsverity digest with the appropriate parameters, but
fsverity measure will take constant time for each file regardless of the size of the file.

fsverity measure does not accept any options.

fsverity sign [OPTION...] FILE OUT_SIGFILE
Sign the given file for fs-verity, in a way that is compatible with the Linux kernel’s fs-
verity built-in signature verification support. The signature will be written to
OUT_SIGFILE in PKCS#7 DER format.

The private key can be specified either by key file or by PKCS#11 token. To use a key
file, provide --key and optionally --cert. To use a PKCS#11 token, provide
--pkcs11-engine, --pkcs11-module, --cert, and optionally --pkcs11-keyid. PKCS#11 token
support is unavailable when fsverity-utils was built with BoringSSL rather than OpenSSL.

fsverity sign should only be used if you need compatibility with fs-verity built-in
signatures. It is not the only way to do signatures with fs-verity. For more
information, see the fsverity-utils README.

Options accepted by fsverity sign:

--block-size=BLOCK_SIZE
Same as for fsverity digest.

--cert=CERTFILE
Specifies the file that contains the certificate, in PEM format. This option is
required if KEYFILE contains only the private key and not also the certificate, or
if a PKCS#11 token is used.

--hash-alg=HASH_ALG
Same as for fsverity digest.

--key=KEYFILE
Specifies the file that contains the private key, in PEM format. This option is
required when not using a PKCS#11 token.

--out-descriptor=FILE
Same as for fsverity digest.

--out-merkle-tree=FILE
Same as for fsverity digest.

--pkcs11-engine=SOFILE
Specifies the path to the OpenSSL PKCS#11 engine file. This typically will be a
path to the libp11 .so file. This option is required when using a PKCS#11 token.

--pkcs11-keyid=KEYID
Specifies the key identifier in the form of a PKCS#11 URI. If not provided, the
default key associated with the token is used. This option is only applicable when
using a PKCS#11 token.