This is fsverity, a userspace utility for fs-verity. fs-verity is
a Linux kernel feature that does transparent on-demand
integrity/authenticity verification of the contents of read-only
files, using a Merkle tree (hash tree) hidden after the end of the
file. The mechanism is similar to dm-verity, but implemented at the
file level rather than at the block device level. The fsverity
utility allows you to set up fs-verity protected files.
fs-verity will initially be supported by the ext4 and f2fs
filesystems, but it may later be supported by other filesystems too.
The fsverity utility uses the OpenSSL library, so you first must
install the needed development files. For example, on Debian-based
systems, run:
sudo apt-get install libssl-dev
OpenSSL must be version 1.0.0 or later.
Then, to build and install:
make
sudo make install
mkfs.f2fs -O verity /dev/vdc
mount /dev/vdc /vdc
cd /vdc
# Create a test file
head -c 1000000 /dev/urandom > file
md5sum file
# Append the Merkle tree and other metadata to the file:
fsverity setup file
# Enable fs-verity on the file
fsverity enable file
# Should show the same hash that 'fsverity setup' printed.
# This hash can be logged, or compared to a trusted value.
fsverity measure file
# Contents are now transparently verified and should match the
# original file contents, i.e. the metadata is hidden.
md5sum file
Note that in the above example, the file isn't signed. Therefore, to
get any authenticity protection (as opposed to just integrity
protection), the output of fsverity measure needs to be compared
against a trusted value.
With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, the filesystem supports
automatically verifying a signed file measurement that has been
included in the fs-verity metadata. The signature is verified against
the set of X.509 certificates that have been loaded into the
".fs-verity" kernel keyring. Here's an example:
# Generate a new certificate and private key:
openssl req -newkey rsa:4096 -nodes -keyout key.pem -x509 -out cert.pem
# Convert the certificate from PEM to DER format:
openssl x509 -in cert.pem -out cert.der -outform der
# Load the certificate into the fs-verity keyring:
keyctl padd asymmetric '' %keyring:.fs-verity < cert.der
# Optionally, lock the keyring so that no more keys can be added
# (requires keyctl v1.5.11 or later):
keyctl restrict_keyring %keyring:.fs-verity
# Optionally, require that all fs-verity files be signed:
sysctl fs.verity.require_signatures=1
# Now set up fs-verity on a test file:
md5sum file
fsverity setup file --signing-key=key.pem --signing-cert=cert.pem
fsverity enable file
md5sum file
By default, it's not required that fs-verity files have a signature.
This can be changed with sysctl fs.verity.require_signatures=1.
When set, it's guaranteed that the contents of every fs-verity file
has been signed by one of the certificates in the keyring.
Note: applications generally still need to check whether the file
they're accessing really is a fs-verity file, since an attacker could
replace a fs-verity file with a regular one.
IMA support for fs-verity is planned.
This project is provided under the terms of the GNU General Public
License, version 2; or at your option, any later version. A copy of the
GPLv2 can be found in the file named COPYING.
Permission to link to OpenSSL (libcrypto) is granted.
Send questions and bug reports to linux-fscrypt@vger.kernel.org.
Send patches to linux-fscrypt@vger.kernel.org. Patches should follow
the Linux kernel's coding style. Additionally, like the Linux kernel
itself, patches require the following "sign-off" procedure:
The sign-off is a simple line at the end of the explanation for the
patch, which certifies that you wrote it or otherwise have the right
to pass it on as an open-source patch. The rules are pretty simple:
if you can certify the below:
Developer's Certificate of Origin 1.1
By making a contribution to this project, I certify that:
(a) The contribution was created in whole or in part by me and I
have the right to submit it under the open source license
indicated in the file; or
(b) The contribution is based upon previous work that, to the best
of my knowledge, is covered under an appropriate open source
license and I have the right under that license to submit that
work with modifications, whether created in whole or in part
by me, under the same open source license (unless I am
permitted to submit under a different license), as indicated
in the file; or
(c) The contribution was provided directly to me by some other
person who certified (a), (b) or (c) and I have not modified
it.
(d) I understand and agree that this project and the contribution
are public and that a record of the contribution (including all
personal information I submit with it, including my sign-off) is
maintained indefinitely and may be redistributed consistent with
this project or the open source license(s) involved.
then you just add a line saying::
Signed-off-by: Random J Developer <random@developer.example.org>
using your real name (sorry, no pseudonyms or anonymous contributions.)