fsverity-utils

Introduction

This is fsverity-utils, a set of userspace utilities 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 hidden Merkle tree (hash tree) associated with the file. It is similar to dm-verity, but implemented at the file level rather than at the block device level. See the kernel documentation for more information about fs-verity, including which filesystems support it.

fsverity-utils currently contains just one program, fsverity. The fsverity program allows you to set up fs-verity protected files. In addition, the file digest computation and signing functionality of fsverity is optionally exposed through a C library libfsverity. See libfsverity.h for the API of this library.

Building and installing

To build fsverity-utils, first install the needed build dependencies. For example, on Debian-based systems, run:

    sudo apt-get install libssl-dev

OpenSSL must be version 1.0.0 or later. This is the only runtime dependency.

Then, to build and install fsverity-utils:

    make
    sudo make install

By default, the following targets are built and installed: the program fsverity, the static library libfsverity.a, the shared library libfsverity.so, and the manual page fsverity.1. You can also run make check to build and run the tests, or make help to display all available build targets.

By default, fsverity is statically linked to libfsverity. You can use make USE_SHARED_LIB=1 to use dynamic linking instead.

See the Makefile for other supported build and installation options.

Building on Windows

There is minimal support for building Windows executables using MinGW.

    make CC=x86_64-w64-mingw32-gcc

fsverity.exe will be built, and it supports the digest and sign commands.

A Windows build of OpenSSL/libcrypto needs to be available.

Examples

Full usage information for fsverity can be found in the manual page (man fsverity). Here, we just show some typical examples.

Basic use

    mkfs.ext4 -O verity /dev/vdc
    mount /dev/vdc /vdc
    cd /vdc

    # Create a test file
    head -c 1000000 /dev/urandom > file
    sha256sum file

    # Enable verity on the file
    fsverity enable file

    # Show the verity file digest
    fsverity measure file

    # File should still be readable as usual.  However, all data read
    # is now transparently checked against a hidden Merkle tree, whose
    # root hash is incorporated into the verity file digest.  Reads of
    # any corrupted parts of the data will fail.
    sha256sum 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 IMA

Since Linux v5.19, the kernel's IMA (Integrity Measurement Architecture) subsystem supports using fs-verity file digests in lieu of traditional file digests. This must be configured in the IMA policy. For more information, see the IMA documentation.

Using builtin signatures

First, note that fs-verity is essentially just a way of hashing a file; it doesn't mandate a specific way of handling signatures. There are several possible ways that signatures could be handled:

• Do it entirely in userspace
• Use IMA appraisal
• Use fs-verity built-in signatures

Any such solution needs two parts: (a) a policy that determines which files are required to have fs-verity enabled and have a valid signature, and (b) enforcement of the policy. Each part could happen either in a trusted userspace program(s) or in the kernel.

fs-verity built-in signatures (which are supported when the kernel was built with CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y) are a hybrid solution where the policy of which files are required to be signed is determined and enforced by a trusted userspace program, but the actual signature verification happens in the kernel. Specifically, with built-in signatures, the filesystem supports storing a signed file digest in each file's verity metadata. Before allowing access to the file, the filesystem will automatically verify the signature against the set of X.509 certificates in the ".fs-verity" kernel keyring. If set, the sysctl fs.verity.require_signatures=1 will make the kernel enforce that every verity file has a valid built-in signature.

fs-verity built-in signatures are primarily intended as a proof-of-concept; they reuse the kernel code that verifies the signatures of loadable kernel modules. This solution still requires a trusted userspace program to enforce that particular files have fs-verity enabled. Also, this solution uses PKCS#7 signatures, which are complex and prone to security bugs.

Thus, if possible one of the other solutions should be used instead. For example, the trusted userspace program could verify signatures itself, using a simple signature format using a modern algorithm such as Ed25519.

That being said, here are some examples of using built-in signatures:

    # 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 verity files be signed:
    sysctl fs.verity.require_signatures=1

    # Now set up fs-verity on a test file:
    sha256sum file
    fsverity sign file file.sig --key=key.pem --cert=cert.pem
    fsverity enable file --signature=file.sig
    rm -f file.sig
    sha256sum file

    # The digest to be signed can also be printed separately, hex
    # encoded, in case the integrated signing cannot be used:
    fsverity digest file --compact --for-builtin-sig | tr -d '\n' | xxd -p -r | openssl smime -sign -in /dev/stdin ...

Notices

fsverity-utils is provided under the terms of the MIT license. A copy of this license can be found in the file named LICENSE.

Send questions and bug reports to fsverity@lists.linux.dev.

Signed release tarballs for fsverity-utils can be found on kernel.org.

Contributing

Send patches to fsverity@lists.linux.dev with the additional tag fsverity-utils in the subject, i.e. [fsverity-utils PATCH]. Patches should follow the Linux kernel's coding style. A .clang-format file is provided to approximate this coding style; consider using git clang-format. 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.)