[v8,0/7] ima: support fs-verity digests and signatures

Message

Mimi Zohar April 29, 2022, 11:25 a.m. UTC

Support for including fs-verity file digests and signatures in the IMA
measurement list as well as verifying the fs-verity file digest based
signatures was discussed prior to fs-verity being upstreamed[1,2].

Including fs-verity file digests and signatures in the IMA measurement
list need to be based on policy and be identifiable.  To address being
based on policy, a new policy rule option "digest_type=verity", applicable
to both "measure" and "appraise" policy rules, is defined.  To address
being identifiable, a new template field 'd-ngv2' and two new template
formats 'ima-ngv2' and 'ima-sigv2' are defined.

d-ngv2:  prefixes the digest type ("ima", "verity") to the digest
algorithm and digest.

ima-ngv2', ima-sigv2: templates with the new d-ngv2 field defined.

In addition the signatures stored in 'security.ima' xattr need to be
disambiguated.  So instead of directly signing the fs-verity digest, the
fs-verity digest is indirectly signed, by signing the hash of the new
ima_file_id structure data (signature version 3) containing the fs-verity
digest and other metadata.

New policy rule option:
appraise_type=sigv3: support for new IMA signature version 3


[1] https://events19.linuxfoundation.org/wp-content/uploads/2017/11/fs-verify_Mike-Halcrow_Eric-Biggers.pdf
[2] Documentation/filesystems/fsverity.rst

Changelog v8:
- Based on Eric Bigger's review, fixed the original 'd-ng' and the new
  DATA_FMT_DIGEST_WITH_TYPE_ALGO comments and documentation, moved the
  buffer measurement test to after fs-verity measurements, renamed
  appraise_type "sigv2" to "sigv3", require the new "digest_type=verity"
  policy option be specified prior to "appraise_type=sigv3", and updated
  the fs-verity documentation.

Changelog v7:
- Based on Stefan Berger's review, cleaned up code by defining an enum,
  removed unnecessary memcpy, fs-verity documentation suggestions.
- Add comment in ima_get_verify_digest() with explanation for always
  returning the fs-verity digest.

Changelog v6:
- As suggested by Eric Bigger's, instead of defining a new field to
  differentiate between IMA and fs-verity signatures, prepend the
  digest type to the digest field.
- Addressed Eric Bigger's comments: updated the patch description,
  corrected comment, squashed patches, fixed enumeration usage,and
  added assumption to fsverity_get_digest.
- Removed the now unnecessary IMA_VERITY_DIGEST flag
- Updated kernel-parameters.txt

Changelog v5:
- Define ima_max_digest_size struct, removing the locally defined versions.
- Don't overload the 'digest_type=verity' to imply a verity signature,
  but extend the 'appraise_type' policy rule option to define 'sigv3'.

Changelog v4:
- Based on Eric Bigger's signature verification concerns of replacing the
  contents of a file with the ima_file_id struct hash, require per policy
  rule signature versions.
- Addressed Eric Bigger's other comments.
- Added new audit messages "causes".
- Updated patch descriptions.

Changelog v3:
- Addressed Eric Bigger's comments: included Ack, incremented the
  signature format version, the crypto issues are generic and will be
  addressed by him separately.
- Addressed Vitaly Chikunov's comments: hard coded maximum digest size
  rather than using a flexible array, removed unnecessary assignment, and
  fixed comment to match variable name.
- Defined new "ima_max_digest_size" struct to avoid wrapping the
  "ima_digest_data" struct inside a function local structure or
  having to dynamically allocate it with enough memory for the specific
  hash algo size.

Changelog v2:
- Addressed Eric Bigger's comments: sign the hash of fsverity's digest
  and the digest's metadata, use match_string, use preferred function
  name fsverity_get_digest(), support including unsigned fs-verity's
  digests in the IMA measurement list.
- Remove signatures requirement for including fs-verity's file digests in
  the 'd-ng' field of the measurement list.

Changelog v1:
- Updated both fsverity and IMA documentation.
- Addressed both Eric Bigger's and Lakshmi's comments.

Mimi Zohar (7):
  ima: fix 'd-ng' comments and documentation
  ima: use IMA default hash algorithm for integrity violations
  fs-verity: define a function to return the integrity protected file
    digest
  ima: define a new template field named 'd-ngv2' and templates
  ima: permit fsverity's file digests in the IMA measurement list
  ima: support fs-verity file digest based version 3 signatures
  fsverity: update the documentation

 Documentation/ABI/testing/ima_policy          |  45 ++++++-
 .../admin-guide/kernel-parameters.txt         |   3 +-
 Documentation/filesystems/fsverity.rst        |  35 ++++--
 Documentation/security/IMA-templates.rst      |  11 +-
 fs/verity/Kconfig                             |   1 +
 fs/verity/fsverity_private.h                  |   7 --
 fs/verity/measure.c                           |  43 +++++++
 include/linux/fsverity.h                      |  18 +++
 security/integrity/digsig.c                   |   3 +-
 security/integrity/ima/ima_api.c              |  49 +++++++-
 security/integrity/ima/ima_appraise.c         | 114 +++++++++++++++++-
 security/integrity/ima/ima_main.c             |   2 +-
 security/integrity/ima/ima_policy.c           |  79 ++++++++++--
 security/integrity/ima/ima_template.c         |   4 +
 security/integrity/ima/ima_template_lib.c     |  97 ++++++++++++---
 security/integrity/ima/ima_template_lib.h     |   4 +
 security/integrity/integrity.h                |  27 ++++-
 17 files changed, 480 insertions(+), 62 deletions(-)