[v5,0/8] ima: support fs-verity digests and signatures

Message

Mimi Zohar Feb. 11, 2022, 9:43 p.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, both based on IMA policy rules, was discussed prior to
fs-verity being upstreamed[1,2].

Support for including fs-verity file digests in the 'd-ng' template
field is based on a new policy rule option named 'digest_type=verity'.
A new template field named 'd-type' as well as a new template named
'ima-ngv2' are defined to differentiate between the regular IMA file
hashes from the fs-verity file digests (tree-hash based file hashes)
stored in the 'd-ng' template field of the measurement list.

A new signature version (v3) is defined as a hash of the 'ima_file_id'
struct, to disambiguate the signatures stored as 'security.ima' xattr.
The policy rule 'appraise_type=' option is extended to support 'sigv3',
which is initially limited to fs-verity.

The fs-verity 'appraise' rules are identified by the 'digest-type=verity'
option and require the 'appraise_type=sigv3' option.

Lastly, for IMA to differentiate between the original IMA signature
from an fs-verity signature a new 'xattr_type' named IMA_VERITY_DIGSIG
is defined.


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

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 (8):
  ima: rename IMA_ACTION_FLAGS to IMA_NONACTION_FLAGS
  ima: define ima_max_digest_data struct without a flexible array
    variable
  fs-verity: define a function to return the integrity protected file
    digest
  ima: define a new template field 'd-type' and a new template
    'ima-ngv2'
  ima: permit fsverity's file digests in the IMA measurement list
  ima: define signature version 3
  ima: support fs-verity file digest based version 3 signatures
  fsverity: update the documentation

 Documentation/ABI/testing/ima_policy      |  30 +++++-
 Documentation/filesystems/fsverity.rst    |  22 +++--
 Documentation/security/IMA-templates.rst  |  11 ++-
 fs/verity/Kconfig                         |   1 +
 fs/verity/fsverity_private.h              |   7 --
 fs/verity/measure.c                       |  41 ++++++++
 include/linux/fsverity.h                  |  18 ++++
 security/integrity/digsig.c               |   3 +-
 security/integrity/ima/ima_api.c          |  49 ++++++++--
 security/integrity/ima/ima_appraise.c     | 112 +++++++++++++++++++++-
 security/integrity/ima/ima_init.c         |   5 +-
 security/integrity/ima/ima_main.c         |   7 +-
 security/integrity/ima/ima_policy.c       |  66 +++++++++++--
 security/integrity/ima/ima_template.c     |   3 +
 security/integrity/ima/ima_template_lib.c |  28 +++++-
 security/integrity/ima/ima_template_lib.h |   2 +
 security/integrity/integrity.h            |  39 +++++++-
 17 files changed, 388 insertions(+), 56 deletions(-)