[RFC][PATCH v2 0/9] ima: Integrate with digest_cache LSM

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

One of the IMA shortcomings over the years has been the availability of
reference digest values for appraisal. Recently, the situation improved
and some Linux distributions are including file signatures.

The digest_cache LSM takes a different approach. Instead of requiring
Linux distributions to include file signatures in their packages, it parses
the digests from signed RPM package headers and exposes an API for
integrity providers to query a digest.

That enables Linux distributions to immediately gain the ability to do
integrity checks with the existing packages, lowering the burden for
software vendors.

In addition, integrating IMA with the digest_cache LSMs has even more
benefits.

First, it allows generating a new-style masurement list including the RPM
package headers and the unknown files, which improves system performance
due to the lower usage of the TPM. The cost is the less accuracy of the
information reported, which might not suitable for everyone.

Second, performance improve for appraisal too. It has been found that
verifying the signatures of only the RPM package headers and doing a digest
lookup is much less computationally expensive than verifying individual
file signatures.

For reference, a preliminary performance evaluation has been published
here:

https://lore.kernel.org/linux-integrity/20240415142436.2545003-15-roberto.sassu@xxxxxxxxxxxxxxx/


Third, it makes a PCR predictable and suitable for TPM key sealing
policies.

Finally, it allows IMA to maintain a predictable PCR and to perform
appraisal from the very beginning of the boot, in the initial ram disk
(of course, it won't recognize automatically generated files, that don't
exist in the RPM packages).


Integration of IMA with the digest_cache LSM is straightforward.

Patch 1 lets IMA know when the digest_cache LSM is reading a digest list,
to populate a digest cache.

Patch 2 allows nested IMA verification of digest lists read by the
digest_cache LSM.

Patch 3 allows the usage of digest caches with the IMA policy.

Patch 4 introduces new boot-time built-in policies, to use digest caches
from the very beginning (it allows measurement/appraisal from the initial
ram disk).

Patch 5 modifies existing boot-time built-in policies if the digest_cache
LSM-specific policies have been selected at boot.

Patch 6 attaches the verification result of the digest list to the digest
cache being populated with that digest list.

Patch 7-8 enable the usage of digest caches respectively for measurement
and appraisal, at the condition that it is authorized with the IMA policy
and that the digest list itself was measured and appraised too.

Patch 9 subscribes to digest cache events and invalidates cached integrity
results on digest cache reset (file or directory modification).

Open points:
- Mimi prefers to extend flags in ima_iint_cache, rather than passing the
  parameter down to process_measurement() - will do in a next version
- Prefetching of digest lists should not be done if there is no
  measurement rule (not relevant for appraisal)

Changelog

v1:
- Change digest_cache= policy keyword value from 'content' to 'data'
  (suggested by Mimi)
- Move digest_cache LSM integration code to ima_digest_cache.c (suggested
  by Mimi)
- Don't store digest cache pointer in integrity metadata
- Rename 'digest_cache_mask' parameter of ima_get_action() and
  ima_match_policy() to 'digest_cache_usage'
- Rename 'digest_cache_mask' parameter of ima_store_measurement() and
  ima_appraise_measurement() to 'allowed_usage'
- Try digest cache method as first in ima_appraise_measurement() (suggested
  by Mimi)
- Introduce ima_digest_cache_change() to be called on digest cache reset
- Subscribe to digest cache events
- Add forgotten modification in ima_iint_lockdep_annotate() (reported by
  Mimi)
- Replace 'digest_cache_mask' member of the ima_rule_entry structure with
  'digest_cache_usage' (suggested by Mimi)
- Split patch introducing digest_cache LSM-specific boot-time built-in
  policies and modifying existing rules
- Add digest_cache LSM-specific boot-time built-in policies if the
  digest_cache LSM is enabled in the kernel configuration
- Rename IMA_DIGEST_CACHE_MEASURE_CONTENT and
  IMA_DIGEST_CACHE_APPRAISE_CONTENT to IMA_DIGEST_CACHE_MEASURE_DATA and
  IMA_DIGEST_CACHE_APPRAISE_DATA

Roberto Sassu (9):
  ima: Introduce hook DIGEST_LIST_CHECK
  ima: Nest iint mutex for DIGEST_LIST_CHECK hook
  ima: Add digest_cache policy keyword
  ima: Add digest_cache_measure/appraise boot-time built-in policies
  ima: Modify existing boot-time built-in policies with digest cache
    policies
  ima: Store allowed usage in digest cache based on integrity metadata
    flags
  ima: Use digest caches for measurement
  ima: Use digest caches for appraisal
  ima: Register to the digest_cache LSM notifier and process events

 Documentation/ABI/testing/ima_policy          |   6 +-
 .../admin-guide/kernel-parameters.txt         |  15 ++-
 security/integrity/ima/Kconfig                |  10 ++
 security/integrity/ima/Makefile               |   1 +
 security/integrity/ima/ima.h                  |  22 +++-
 security/integrity/ima/ima_api.c              |  21 ++-
 security/integrity/ima/ima_appraise.c         |  32 +++--
 security/integrity/ima/ima_digest_cache.c     | 123 ++++++++++++++++++
 security/integrity/ima/ima_digest_cache.h     |  36 +++++
 security/integrity/ima/ima_iint.c             |  17 ++-
 security/integrity/ima/ima_main.c             |  46 +++++--
 security/integrity/ima/ima_policy.c           | 122 ++++++++++++++++-
 12 files changed, 413 insertions(+), 38 deletions(-)
 create mode 100644 security/integrity/ima/ima_digest_cache.c
 create mode 100644 security/integrity/ima/ima_digest_cache.h

-- 
2.34.1