[RFC,0/8] ima: Integrate with digest_cache LSM

Message

Roberto Sassu Feb. 14, 2024, 2:35 p.m. UTC

From: Roberto Sassu <[e-mail]>

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/[e-mail]/


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).


This patch set has some prerequisites:
- KEYS: Introduce user asymmetric keys and signatures (PGP keys and sigs)
- security: Move IMA and EVM to the LSM infrastructure
- security: digest_cache LSM (+digest_cache_changed(), introduced later)


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 policies, to use digest caches from the
very beginning (it allows measurement/appraisal from the initial ram disk).

Patch 5 attaches the verification result of the digest list to the digest
cache being populated with that digest list.

Patch 6-7 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 8 detects digest cache changes and consequently resets the IMA
cached verification result.

Roberto Sassu (8):
  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 and digest_cache_appraise boot-time
    policies
  ima: Record IMA verification result of digest lists in digest cache
  ima: Use digest cache for measurement
  ima: Use digest cache for appraisal
  ima: Detect if digest cache changed since last measurement/appraisal

 Documentation/ABI/testing/ima_policy          |   6 +-
 .../admin-guide/kernel-parameters.txt         |  15 ++-
 security/integrity/ima/Kconfig                |  10 ++
 security/integrity/ima/ima.h                  |  24 +++-
 security/integrity/ima/ima_api.c              |  21 +++-
 security/integrity/ima/ima_appraise.c         |  33 +++--
 security/integrity/ima/ima_iint.c             |  14 ++-
 security/integrity/ima/ima_main.c             |  81 ++++++++++--
 security/integrity/ima/ima_policy.c           | 118 +++++++++++++++++-
 9 files changed, 285 insertions(+), 37 deletions(-)