[PATCH 00/12] ima: measure digest lists instead of individual files

[Roberto Sassu]

This patch set applies on top of kernel v4.13-rc2.

IMA, for each file matching policy rules, calculates a digest, creates
a new entry in the measurement list and extends a TPM PCR with the digest
of entry data. The last step causes a noticeable performance reduction.

Since systems likely access the same files, repeating the above tasks at
every boot can be avoided by replacing individual measurements of likely
accessed files with only one measurement of their digests: the advantage
is that the system performance significantly improves due to less PCR
extend operations; on the other hand, the information about which files
have exactly been accessed and in which sequence is lost.

If this new measurement reports only good digests (e.g. those of
files included in a Linux distribution), and if verifiers only check
that a system executed good software and didn't access malicious data,
the disadvantages reported earlier would be acceptable.

The Trusted Computing paradigm measure & load is still respected by IMA
with the proposed optimization. If a file being accessed is not in a
measured digest list, a measurement will be recorded as before. If it is,
the list has already been measured, and the verifier must assume that
files with digest in the list have been accessed.

Measuring digest lists gives the following benefits:

- boot time reduction
  For a minimal Linux installation with 1400 measurements, the boot time
  decreases from 1 minute 30 seconds to 15 seconds, after loading to IMA
  the digest of all files packaged by the distribution (32000). The new
  list contains 92 entries. Without IMA, the boot time is 8.5 seconds.

- lower network and CPU requirements for remote attestation
  With the IMA optimization, both the measurement and digest lists
  must be verified for a complete evaluation. However, since the lists
  are fixed, they could be sent to and checked by the verifier only once.
  Then, during a remote attestation, the only remaining task is to verify
  the short measurement list.

- signature-based remote attestation
  Digest list signature can be used as a proof of the provenance for the
  files whose digest is in the list. Then, if verifiers trust the signer
  and only check provenance, remote attestation verification would simply
  consist on checking digest lists signatures and that the measurement
  list only contain list metadata digests (reference measurement databases
  would be no longer required). An example of a signed digest list,
  that can be parsed with this patch set, is the RPM package header.

Digest lists are loaded in two stages by IMA through the new securityfs
interface called 'digest_lists'. Users supply metadata, for the digest
lists they want to load: path, format, digest, signature and algorithm
of the digest.

Then, after the metadata digest is added to the measurement list, IMA
reads the digest lists at the path specified and loads the digests in
a hash table (digest lists are not measured, since their digest is already
included in the metadata). With metadata measurement instead of digest list
measurement, it is possible to avoid a performance reduction that would
occur by measuring many digest lists (e.g. RPM headers) individually.
If, alternatively, digest lists are loaded together, their signature
cannot be verified.

Lastly, when a file is accessed, IMA searches the calculated digest in
the hash table. Only if the digest is not found a new entry is added
to the measurement list.


Roberto Sassu (12):
  ima: generalize ima_read_policy()
  ima: generalize ima_write_policy()
  ima: generalize policy file operations
  ima: use ima_show_htable_value to show hash table data
  ima: add functions to manage digest lists
  ima: added parser of digest lists metadata
  ima: added parser for compact digest list
  ima: added parser for RPM data type
  ima: introduce securityfs interfaces for digest lists
  ima: disable digest lookup if digest lists are not measured
  ima: don't report measurements if digests are included in the loaded
    lists
  ima: added Documentation/security/IMA-digest-lists.txt

 Documentation/security/IMA-digest-lists.txt | 150 +++++++++++++++++
 include/linux/fs.h                          |   1 +
 security/integrity/ima/Kconfig              |  11 ++
 security/integrity/ima/Makefile             |   1 +
 security/integrity/ima/ima.h                |  17 ++
 security/integrity/ima/ima_digest_list.c    | 247 ++++++++++++++++++++++++++++
 security/integrity/ima/ima_fs.c             | 178 ++++++++++++--------
 security/integrity/ima/ima_main.c           |  23 ++-
 security/integrity/ima/ima_policy.c         |   1 +
 security/integrity/ima/ima_queue.c          |  39 +++++
 10 files changed, 602 insertions(+), 66 deletions(-)
 create mode 100644 Documentation/security/IMA-digest-lists.txt
 create mode 100644 security/integrity/ima/ima_digest_list.c

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2.9.3