[RFC,v1,0/7] ima: get rid of hard dependency on SHA-1

Message

Nicolai Stange March 13, 2025, 5:33 p.m. UTC

Hi all,

if no SHA-1 implementation was available to the kernel, IMA init would
currently fail, rendering the whole subsystem unusable.

This patch series is an attempt to make SHA-1 availability non-mandatory
for IMA. The main motivation is that NIST announced to sunset SHA-1 by
2030 ([1]), whereby any attempt to instantiate it when booted in FIPS mode
would have to be made to fail with -ENOENT. As this does potentially have
an impact on lifetimes for FIPS certifications issued today, distros might
be interested in disabling SHA-1 downstream soon already.

Anyway, making IMA to work without a SHA-1 implementation available is not
so straightforward, mainly due to that established scheme to substitute
padded SHA-1 template hashes for PCR banks with unmapped/unavailable algos.
There is some userspace around expecting that existing behavior, e.g. the
ima_measurement command from ([2]), and breaking that in certain scenarios
is inevitable.

I tried to make it the least painful possible, and I think I arrived at
a not completely unreasonable solution in the end, but wouldn't be too
surprised if you had a different stance on that. So I would be curious
about your feedback on whether this is a route worth pursuing any further.
FWIW, the most controversial parts are probably
 - [1/7] ima: don't expose runtime_measurements for unsupported hashes
 - [6/7] ima: invalidate unsupported PCR banks once at first use

Note that I haven't tested this series thoroughly yet -- for the time being
I only ran a couple of brief smoke tests in a VM w/o a TPM  (w/ and w/o
SHA-1 disabled of course).

Many thanks!

Nicolai

[1] https://www.nist.gov/news-events/news/2022/12/nist-retires-sha-1-cryptographic-algorithm
[2] https://github.com/linux-integrity/ima-evm-utils.git

Nicolai Stange (7):
  ima: don't expose runtime_measurements for unsupported hashes
  ima: always create runtime_measurements sysfs file for ima_hash
  ima: move INVALID_PCR() to ima.h
  ima: track the set of PCRs ever extended
  tpm: enable bank selection for PCR extend
  ima: invalidate unsupported PCR banks once at first use
  ima: make SHA1 non-mandatory

 drivers/char/tpm/tpm-interface.c      | 29 +++++++++-
 drivers/char/tpm/tpm.h                |  3 +-
 drivers/char/tpm/tpm2-cmd.c           | 29 +++++++++-
 include/linux/tpm.h                   |  3 +
 security/integrity/ima/Kconfig        | 14 +++++
 security/integrity/ima/ima.h          |  9 +++
 security/integrity/ima/ima_crypto.c   | 83 ++++++++++++++++-----------
 security/integrity/ima/ima_fs.c       | 41 +++++++------
 security/integrity/ima/ima_policy.c   |  5 +-
 security/integrity/ima/ima_queue.c    | 26 ++++++++-
 security/integrity/ima/ima_template.c |  7 +++
 11 files changed, 190 insertions(+), 59 deletions(-)