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

Nicolai Stange
2025-03-13T17:34:50Z

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(-)

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2.47.1