Re: [PATCH v3 00/11] evm: Improve usability of portable signatures

Hi Roberto,

On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:

EVM portable signatures are particularly suitable for the protection of
metadata of immutable files where metadata is signed by a software vendor.
They can be used for example in conjunction with an IMA policy that
appraises only executed and memory mapped files.

The existing "appraise_tcb" builtin policy verify all root owned files.
Defining a new builtin policy to verify only executed and memory
mmapped files would make a nice addition and would probably simplify
testing.

However, some usability issues are still unsolved, especially when EVM is
used without loading an HMAC key. This patch set attempts to fix the open
issues.

We need regression tests for each of these changes.

To prevent affecting the running system, the appraise policy rules
could be limited to a loopback mounted filesystem. 

Patch 1 allows EVM to be used without loading an HMAC key. Patch 2 avoids
appraisal verification of public keys (they are already verified by the key
subsystem).

Loading the EVM key(s) occurs early, either the builtin x509 EVM key or
during the initramfs, makes testing difficult.  Based on
security/evm/evm, different tests could be defined for when only x509
keys, only HMAC key, or both EVM key types are loaded.

Patches 3-5 allow metadata verification to be turned off when no HMAC key
is loaded and to use this mode in a safe way (by ensuring that IMA
revalidates metadata when there is a change).

Patches 6-8 make portable signatures more usable if metadata verification
is not turned off, by ignoring the INTEGRITY_NOLABEL error when no HMAC key
is loaded, by accepting any metadata modification until signature
verification succeeds (useful when xattrs/attrs are copied sequentially
from a source) and by allowing operations that don't change metadata.

Patch 9 makes it possible to use portable signatures when the IMA policy
requires file signatures and patch 10 shows portable signatures in the
measurement list when the ima-sig template is selected.

ima-evm-utils needs to be updated to support EVM portable & immutable
signatures.

Lastly, patch 11 avoids undesired removal of security.ima when a file is
not selected by the IMA policy.

thanks,

Mimi