ima: Add enforce-evm and log-evm modes to strictly check EVM status

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

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IMA and EVM have been designed as two independent subsystems: the first for
checking the integrity of file data; the second for checking file metadata.
Making them independent allows users to adopt them incrementally.

The point of intersection is in IMA-Appraise, which calls evm_verifyxattr()
to ensure that security.ima wasn't modified during an offline attack. The
design choice, to ensure incremental adoption, was to continue appraisal
verification if evm_verifyxattr() returns INTEGRITY_UNKNOWN. This value is
returned when EVM is not enabled in the kernel configuration, or if the
HMAC key has not been loaded yet.

Although this choice appears legitimate, it might not be suitable for
hardened systems, where the administrator expects that access is denied if
there is any error. An attacker could intentionally delete the EVM keys
from the system and set the file digest in security.ima to the actual file
digest so that the final appraisal status is INTEGRITY_PASS.

This patch allows such hardened systems to strictly enforce an access
control policy based on the validity of signatures/HMACs, by introducing
two new values for the ima_appraise= kernel option: enforce-evm and
log-evm.

Cc: stable@vger.kernel.org
Fixes: 2fe5d6de ("ima: integrity appraisal extension")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>