ima: Add support for measurement with digest lists

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

-------------------------------------------------

IMA-Measure creates a new measurement entry every time a file is measured,
unless the same entry is already in the measurement list.

This patch introduces a new type of measurement list, recognizable by the
PCR number specified with the new ima_digest_list_pcr= kernel option. This
type of measurement list includes measurements of digest lists and files
not found in those lists.

The benefit of this patch is the availability of a predictable PCR that
can be used to seal data or TPM keys to the OS software. Unlike standard
measurements, digest list measurements only indicate that files with a
digest in those lists could have been accessed, but not if and when. With
standard measurements, however, the chosen PCR is unlikely predictable.

Both standard and digest list measurements can be generated at the same
time by adding '+' as a prefix to the value of ima_digest_list_pcr=
(example: with ima_digest_list_pcr=+11, IMA generates standard measurements
with PCR 10 and digest list measurements with PCR 11).

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>