ima: Introduce exec_tcb policy

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

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This patch introduces a new hard-coded policy to measure executable code:

dont_measure fsmagic=0x9fa0
dont_measure fsmagic=0x62656572
dont_measure fsmagic=0x64626720
dont_measure fsmagic=0x1cd1
dont_measure fsmagic=0x42494e4d
dont_measure fsmagic=0x73636673
dont_measure fsmagic=0xf97cff8c
dont_measure fsmagic=0x43415d53
dont_measure fsmagic=0x27e0eb
dont_measure fsmagic=0x63677270
dont_measure fsmagic=0x6e736673
measure func=MMAP_CHECK mask=MAY_EXEC
measure func=BPRM_CHECK mask=MAY_EXEC
measure func=MODULE_CHECK
measure func=FIRMWARE_CHECK
measure func=POLICY_CHECK
measure func=DIGEST_LIST_CHECK

It can be selected by specifying ima_policy=exec_tcb in the kernel command
line. Files in tmpfs are not excluded from measurement.

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>