ima: Introduce ima namespace

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I49KW1


CVE: NA

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IMA namespace wraps global ima resources in an abstraction, to enable ima
to work with the containers. Currently, ima namespace contains no useful
data but a dummy interface. IMA resources related to different aspects of
IMA, namely IMA-audit, IMA-measurement, IMA-appraisal will be added in the
following patches.

The way how ima namespace is created is analogous to the time namespace:
unshare(CLONE_NEWIMA) system call creates a new ima namespace but doesn't
assign it to the current process. All children of the process will be born
in the new ima namespace, or a process can use setns() system call to join
the new ima namespace. Call to clone3(CLONE_NEWIMA) system call creates a
new namespace, which the new process joins instantly.

This scheme, allows to configure the new ima namespace before any process
appears in it. If user initially unshares the new ima namespace, ima can
be configured using ima entries in the securityfs. If user calls clone3()
system call directly, the new ima namespace can be configured using clone
arguments. To allow this, new securityfs entries have to be added, and
structures clone_args and kernel_clone_args have to be extended.

Early configuration is crucial. The new ima polices must apply to the
first process in the new namespace, and the appraisal key has to be loaded
beforehand.

Add a new CONFIG_IMA_NS option to the kernel configuration, that enables
one to create a new IMA namespace. IMA namespace functionality is disabled
by default.

Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski@huawei.com>
Reviewed-by: Zhang Tianxing <zhangtianxing3@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>