[PATCH v8 19/24] ima: Move to LSM infrastructure

[Casey Schaufler]

On 12/26/2023 10:14 AM, Mimi Zohar wrote:
> On Thu, 2023-12-14 at 18:08 +0100, Roberto Sassu wrote:
>> From: Roberto Sassu <roberto.sassu at huawei.com>
>>
>> Move hardcoded IMA function calls (not appraisal-specific functions) from
>> various places in the kernel to the LSM infrastructure, by introducing a
>> new LSM named 'ima' (at the end of the LSM list and always enabled like
>> 'integrity').
>>
>> Having IMA before EVM in the Makefile is sufficient to preserve the
>> relative order of the new 'ima' LSM in respect to the upcoming 'evm' LSM,
>> and thus the order of IMA and EVM function calls as when they were
>> hardcoded.
>>
>> Make moved functions as static (except ima_post_key_create_or_update(),
>> which is not in ima_main.c), and register them as implementation of the
>> respective hooks in the new function init_ima_lsm().
>>
>> A slight difference is that IMA and EVM functions registered for the
>> inode_post_setattr, inode_post_removexattr, path_post_mknod,
>> inode_post_create_tmpfile, inode_post_set_acl and inode_post_remove_acl
>> won't be executed for private inodes. Since those inodes are supposed to be
>> fs-internal, they should not be of interest of IMA or EVM. The S_PRIVATE
>> flag is used for anonymous inodes, hugetlbfs, reiserfs xattrs, XFS scrub
>> and kernel-internal tmpfs files.
>>
>> Conditionally register ima_post_path_mknod() if CONFIG_SECURITY_PATH is
>> enabled, otherwise the path_post_mknod hook won't be available.
> Up to this point, enabling CONFIG_SECURITY_PATH was not required.  By
> making it conditional on CONFIG_SECURITY_PATH, anyone enabling IMA will
> also need to enable CONFIG_SECURITY_PATH.  Without it, new files will
> not be tagged as a "new" file.
>
> Casey, Paul, how common is it today not to enable CONFIG_SECURITY_PATH?
> Will enabling it just for IMA be a problem?

Landlock, AppArmor and TOMOYO require it. Fedora enables Landlock and Ubuntu
enables AppArmor. I expect that, except for "minimal" distributions, you
won't get any push back. If a distribution is striving for minimal, it's not
going to use IMA.

It makes me wonder if eliminating CONFIG_SECURITY_PATH might not be a
rational alternative.