linux-kernel - Re: [PATCH v4 04/11] ima: Move ima_reset_appraise

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Date:   Mon, 26 Apr 2021 15:49:00 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Casey Schaufler <casey@...aufler-ca.com>,
        Roberto Sassu <roberto.sassu@...wei.com>, mjg59@...gle.com
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 04/11] ima: Move ima_reset_appraise_flags() call to
 post hooks

On Fri, 2021-03-05 at 09:30 -0800, Casey Schaufler wrote:
> On 3/5/2021 7:19 AM, Roberto Sassu wrote:
> > ima_inode_setxattr() and ima_inode_removexattr() hooks are called before an
> > operation is performed. Thus, ima_reset_appraise_flags() should not be
> > called there, as flags might be unnecessarily reset if the operation is
> > denied.
> >
> > This patch introduces the post hooks ima_inode_post_setxattr() and
> > ima_inode_post_removexattr(), and adds the call to
> > ima_reset_appraise_flags() in the new functions.
> 
> I don't see anything wrong with this patch in light of the way
> IMA and EVM have been treated to date.
> 
> However ...
> 
> The special casing of IMA and EVM in security.c is getting out of
> hand, and appears to be unnecessary. By my count there are 9 IMA
> hooks and 5 EVM hooks that have been hard coded. Adding this IMA
> hook makes 10. It would be really easy to register IMA and EVM as
> security modules. That would remove the dependency they currently
> have on security sub-system approval for changes like this one.
> I know there has been resistance to "IMA as an LSM" in the past,
> but it's pretty hard to see how it wouldn't be a win.

Somehow I missed the new "lsm=" boot command line option, which
dynamically allows enabling/disabling LSMs, being upstreamed.  This
would be one of the reasons for not making IMA/EVM full LSMs.

Both IMA and EVM file data/metadata is persistent across boots.  If
either one or the other is not enabled the file data hash or file
metadata HMAC will not properly be updated, potentially preventing the
system from booting when re-enabled.  Re-enabling IMA and EVM would
require "fixing" the mutable file data hash and HMAC, without any
knowledge of what the "fixed" values should be.  Dave Safford referred
to this as "blessing" the newly calculated values.

Mimi