[PATCH v4 04/11] ima: Move ima_reset_appraise_flags() call to post hooks

[Roberto Sassu]

> From: Mimi Zohar [mailto:zohar at linux.ibm.com]
> Sent: Monday, April 26, 2021 9:49 PM
> 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.

Hi Mimi

one could argue why IMA/EVM should receive a special
treatment. I understand that this was a necessity without
LSM stacking. Now that LSM stacking is available, I don't
see any valid reason why IMA/EVM should not be managed
by the LSM infrastructure.

> 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.

IMA/EVM can be easily disabled in other ways, for example
by moving the IMA policy or the EVM keys elsewhere.

Also other LSMs rely on a dynamic and persistent state
(for example for file transitions in SELinux), which cannot be
trusted anymore if LSMs are even temporarily disabled.

If IMA/EVM have to be enabled to prevent misconfiguration,
I think the same can be achieved if they are full LSMs, for
example by preventing that the list of enabled LSMs changes
at run-time.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli