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

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Date:   Tue, 27 Apr 2021 12:03:58 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "mjg59@...gle.com" <mjg59@...gle.com>
Cc:     "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...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 Tue, 2021-04-27 at 15:57 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@...ux.ibm.com]
> > Sent: Tuesday, April 27, 2021 5:35 PM
> > On Tue, 2021-04-27 at 09:25 +0000, Roberto Sassu wrote:
> > > > From: Mimi Zohar [mailto:zohar@...ux.ibm.com]
> > > > Sent: Monday, April 26, 2021 9:49 PM
> > > > On Fri, 2021-03-05 at 09:30 -0800, Casey Schaufler wrote:
> > 
> > > > > 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.
> > 
> > It sholdn't be one way.  Are you willing to also make the existing
> > IMA/EVM hooks that are not currently security hooks, security hooks
> > too?   And accept any new IMA/EVM hooks would result in new security
> > hooks?  Are you also willing to add dependency tracking between LSMs?
> 
> I already have a preliminary branch where IMA/EVM are full LSMs.
> 
> Indeed, the biggest problem would be to have the new hooks
> accepted. I can send the patch set for evaluation to see what
> people think.

Defining new security hooks is pretty straight forward.   Perhaps at
least wait until Casey responds before posting the patches.

> 
> > > > 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.
> > 
> > Dynamically disabling IMA/EVM is very different than removing keys and
> > preventing the system from booting.  Restoring the keys should result
> > in being able to re-boot the system.  Re-enabling IMA/EVM, requires re-
> > labeling the filesystem in "fix" mode, which "blesses" any changes made
> > when IMA/EVM were not enabled.
> 
> Uhm, I thought that if you move the HMAC key for example
> and you boot the system, you invalidate all files that change,
> because the HMAC is not updated.

More likely you wouldn't be able to boot the system without the HMAC
key.

Mimi

> 
> > > 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.
> > 
> > Your argument is because this is a problem for SELinux, make it also a
> > problem for IMA/EVM too?!   ("Two wrongs make a right")
> 
> To me it seems reasonable to give the ability to people to
> disable the LSMs if they want to do so, and at the same time
> to try to prevent accidental disable when the LSMs should be
> enabled.
> 
> > > 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.
> > 
> > That ship sailed when "security=" was deprecated in favor of "lsm="
> > support, which dynamically enables/disables LSMs at runtime.
> 
> Maybe this possibility can be disabled with a new kernel option.
> I will think a more concrete solution.
> 
> Roberto
> 
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