From: Mimi Zohar <zohar@linux.ibm.com>
To: Roberto Sassu <roberto.sassu@huawei.com>,
Casey Schaufler <casey@schaufler-ca.com>,
"mjg59@google.com" <mjg59@google.com>
Cc: "linux-integrity@vger.kernel.org"
<linux-integrity@vger.kernel.org>,
"linux-security-module@vger.kernel.org"
<linux-security-module@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 04/11] ima: Move ima_reset_appraise_flags() call to post hooks
Date: Tue, 27 Apr 2021 12:03:58 -0400 [thread overview]
Message-ID: <821796ff548c58138be547e0e5f4d7ba432356a7.camel@linux.ibm.com> (raw)
In-Reply-To: <d783e2703248463f9af68e155ee65c38@huawei.com>
On Tue, 2021-04-27 at 15:57 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@linux.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@linux.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
>
> HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
> Managing Director: Li Peng, Li Jian, Shi Yanli
next prev parent reply other threads:[~2021-04-27 16:04 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-05 15:19 [PATCH v4 00/11] evm: Improve usability of portable signatures Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 01/11] evm: Execute evm_inode_init_security() only when an HMAC key is loaded Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 02/11] evm: Load EVM key in ima_load_x509() to avoid appraisal Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 04/11] ima: Move ima_reset_appraise_flags() call to post hooks Roberto Sassu
2021-03-05 17:30 ` Casey Schaufler
2021-04-26 19:49 ` Mimi Zohar
2021-04-27 9:25 ` Roberto Sassu
2021-04-27 15:34 ` Mimi Zohar
2021-04-27 15:57 ` Roberto Sassu
2021-04-27 16:03 ` Mimi Zohar [this message]
2021-04-27 16:39 ` Casey Schaufler
2021-04-27 16:48 ` Mimi Zohar
2021-04-28 7:48 ` Roberto Sassu
2021-04-28 15:35 ` Roberto Sassu
2021-04-30 18:00 ` Mimi Zohar
2021-05-03 7:41 ` Roberto Sassu
2021-05-03 13:21 ` Mimi Zohar
2021-03-05 15:19 ` [PATCH v4 05/11] evm: Introduce evm_status_revalidate() Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 06/11] evm: Ignore INTEGRITY_NOLABEL/INTEGRITY_NOXATTRS if conditions are safe Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 07/11] evm: Allow xattr/attr operations for portable signatures Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 08/11] evm: Allow setxattr() and setattr() for unmodified metadata Roberto Sassu
2021-03-25 10:53 ` Roberto Sassu
2021-03-25 12:13 ` Christian Brauner
2021-03-25 12:21 ` Christian Brauner
2021-03-25 12:40 ` Roberto Sassu
2021-03-25 12:39 ` Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 09/11] ima: Allow imasig requirement to be satisfied by EVM portable signatures Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 10/11] ima: Introduce template field evmsig and write to field sig as fallback Roberto Sassu
2021-03-05 15:19 ` [PATCH v4 11/11] ima: Don't remove security.ima if file must not be appraised Roberto Sassu
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