Re: [RFC PATCH 1/5] ima: extend clone() with IMA namespace support

From: Stefan Berger
Date: Fri Mar 09 2018 - 08:52:57 EST

On 03/08/2018 09:59 PM, Serge E. Hallyn wrote:
On Thu, Mar 08, 2018 at 09:04:52AM -0500, Stefan Berger wrote:
On 07/25/2017 04:46 PM, Serge E. Hallyn wrote:
On Tue, Jul 25, 2017 at 04:11:29PM -0400, Stefan Berger wrote:
On 07/25/2017 03:48 PM, Mimi Zohar wrote:
On Tue, 2017-07-25 at 12:08 -0700, James Bottomley wrote:
On Tue, 2017-07-25 at 14:04 -0500, Serge E. Hallyn wrote:
On Tue, Jul 25, 2017 at 11:49:14AM -0700, James Bottomley wrote:
On Tue, 2017-07-25 at 12:53 -0500, Serge E. Hallyn wrote:
On Thu, Jul 20, 2017 at 06:50:29PM -0400, Mehmet Kayaalp wrote:
From: Yuqiong Sun <suny@xxxxxxxxxx>

Add new CONFIG_IMA_NS config option. Let clone() create a new
IMA namespace upon CLONE_NEWNS flag. Add ima_ns data structure
in nsproxy. ima_ns is allocated and freed upon IMA namespace
creation and exit. Currently, the ima_ns contains no useful IMA
data but only a dummy interface. This patch creates the
framework for namespacing the different aspects of IMA (eg.
IMA-audit, IMA-measurement, IMA-appraisal).

Signed-off-by: Yuqiong Sun <suny@xxxxxxxxxx>

Changelog:
* Use CLONE_NEWNS instead of a new CLONE_NEWIMA flag
Hi,

So this means that every mount namespace clone will clone a new
IMA namespace. Is that really ok?
Based on what: space concerns (struct ima_ns is reasonably small)?
or whether tying it to the mount namespace is the correct thing to
do. On
Mostly the latter. The other would be not so much space concerns as
time concerns. Many things use new mounts namespaces, and we
wouldn't want multiple IMA calls on all file accesses by all of
those.

the latter, it does seem that this should be a property of either
the mount or user ns rather than its own separate ns. I could see
a use where even a container might want multiple ima keyrings
within the container (say containerised apache service with
multiple tenants), so instinct tells me that mount ns is the
correct granularity for this.
I wonder whether we could use echo 1 > /sys/kernel/security/ima/newns
as the trigger for requesting a new ima ns on the next
clone(CLONE_NEWNS).
I could go with that, but what about the trigger being installing or
updating the keyring? That's the only operation that needs namespace
separation, so on mount ns clone, you get a pointer to the old ima_ns
until you do something that requires a new key, which then triggers the
copy of the namespace and installing it?
It isn't just the keyrings that need to be namespaced, but the
measurement list and policy as well.

IMA-measurement, IMA-appraisal and IMA-audit are all policy based.

As soon as the namespace starts, measurements should be added to the
namespace specific measurement list, not it's parent.
Shouldn't it be both?

If not, then it seems to me this must be tied to user namespace.

IMA is about measuring things, logging what was executed, and
finally someone looking at the measurement log and detecting
'things'. So at least one attack that needs to be prevented is a
malicious person opening an IMA namespace, executing something
malicious, and not leaving any trace on the host because all the
logs went into the measurement list of the IMA namespace, which
disappeared. That said, I am wondering whether there has to be a
minimum set of namespaces (PID, UTS) providing enough 'isolation'
that someone may actually open an IMA namespace and run their code.
To avoid leaving no traces one could argue to implement recursive
logging, so something that is logged inside the namespace will be
detected in all parent containers up to the init_ima_ns (host)
because it's logged (and TPM extended) there as well. The challenge
with that is that logging costs memory and that can be abused as
well until the machine needs a reboot... I guess the solution could
be requesting an IMA namespace in one way or another but requiring
several other namespace flags in the clone() to actually 'get' it.
Jumping namespaces with setns() may have to be restricted as well
once there is an IMA namespace.
Wait. So if I create a new IMA namespace, the things I run in
that namespace are not subject to the parent namespace policy?
We'll let an IMA namespace set its own policy and rules in that
policy will decide whether the child namespaces' measurements will
also be logged. This is to avoid a potentially huge log on the host.
However, the activities of root in namespaces may need to be logged
independently of what the policy rules say so that root's activities
in the TCB will always be tracked also if he operates in a temporary
mount/IMA namespace pair (and sharing the rest of the namespaces
with the host).
Thanks, Stefan. Is there a particular paper where I can get a good
idea of what is and is not part of the goals and threat model here?
Yuqiong is publishing a paper in this area. I believe the conference is only later this year.

Our goals are to enable IMA measurements, appraisal, and auditing inside a container using namespaces. IMA measurements are about logging files that were read or executables that were started on a machine, appraisal is about locking down the machine and only allowing files to be accessed that have been properly signed. We certainly want to prevent the abuse of namespaces by users to do things that go undetected. A primary concern are activities of root in the TCB.

Support for IMA in namespaces should enable the following:

- IMA policy for container (similar to the host):
- there should be an initial default policy for every IMA namespace that measures activities inside the container
- the policy can be overwritten once with a user-defined policy that may activate appraisal

- IMA policy extensions due to namespacing:
- an IMA policy should allow rules that define whether activities in (all) child namespaces is to be measured (prevents huge logs on the host)
- to prevent root from spawning new IMA namespaces and doing things undetected in the TCB, all activities of root are (recursively) measured in all IMA namespaces independent of whether the policy enables logging in child namespaces

- IMA appraisal and keys:
- each IMA namespace should have its own keyring so that each container can have its files signed with different keys
- it should be possible to enforce that only certified keys are loaded onto a keyring, similar to .ima on the host

- IMA auditing:
- auditing should report activity in namespaces following the IMA policy; root's activities in containers should be audited

- TPM and measurements:
- The IMA namespace that holds the logs should be configurable to extend PCRs; since the single TPM of the host cannot be shared by containers, each IMA namespace would have to be associated with its own TPM instance (vTPM); measurement on the initial IMA namespace are extended into the hardware TPM asdone today


A concrete 'ab-use' case that we are trying to avoid is the following:
- a user creating a privileged container that shares the host's mount namespace: it would be unexpected if there is an IMA policy active on the host that enforces file appraisal but in this case the IMA policy is not enforced since a (hypothetical) IMA namespace of the host was not joined since only the mount namespace of the host was joined. Now we have two choices here: We tie the mount and IMA namespaces together via sinlge clone flag, as proposed, and joining the mount namespace automatically joins the associated IMA namespace (single setns()). Or we make user space responsible for it and say if a mount namespace is joined, find the associated IMA namespace (how to do that?), and join both of them (2 setns() calls). Well, I think the former would preferred over the latter.

Let's assume we tie MNT and IMA together, then there are other scenarios with switching through the other namespaces (UTS, PID, IPC, NET, USER, CGROUP). I am not sure what is supposed to happen other than logging the activity active in the current IMA namespace:

What should happen with IMA logging, appraisal, and auditing if we setns() through all available
- PID namespaces and send signals: log, appraise, and audit file activity following IMA policy
- IPC namespaces and send messages via IPC: same as for PID
- UTS namespaces and setting hostname: same as for PID
- NET namespaces and sending network traffic: same as for PID?
- CGROUP namespaces and configuring cgroups: same as for PID?
- USER: should now the keys of this USER namespace be active or the keys of the original user namespace used during the clone()? other than that, same as for PID?



My impression was that you are measuring things that are executed in an
effort to make sure that anything that can affect resource $x will
be at some point detectable. But if you allow containers (not in
user namespace) to evade the ima measurements that seems to undermine
that, so that must not be your goal. And even if you insist on a

Yes, we want to prevent 'abuse', especially by root. See above.

user namespace, if some resource is owned by $uid, then $uid can create
a new namespace, evade the detection, and run malicious code to affect
the resource.

Unless you're counting on the container runtime to set a proper new
ima policy? But if that's the case then you can't have every CLONE_NEWNS
start a new ima ns.

The container runtime will be able to overwrite a default IMA policy that is active upon spawning an IMA namespace. This policy has to be useful in so far that it must enable a gapless measurement chain and for example show keys that were loaded into keyrings or the IMA policy that was loaded.

Stefan


So I think I need to start from scratch.

thanks,
-serge
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