[RFC PATCH v2 1/3] ima: extend clone() with IMA namespace support

On Thu, 2018-03-15 at 14:26 -0400, Stefan Berger wrote:

On 03/15/2018 01:33 PM, James Bottomley wrote:

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On Thu, 2018-03-15 at 11:26 -0400, Stefan Berger wrote:

[...]

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IMA measures the files described by these paths. The files also
may hold signatures (security.ima xattr) needed for IMA
appraisal.

The xattr is an inode property, which isn't namespaced by the
mount_ns.

When we had this discussion last year, we talked about possibly
using the user_ns instead.??It makes sense because for IMA
signatures you're

'using the user_ns' I suppose means hooking IMA namespace to it...

Yes, making it belong to the user ns instead of the mnt ns.

I'm not saying it has to, but I equally don't see a good reason IMA has
to have its own namespace, especially as the keyrings are tied to the
user_ns.

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going to need some type of keyring namespace and there's already
one hanging off the user_ns:

commit f36f8c75ae2e7d4da34f4c908cebdb4aa42c977e
Author: David Howells [off-list ref]
Date:???Tue Sep 24 10:35:19 2013 +0100

?????KEYS: Add per-user_namespace registers for persistent per-UID
kerberos caches

The benefit for IMA would be that this would then tie the keys needed
for appraising to the IMA namespace's policy.
However, if you have an appraise policy in your IMA namespace, which
is now hooked to the user namespace, and you join that user namespace
but your files don't have signatures, nothing will execute anymore.
That's now a side effect of joining this user namespace unless we
have a magic ?exception. My feeling is, people may not like that...

Agree, but I think the magic might be to populate the ima keyring with
the parent on user_ns creation. ?That way the user_ns owner can delete
the parent keys if they don't like them, but by default the parent
appraisal policy should just work.

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I saw that Serge even recently mentioned that you need to take
this aspect of the changes back to the drawing board.??With my
namespace maintainer hat on I repeat that.

Drawing board is here now (tuning on the text...):

http://kernsec.org/wiki/index.php/IMA_Namespacing_design_consider
ations

You mention an abuse case here which is basically a way of relaxing
security policy.??Cannot we fix that by making policy hierarchical,
so a child namespace must have the same or a more strict policy
than the parent?

I updated the page now with a hopefully better idea. So that root
cannot escape IMA-appraisal by spawning IMA namespaces and setting an
IMA NULL policy, root's activities will *always* be evaluated against
the init_ima_ns policy with keys found in the init_user_ns. In other
word, if there is an appraisal policy (rule) on the host's
init_ima_ns and root does something as uid 0 in any namespace, file
activity for appraising purposes must pass signature checking. So
just spawning a MNT namespace, mounting a filesystem with unknown
apps won't execute any of them unless the stuff is signed.

So this would mean that most orchestration systems' ideas of privileged
containers (i.e. containers which run real root) would be unable to
have their own IMA namespace ... that's also going to be surprising.

James

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