Date:	Tue, 12 Jan 2016 01:38:13 +0000
From:	David Howells <dhowells@...hat.com>
To:	Petko Manolov <petkan@...-labs.com>
Cc:	dhowells@...hat.com, Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org, keyrings@...r.kernel.org,
	linux-kernel@...r.kernel.org, mdb@...iper.net
Subject: Re: [PATCH] X.509: Partially revert patch to add validation against IMA MOK keyring

Petko Manolov <petkan@...-labs.com> wrote:

> > I would like to counter Mimi's NAK:
> > 
> >  (1) Commit 41c89b64d7184a780f12f2cccdabe65cb2408893 doesn't do what it
> >      says.  Given the change I want to revert, this bit of the description:
> > 
> > 	To successfully import a key into .ima_mok it must be signed by a
> > 	key which CA is in .system keyring.
> > 
> >      is *not* true.  A key in the .ima_mok keyring will *also* allow a key
> 
> This is correct, but is also the desired result.

Not really.  What if you want a trusted keyring that is not governed by the
.ima_mok keyring?  You can't have it.

> Assume that you have multi tenant machine where the manufacturer signs the
> owner's/tenant's key and those also need to sign other sub-tenant keys.  One
> can't put them on the system keyring

Why not?  We can enable user-write on the system keyring.  Are you saying that
there should be multiple .ima_mok keyrings?  If so, your change is wrong.

> so they end up in .ima_mok.
> 
> >      into the .ima_mok keyring.  Thus the .ima_mok keyring is redundant and
> >      should be merged into the .system keyring.
> 
> I share Mimi's opinion that .system keyring must be static and ultimately 
> trusted.

I don't necessarily share the opinion that it must remain static.  If you can
change the .ima_mok keyring, then it's as good as being able to change the
.system keyring by your change that I want to revert.

The one thing I grant that enabling the .system keyring will allow is deletion
of trusted keys - and once you've deleted them, you can't necessarily get them
back without rebooting.

> Since .ima_mok is a dynamic keyring, merging them will break the 
> semantics.

So what you're saying is that .ima_mok is just a staging area for changes
that would otherwise go in .system?  In which case, why is it IMA-related at
all?  Why isn't it called ".mok" or ".system_overrides" or something like that
and in certs/system_keyring.c?

Remember: IMA is optional.  We want to be able to disable it.  So if you want
this feature, it really needs to be separated from IMA.

And why shouldn't we change these semantics?

And why can't .system be a dynamic keyring?

> >  (2) You can use KEYCTL_LINK to link trusted keys between trusted keyrings
> >      if the key being linked grants permission.  Add a new key to one open
> >      keyring and you can then link it across to another.
> > 
> >      Keyrings need to guard against *link* as per my recently posted
> >      patches.
> 
> I'd rather rely on a certificate being properly signed in order to land in a 
> particular keyring, rather than being linked based on permissions model.

Then the patches I posted *are* necessary.  Currently KEYCTL_LINK will let you
link a "trusted" key between trusted-only keyrings if the Link permission is
set because trustedness is currently evaluated only once - at key preparse
time - because we currently throw away all the metadata you need to do further
checks.

The patches I posted validate the certificate signature on addition of a
*link* into a keyring (add_key(), if it doesn't update a key, creates a key
and then does a link operation).  The gatekeeper function is settable
per-keyring.

> >  (3) In the current model, the trusted-only keyring and trusted-key concept
> >      ought really to apply only to the .system keyring as the concept of
> >      'trust' is boolean in this implementation.
> 
> The .system keyring should be read-only, IOW static.  Only keys present at
> build time should go there.

Why?  You haven't given a reason why .ima_mok shouldn't be integrated into
.system and that opened up.  Because you prefer it the way you've done it
isn't necessarily a good reason.

> Everything else goes to the machine owner keyring (MOK) or whatever the
> name.  MOK should be read-write and (maybe) hold only second level CAs,
> signed by CA in the system keyring.  I introduced .ima_mok just because my
> work had limited scope at the time and i consider the name as misleading.

What would you call it then, if not .ima_mok?  Given its current name, it
seems that it should relate to the UEFI BIOS data if such is available.

> The way i see kernel's keyrings:
> 
>                            /---> .ima
>             /----> MOK ---<
> .system ---<               \---> .evm
>             \----> BL       \---> .whatever need to be "trusted"
> 
> The graph could be a lot more complex, but to wrap your head around the idea
> think of big ass machine with years of uptime and multiple simultaneous
> users, all pre-installed files IMA signed, ability to add other IMA signed
> packages on the fly.  The machine must be FIPS certifiable, etc.  A terabit
> switching machine should be able to do that and there are real users for
> this scenario out there.

Given you seem to have one .system ring and one MOK ring, why do you need
separate rings?

> The black-list keyring is equally important so one can revoke CAs if need
> be.  On the fly.

I've no objection to a blacklist.  I can see that you might want it to be a
separate keyring to have the no-removal clause in place.  I would consider
making a blacklist key type and have it contain a bundle of key IDs to reduce
resource consumption, but that's an implementation detail (the match function
can check against all the IDs in a bundle).

However, the blacklist also should be separated from the IMA subsystem and
moved to system_keyring.c.  I can probably backport the code we use in Fedora
for this.

David

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