linux-kernel - Re: [PATCH v9 2/8] integrity: Introduce a Linux keyring called machine

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Date:   Wed, 12 Jan 2022 14:41:12 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Eric Snowberg <eric.snowberg@...cle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        "nayna@...ux.ibm.com" <nayna@...ux.ibm.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "weiyongjun1@...wei.com" <weiyongjun1@...wei.com>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "James.Bottomley@...senpartnership.com" 
        <James.Bottomley@...senPartnership.com>,
        "pjones@...hat.com" <pjones@...hat.com>,
        Konrad Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v9 2/8] integrity: Introduce a Linux keyring called
 machine

On Tue, 2022-01-11 at 20:14 -0500, Mimi Zohar wrote:
> On Tue, 2022-01-11 at 21:26 +0000, Eric Snowberg wrote:
> > 
> > > On Jan 11, 2022, at 11:16 AM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
> > > 
> > > On Mon, 2022-01-10 at 23:25 +0000, Eric Snowberg wrote:
> > >>> Jarkko, my concern is that once this version of the patch set is
> > >>> upstreamed, would limiting which keys may be loaded onto the .machine
> > >>> keyring be considered a regression?
> > >> 
> > >> 
> > >> Currently certificates built into the kernel do not have a CA restriction on them.  
> > >> IMA will trust anything in this keyring even if the CA bit is not set.  While it would 
> > >> be advisable for a kernel to be built with a CA, nothing currently enforces it. 
> > >> 
> > >> My thinking for the dropped CA restriction patches was to introduce a new Kconfig.  
> > >> This Kconfig would do the CA enforcement on the machine keyring.  However if the 
> > >> Kconfig option was not set for enforcement, it would work as it does in this series, 
> > >> plus it would allow IMA to work with non-CA keys.  This would be done by removing 
> > >> the restriction placed in this patch. Let me know your thoughts on whether this would 
> > >> be an appropriate solution.  I believe this would get around what you are identifying as 
> > >> a possible regression.
> > > 
> > > True the problem currently exists with the builtin keys, but there's a
> > > major difference between trusting the builtin keys and those being
> > > loading via MOK.  This is an integrity gap that needs to be closed and
> > > shouldn't be expanded to keys on the .machine keyring.
> > > 
> > > "plus it would allow IMA to work with non-CA keys" is unacceptable.
> > 
> > Ok, I’ll leave that part out.  Could you clarify the wording I should include in the future 
> > cover letter, which adds IMA support, on why it is unacceptable for the end-user to
> > make this decision?
> 
> The Kconfig IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
> "help" is very clear:

[Reposting the text due to email formatting issues.]

help
  Keys may be added to the IMA or IMA blacklist keyrings, if the
  key is validly signed by a CA cert in the system built-in or
  secondary trusted keyrings.

  Intermediate keys between those the kernel has compiled in and the 
  IMA keys to be added may be added to the system secondary keyring,
  provided they are validly signed by a key already resident in the
  built-in or secondary trusted keyrings.


The first paragraph requires "validly signed by a CA cert in the system
built-in or secondary trusted keyrings" for keys to be loaded onto the
IMA keyring.  This Kconfig is limited to just the builtin and secondary
keyrings.  Changing this silently to include the ".machine" keyring
introduces integrity risks that previously did not exist.  A new IMA
Kconfig needs to be defined to allow all three keyrings - builtin,
machine, and secondary.

The second paragraph implies that only CA and intermediate CA keys are
on secondary keyring, or as in our case the ".machine" keyring linked
to the secondary keyring.

Mimi