linux-kernel - Re: [PULL REQUEST] Lock down patches

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Message-ID: <CACdnJuvwnh0PKr5jUTm2h=P5-u-oLgBjvThX+DxcegShXReCdA@mail.gmail.com>
Date:   Thu, 28 Feb 2019 15:13:05 -0800
From:   Matthew Garrett <mjg59@...gle.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     jmorris@...ei.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PULL REQUEST] Lock down patches

On Thu, Feb 28, 2019 at 2:20 PM Mimi Zohar <zohar@...ux.ibm.com> wrote:
> On Thu, 2019-02-28 at 13:28 -0800, Matthew Garrett wrote:
> > This PR is mostly the same as the previous attempt, but with the
> > following changes:
>
> Where/when was this latest version of the patches posted?

They should have followed this, but git-send-email choked on some
reviewed-by: lines so I'm just trying to sort that out.

> >
> > 1) The integration between EFI secure boot and the lockdown state has
> > been removed
> > 2) A new CONFIG_KERNEL_LOCK_DOWN_FORCE kconfig option has been added,
> > which will always enable lockdown regardless of the kernel command
> > line
> > 3) The integration with IMA has been dropped for now. Requiring the
> > use of the IMA secure boot policy when lockdown is enabled isn't
> > practical for most distributions at the moment, as there's still not a
> > great deal of infrastructure for shipping packages with appropriate
> > IMA signatures, and it makes it complicated for end users to manage
> > custom IMA policies.
>
> I'm all in favor of dropping the original attempt to coordinate
> between the kexec PE and IMA kernel image signatures and the kernel
> appended and IMA modules signatures, but there has been quite a bit of
> work recently coordinating the different types of signatures.
>
> Preventing systems which do use IMA for signature verification, should
> not limit their ability to enable "lock down".  Does this version of
> the "lock down" patches coordinate the different kexec kernel image
> and kernel module signature verification methods?

It's a little more complicated than this. We can't just rely on IMA
appraisal - it has to be based on digital signatures, and the existing
patch only made that implicit by enabling the secure_boot policy. I
think we do want to integrate these, but there's a few things we need
to take into account:

1) An integrated solution can't depend on xattrs, both because of the
lagging support for distributing those signatures but also because we
need to support filesystems that don't support xattrs
2) An integrated solution can't depend on the current secure_boot
policy because that requires signed IMA policy updates, but
distributions have no way of knowing what IMA policy end users require

In any case, I do agree that we should aim to make this more
reasonable - having orthogonal signing code doesn't benefit anyone.
Once there's solid agreement on that we can extend this support.