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Subject: Re: [PATCH v2 0/3] initramfs: add support for xattrs in the initial
 ram disk
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Arvind Sankar <niveditas98@gmail.com>,
        Roberto Sassu <roberto.sassu@huawei.com>,
        Rob Landley <rob@landley.net>, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-integrity@vger.kernel.org, initramfs@vger.kernel.org
Date:   Mon, 13 May 2019 18:09:11 -0400
In-Reply-To: <20190513184744.GA12386@rani.riverdale.lan>
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On Mon, 2019-05-13 at 14:47 -0400, Arvind Sankar wrote:
> On Mon, May 13, 2019 at 02:36:24PM -0400, Mimi Zohar wrote:
> > 
> > > > How does this work today then? Is it actually the case that initramfs
> > > > just cannot be used on an IMA-enabled system, or it can but it leaves
> > > > the initramfs unverified and we're trying to fix that? I had assumed the
> > > > latter.
> > > Oooh, it's done not by starting IMA appraisal later, but by loading a
> > > default policy to ignore initramfs?
> > 
> > Right, when rootfs is a tmpfs filesystem, it supports xattrs, allowing
> > for finer grained policies to be defined.  This patch set would allow
> > a builtin IMA appraise policy to be defined which includes tmpfs.

Clarification: finer grain IMA policy rules are normally defined in
terms of LSM labels.  The LSMs need to enabled, before writing IMA
policy rules in terms of the LSM labels.

> > 
> Ok, but wouldn't my idea still work? Leave the default compiled-in
> policy set to not appraise initramfs. The embedded /init sets all the
> xattrs, changes the policy to appraise tmpfs, and then exec's the real
> init? Then everything except the embedded /init and the file with the
> xattrs will be appraised, and the embedded /init was verified as part of
> the kernel image signature. The only additional kernel change needed
> then is to add a config option to the kernel to disallow overwriting the
> embedded initramfs (or at least the embedded /init).

Yes and no.  The current IMA design allows a builtin policy to be
specified on the boot command line ("ima_policy="), so that it exists
from boot, and allows it to be replaced once with a custom policy.
 After that, assuming that CONFIG_IMA_WRITE_POLICY is configured,
additional rules may be appended.  As your embedded /init solution
already replaces the builtin policy, the IMA policy couldn't currently
be replaced a second time with a custom policy based on LSM labels.

Mimi