Re: [PATCH v2 0/3] initramfs: add support for xattrs in the initial ram disk

From: Mimi Zohar
Date: Mon May 13 2019 - 18:11:07 EST

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. ÂThis 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. ÂThe 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. ÂThe 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.
ÂAfter that, assuming that CONFIG_IMA_WRITE_POLICY is configured,
additional rules may be appended. ÂAs 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.