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Date:   Wed, 6 Jul 2022 11:08:08 -0400
From:   Mike Snitzer <snitzer@kernel.org>
To:     Matthias Kaehlcke <mka@chromium.org>
Cc:     Alasdair Kergon <agk@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        James Morris <jmorris@namei.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        linux-kernel@vger.kernel.org, dm-devel@redhat.com,
        Douglas Anderson <dianders@chromium.org>,
        linux-raid@vger.kernel.org, Song Liu <song@kernel.org>,
        linux-security-module@vger.kernel.org,
        Milan Broz <gmazyland@gmail.com>
Subject: Re: [PATCH v7 2/3] LoadPin: Enable loading from trusted dm-verity
 devices
Message-ID: <YsWlWIbzJtUQUaWy@redhat.com>
References: <20220627153526.3750341-1-mka@chromium.org>
 <20220627083512.v7.2.I01c67af41d2f6525c6d023101671d7339a9bc8b5@changeid>
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On Mon, Jun 27 2022 at 11:35P -0400,
Matthias Kaehlcke <mka@chromium.org> wrote:

> Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
> devices.
> 
> This change adds the concept of trusted verity devices to LoadPin. LoadPin
> maintains a list of root digests of verity devices it considers trusted.
> Userspace can populate this list through an ioctl on the new LoadPin
> securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
> a file with verity digests as parameter. Verity reads the digests from
> this file after confirming that the file is located on the pinned root.
> The digest file must contain one digest per line. The list of trusted
> digests can only be set up once, which is typically done at boot time.
> 
> When a kernel file is read LoadPin first checks (as usual) whether the file
> is located on the pinned root, if so the file can be loaded. Otherwise, if
> the verity extension is enabled, LoadPin determines whether the file is
> located on a verity backed device and whether the root digest of that
> device is in the list of trusted digests. The file can be loaded if the
> verity device has a trusted root digest.
> 
> Background:
> 
> As of now LoadPin restricts loading of kernel files to a single pinned
> filesystem, typically the rootfs. This works for many systems, however it
> can result in a bloated rootfs (and OTA updates) on platforms where
> multiple boards with different hardware configurations use the same rootfs
> image. Especially when 'optional' files are large it may be preferable to
> download/install them only when they are actually needed by a given board.
> Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
> at runtime. As an example a DLC package could contain firmware for a
> peripheral that is not present on all boards. DLCs use dm-verity to verify
> the integrity of the DLC content.
> 
> [1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
> [2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
> 
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> Acked-by: Kees Cook <keescook@chromium.org>

Acked-by: Mike Snitzer <snitzer@kernel.org>