linux-kernel - Re: [PATCH v3 1/3] dm: Add verity helpers for LoadPin

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Date:   Mon, 16 May 2022 20:38:12 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Douglas Anderson <dianders@...omium.org>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 1/3] dm: Add verity helpers for LoadPin

On Mon, May 16, 2022 at 11:51:54AM -0700, Matthias Kaehlcke wrote:
> 'targets' are different types of DM mappings like 'linear' or 'verity'. A
> device mapper table contains has one or more targets that define the mapping
> of the blocks of the mapped device.
> 
> Having spelled that out I realize that the above check is wrong. It would
> consider a device like this trusted:
> 
> 0 10000000 linear 8:1
> 10000000 10001000 verity <params>
> 
> In the above case only a small part of the DM device would be backed by verity.
> 
> I think we want a table with a single entry that is a verity target.

Ah-ha! Okay, that's what I was worried about. Yes, a device made up
of only trusted verity targets should be the only trusted device. (So,
technically it could be more than 1 verity target, but each would need
to be trusted. Supporting that arrangement, though, may be overkill --
I would expect a 1:1 mapping as you suggest.

-- 
Kees Cook