Re: [PATCH v2 2/3] LoadPin: Enable loading from trusted dm-verity devices

From: Kees Cook <keescook@chromium.org>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: Alasdair Kergon <agk@redhat.com>,
	Mike Snitzer <snitzer@kernel.org>,
	James Morris <jmorris@namei.org>,
	"Serge E . Hallyn" <serge@hallyn.com>,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org, dm-devel@redhat.com,
	linux-raid@vger.kernel.org,
	Douglas Anderson <dianders@chromium.org>,
	Song Liu <song@kernel.org>
Subject: Re: [PATCH v2 2/3] LoadPin: Enable loading from trusted dm-verity devices
Date: Sat, 30 Apr 2022 23:21:54 -0700	[thread overview]
Message-ID: <202204302316.AF04961@keescook> (raw)
In-Reply-To: <20220426143059.v2.2.I01c67af41d2f6525c6d023101671d7339a9bc8b5@changeid>

On Tue, Apr 26, 2022 at 02:31:09PM -0700, Matthias Kaehlcke wrote:
> I'm still doubting what would be the best way to configure
> the list of trusted digests. The approach in v2 of writing
> a path through sysctl is flexible, but it also feels a bit
> odd. I did some experiments with passing a file descriptor
> through sysctl, but it's also odd and has its own issues.
> Passing the list through a kernel parameter seems hacky.
> A Kconfig string would work, but can be have issues when
> the same config is used for different platforms, where
> some may have trusted digests and others not.

I prefer the idea of passing an fd, since that can just use LoadPin
itself to verify the origin of the fd.

I also agree, though, that it's weird as a sysctl. Possible thoughts:

- make it a new ioctl on /dev/mapper/control (seems reasonable given
  that it's specifically about dm devices).
- have LoadPin grow a securityfs node, maybe something like
  /sys/kernel/security/loadpin/dm-verify and do the ioctl there (seems
  reasonable given that it's specifically about LoadPin, but is perhaps
  more overhead to built the securityfs).

-- 
Kees Cook