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[68.160.176.52]) by smtp.gmail.com with ESMTPSA id w140-20020a376292000000b006a10aa7908dsm108133qkb.38.2022.05.18.13.03.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 May 2022 13:03:45 -0700 (PDT) Date: Wed, 18 May 2022 16:03:44 -0400 From: Mike Snitzer To: Matthias Kaehlcke Cc: Milan Broz , Alasdair Kergon , Kees Cook , James Morris , "Serge E . Hallyn" , dm-devel@redhat.com, Douglas Anderson , linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, Song Liu , linux-security-module@vger.kernel.org Subject: Re: [PATCH v4 1/3] dm: Add verity helpers for LoadPin Message-ID: References: <20220517233457.1123309-1-mka@chromium.org> <20220517163437.v4.1.I3e928575a23481121e73286874c4c2bdb403355d@changeid> <19149028-ec94-8f64-aed4-1e58f29942a8@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-raid@vger.kernel.org On Wed, May 18 2022 at 11:13P -0400, Matthias Kaehlcke wrote: > Hi Milan, > > On Wed, May 18, 2022 at 09:57:43AM +0200, Milan Broz wrote: > > On 18/05/2022 01:34, Matthias Kaehlcke wrote: > > > LoadPin limits loading of kernel modules, firmware and certain > > > other files to a 'pinned' file system (typically a read-only > > > rootfs). To provide more flexibility LoadPin is being extended > > > to also allow loading these files from trusted dm-verity > > > devices. For that purpose LoadPin can be provided with a list > > > of verity root digests that it should consider as trusted. > > > > > > Add a bunch of helpers to allow LoadPin to check whether a DM > > > device is a trusted verity device. The new functions broadly > > > fall in two categories: those that need access to verity > > > internals (like the root digest), and the 'glue' between > > > LoadPin and verity. The new file dm-verity-loadpin.c contains > > > the glue functions. > > > > > > Signed-off-by: Matthias Kaehlcke > > > > ... > > > > > + > > > + if (dm_verity_get_root_digest(ti, &root_digest, &digest_size)) > > > + return false; > > > > Almost unrelated note, but as there are more and more situations > > that checks verity root digest, shouldn't we export this as read-only > > sysfs attribute for DM verity devices? > > > > Attacker can always calculate (but not change) Merkle tree, so this > > is not something that need to be hidden. > > > > It would allow userspace to easily enumerate trusted DM devices without > > calling kernel ioctls... > > I guess that's an option if there are scenarios where it is useful. It > should probably be a separate patch, since it isn't directly related with > extending LoadPin support to trusted verity devices. > > > > + > > > + table = dm_get_live_table(md, &srcu_idx); > > > + > > > + if (dm_table_get_num_targets(table) != 1) > > > + goto out; > > > + > > > + ti = dm_table_get_target(table, 0); > > > + > > > + if (is_trusted_verity_target(ti)) > > > + trusted = true; > > > > What happens is someone reloads verity table later with > > a different content (or even different target type)? > > Does LoadPin even care here? > > LoadPin cares, but only when new kernel files are loaded. It will then check > against the new verity table, and only allow loading of the file if it comes > from a verity target with a trusted digest. > > > > static struct target_type verity_target = { > > > .name = "verity", > > > .version = {1, 8, 0}, > > > > Please increase the minor version, it is very useful to detect (in logs) > > that the target driver has compatible extensions. > > I can do that, but would like to confirm that this is really needed/desired. > This patch adds kernel-internal APIs which aren't accessible to userspace, > that don't impact verity directly, so I'm not sure an increased minor version > would be useful. Bumping to 1.8.1 is useful to indicate new changes that offer expanded use of the verity target (even if by LoadPin).