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Date:   Fri, 13 May 2022 11:26:44 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Mike Snitzer <snitzer@redhat.com>,
        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>, dm-devel@redhat.com,
        linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org,
        Song Liu <song@kernel.org>,
        Douglas Anderson <dianders@chromium.org>,
        linux-security-module@vger.kernel.org
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_v3_2/3=5D_LoadPin=3A_Enable_l?= =?US-ASCII?Q?oading_from_trusted_dm-verity_devices?=
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On May 13, 2022 9:32:12 AM PDT, Mike Snitzer <snitzer@redhat=2Ecom> wrote:
>On Wed, May 04 2022 at  3:54P -0400,
>Matthias Kaehlcke <mka@chromium=2Eorg> wrote:
>
>> Extend LoadPin to allow loading of kernel files from trusted dm-verity =
[1]
>> devices=2E
>>=20
>> This change adds the concept of trusted verity devices to LoadPin=2E Lo=
adPin
>> maintains a list of root digests of verity devices it considers trusted=
=2E
>> Userspace can populate this list through an ioctl on the new LoadPin
>> securityfs entry 'dm-verity'=2E The ioctl receives a file descriptor of
>> a file with verity digests as parameter=2E Verity reads the digests fro=
m
>> this file after confirming that the file is located on the pinned root=
=2E
>> The list of trusted digests can only be set up once, which is typically
>> done at boot time=2E
>>=20
>> 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=2E Otherwis=
e, 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=2E The file can be loaded if t=
he
>> verity device has a trusted root digest=2E
>>=20
>> Background:
>>=20
>> As of now LoadPin restricts loading of kernel files to a single pinned
>> filesystem, typically the rootfs=2E This works for many systems, howeve=
r it
>> can result in a bloated rootfs (and OTA updates) on platforms where
>> multiple boards with different hardware configurations use the same roo=
tfs
>> image=2E Especially when 'optional' files are large it may be preferabl=
e to
>> download/install them only when they are actually needed by a given boa=
rd=2E
>> Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packag=
es'
>> at runtime=2E As an example a DLC package could contain firmware for a
>> peripheral that is not present on all boards=2E DLCs use dm-verity to v=
erify
>> the integrity of the DLC content=2E
>>=20
>> [1] https://www=2Ekernel=2Eorg/doc/html/latest/admin-guide/device-mappe=
r/verity=2Ehtml
>> [2] https://chromium=2Egooglesource=2Ecom/chromiumos/platform2/+/HEAD/d=
lcservice/docs/developer=2Emd
>>=20
>> Signed-off-by: Matthias Kaehlcke <mka@chromium=2Eorg>
>> ---
>>=20
>> Changes in v3:
>> - added securityfs for LoadPin (currently only populated when
>>   CONFIG_SECURITY_LOADPIN_VERITY=3Dy)
>> - added uapi include for LoadPin
>> - changed the interface for setting up the list of trusted
>>   digests from sysctl to ioctl on securityfs entry
>> - added stub for loadpin_is_fs_trusted() to be used
>>   CONFIG_SECURITY_LOADPIN_VERITY is not select
>> - depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
>> - updated Kconfig help
>> - minor changes in read_trusted_verity_root_digests()
>> - updated commit message
>>=20
>> Changes in v2:
>> - userspace now passes the path of the file with the verity digests
>>   via systcl, instead of the digests themselves
>> - renamed sysctl file to 'trusted_verity_root_digests_path'
>> - have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
>> - updated Kconfig doc
>> - updated commit message
>>=20
>>  include/uapi/linux/loadpin=2Eh |  19 ++++
>>  security/loadpin/Kconfig     |  16 +++
>>  security/loadpin/loadpin=2Ec   | 184 +++++++++++++++++++++++++++++++++=
+-
>>  3 files changed, 218 insertions(+), 1 deletion(-)
>>  create mode 100644 include/uapi/linux/loadpin=2Eh
>
>I would certainly need some Reviewed-by:s from security and/or loadpin
>experts if I were to pick this patch up=2E

Alternatively, since it's mostly touching loadpin, I can carry it in my tr=
ee, as long as you've Acked the dm bits=2E :)

>Did you see the issues the kernel test robot emailed about?
>
>You'd do well to fix those issues up when submitting another revision
>of this patchset=2E

Agreed=2E


--=20
Kees Cook