Re: [PATCH v2] dm verity: Add support for signature verification with 2nd keyring

Mike Snitzer
2020-10-15T16:52:42Z

On Thu, Oct 15 2020 at 11:05am -0400,
Mickaël Salaün <mic@digikod.net> wrote:

> From: Mickaël Salaün <mic@linux.microsoft.com>
>
> Add a new configuration DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
> to enable dm-verity signatures to be verified against the secondary
> trusted keyring. Instead of relying on the builtin trusted keyring
> (with hard-coded certificates), the second trusted keyring can include
> certificate authorities from the builtin trusted keyring and child
> certificates loaded at run time. Using the secondary trusted keyring
> enables to use dm-verity disks (e.g. loop devices) signed by keys which
> did not exist at kernel build time, leveraging the certificate chain of
> trust model. In practice, this makes it possible to update certificates
> without kernel update and reboot, aligning with module and kernel
> (kexec) signature verification which already use the secondary trusted
> keyring.
>
> Cc: Alasdair Kergon <agk@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> Cc: Jaskaran Khurana <jaskarankhurana@linux.microsoft.com>
> Cc: Mike Snitzer <snitzer@redhat.com>
> Cc: Milan Broz <gmazyland@gmail.com>
> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> ---
>
> Previous version:
> https://lore.kernel.org/lkml/20201002071802.535023-1-mic@digikod.net/
>
> Changes since v1:
> * Extend the commit message (asked by Jarkko Sakkinen).
> * Rename the Kconfig "help" keyword according to commit 84af7a6194e4
> ("checkpatch: kconfig: prefer 'help' over '---help---'").

Can you please explain why you've decided to make this a Kconfig CONFIG
knob? Why not either add: a dm-verity table argument? A dm-verity
kernel module parameter? or both (to allow a particular default but then
per-device override)?

Otherwise, _all_ DM verity devices will be configured to use secondary
keyring fallback. Is that really desirable?

Regardless, I really don't see why a Kconfig knob is appropriate.

Mike

> ---
> drivers/md/Kconfig | 13 ++++++++++++-
> drivers/md/dm-verity-verify-sig.c | 9 +++++++--
> 2 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
> index 30ba3573626c..1d68935e45ef 100644
> --- a/drivers/md/Kconfig
> +++ b/drivers/md/Kconfig
> @@ -530,11 +530,22 @@ config DM_VERITY_VERIFY_ROOTHASH_SIG
> bool "Verity data device root hash signature verification support"
> depends on DM_VERITY
> select SYSTEM_DATA_VERIFICATION
> - help
> + help
> Add ability for dm-verity device to be validated if the
> pre-generated tree of cryptographic checksums passed has a pkcs#7
> signature file that can validate the roothash of the tree.
>
> + By default, rely on the builtin trusted keyring.
> +
> + If unsure, say N.
> +
> +config DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
> + bool "Verity data device root hash signature verification with secondary keyring"
> + depends on DM_VERITY_VERIFY_ROOTHASH_SIG
> + depends on SECONDARY_TRUSTED_KEYRING
> + help
> + Rely on the secondary trusted keyring to verify dm-verity signatures.
> +
> If unsure, say N.
>
> config DM_VERITY_FEC
> diff --git a/drivers/md/dm-verity-verify-sig.c b/drivers/md/dm-verity-verify-sig.c
> index 614e43db93aa..29385dc470d5 100644
> --- a/drivers/md/dm-verity-verify-sig.c
> +++ b/drivers/md/dm-verity-verify-sig.c
> @@ -119,8 +119,13 @@ int verity_verify_root_hash(const void *root_hash, size_t root_hash_len,
> }
>
> ret = verify_pkcs7_signature(root_hash, root_hash_len, sig_data,
> - sig_len, NULL, VERIFYING_UNSPECIFIED_SIGNATURE,
> - NULL, NULL);
> + sig_len,
> +#ifdef CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
> + VERIFY_USE_SECONDARY_KEYRING,
> +#else
> + NULL,
> +#endif
> + VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);
>
> return ret;
> }
>
> base-commit: bbf5c979011a099af5dc76498918ed7df445635b
> --
> 2.28.0
>