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

[Mike Snitzer]

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

> From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> Cc: Jaskaran Khurana <jaskarankhurana@xxxxxxxxxxxxxxxxxxx>
> Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
> Cc: Milan Broz <gmazyland@xxxxxxxxx>
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
> ---
> 
> Previous version:
> https://lore.kernel.org/lkml/20201002071802.535023-1-mic@xxxxxxxxxxx/
> 
> 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
>