Re: [PATCH] certs: Restrict blacklist updates to the secondary trusted keyring

[Mimi Zohar]

Hi Eric,

On Fri, 2023-09-08 at 17:34 -0400, Eric Snowberg wrote:
> Currently root can dynamically update the blacklist keyring if the hash
> being added is signed and vouched for by the builtin trusted keyring.
> Currently keys in the secondary trusted keyring can not be used.
> 
> Keys within the secondary trusted keyring carry the same capabilities as
> the builtin trusted keyring.  Relax the current restriction for updating
> the .blacklist keyring and allow the secondary to also be referenced as
> a trust source.  Since the machine keyring is linked to the secondary
> trusted keyring, any key within it may also be used.
> 
> An example use case for this is IMA appraisal.  Now that IMA both
> references the blacklist keyring and allows the machine owner to add
> custom IMA CA certs via the machine keyring, this adds the additional
> capability for the machine owner to also do revocations on a running
> system.
> 
> IMA appraisal usage example to add a revocation for /usr/foo:
> 
> sha256sum /bin/foo | awk '{printf "bin:" $1}' > hash.txt
> 
> openssl smime -sign -in hash.txt -inkey machine-private-key.pem \
>        -signer machine-certificate.pem -noattr -binary -outform DER \
>        -out hash.p7s
> 
> keyctl padd blacklist "$(< hash.txt)" %:.blacklist < hash.p7s
> 
> Signed-off-by: Eric Snowberg <eric.snowberg@xxxxxxxxxx>

The secondary keyring may include both CA and code signing keys.  With
this change any key loaded onto the secondary keyring may blacklist a
hash.  Wouldn't it make more sense to limit blacklisting
certificates/hashes to at least CA keys? 

> ---
>  certs/Kconfig     | 2 +-
>  certs/blacklist.c | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/certs/Kconfig b/certs/Kconfig
> index 1f109b070877..23dc87c52aff 100644
> --- a/certs/Kconfig
> +++ b/certs/Kconfig
> @@ -134,7 +134,7 @@ config SYSTEM_BLACKLIST_AUTH_UPDATE
>  	depends on SYSTEM_DATA_VERIFICATION
>  	help
>  	  If set, provide the ability to load new blacklist keys at run time if
> -	  they are signed and vouched by a certificate from the builtin trusted
> +	  they are signed and vouched by a certificate from the secondary trusted

If CONFIG_SECONDARY_TRUSTED_KEYRING is not enabled, it falls back to
the builtin keyring.  Please update the comment accordingly.

>  	  keyring.  The PKCS#7 signature of the description is set in the key
>  	  payload.  Blacklist keys cannot be removed.
>  
> diff --git a/certs/blacklist.c b/certs/blacklist.c
> index 675dd7a8f07a..0b346048ae2d 100644
> --- a/certs/blacklist.c
> +++ b/certs/blacklist.c
> @@ -102,12 +102,12 @@ static int blacklist_key_instantiate(struct key *key,
>  
>  #ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE
>  	/*
> -	 * Verifies the description's PKCS#7 signature against the builtin
> +	 * Verifies the description's PKCS#7 signature against the secondary
>  	 * trusted keyring.
>  	 */

And similarly here ...

>  	err = verify_pkcs7_signature(key->description,
>  			strlen(key->description), prep->data, prep->datalen,
> -			NULL, VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);
> +			VERIFY_USE_SECONDARY_KEYRING, VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);
>  	if (err)
>  		return err;
>  #else

-- 
thanks,

Mimi