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

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Message-Id: <CVGFDI2VELSN.2E98O6TF5TZ0E@suppilovahvero>
Date:   Tue, 12 Sep 2023 01:01:31 +0300
From:   "Jarkko Sakkinen" <jarkko@...nel.org>
To:     "Eric Snowberg" <eric.snowberg@...cle.com>, <zohar@...ux.ibm.com>,
        <dhowells@...hat.com>, <dwmw2@...radead.org>
Cc:     <mic@...ux.microsoft.com>, <kanth.ghatraju@...cle.com>,
        <konrad.wilk@...cle.com>, <linux-integrity@...r.kernel.org>,
        <keyrings@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] certs: Restrict blacklist updates to the secondary
 trusted keyring

On Sat Sep 9, 2023 at 12:34 AM EEST, 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@...cle.com>
> ---
>  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
>  	  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.
>  	 */
>  	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
> -- 
> 2.39.3

What if a live system in the wild assumes the old policy? I feel that
this is "sort of" breaking backwards compatibility but please prove me
wrong.

BR, Jarkko