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

[Mickaël Salaün]

On Mon, Sep 11, 2023 at 09:29:07AM -0400, Mimi Zohar wrote:
> 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? 

Some operational constraints may limit what a CA can sign.

This change is critical and should be tied to a dedicated kernel config
(disabled by default), otherwise existing systems using this feature
will have their threat model automatically changed without notice.

> 
> > ---
> >  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
>