Commit 6364d106 authored by Mickaël Salaün's avatar Mickaël Salaün Committed by Jarkko Sakkinen
Browse files

certs: Allow root user to append signed hashes to the blacklist keyring 



Add a kernel option SYSTEM_BLACKLIST_AUTH_UPDATE to enable the root user
to dynamically add new keys to the blacklist keyring.  This enables to
invalidate new certificates, either from being loaded in a keyring, or
from being trusted in a PKCS#7 certificate chain.  This also enables to
add new file hashes to be denied by the integrity infrastructure.

Being able to untrust a certificate which could have normaly been
trusted is a sensitive operation.  This is why adding new hashes to the
blacklist keyring is only allowed when these hashes are signed and
vouched by the builtin trusted keyring.  A blacklist hash is stored as a
key description.  The PKCS#7 signature of this description must be
provided as the key payload.

Marking a certificate as untrusted should be enforced while the system
is running.  It is then forbiden to remove such blacklist keys.

Update blacklist keyring, blacklist key and revoked certificate access
rights:
* allows the root user to search for a specific blacklisted hash, which
  make sense because the descriptions are already viewable;
* forbids key update (blacklist and asymmetric ones);
* restricts kernel rights on the blacklist keyring to align with the
  root user rights.

See help in tools/certs/print-cert-tbs-hash.sh .

Cc: David Howells <dhowells@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Eric Snowberg <eric.snowberg@oracle.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: default avatarMickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210712170313.884724-6-mic@digikod.net


Reviewed-by: default avatarJarkko Sakkinen <jarkko@kernel.org>
Tested-by: default avatarJarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: default avatarJarkko Sakkinen <jarkko@kernel.org>
parent addf4663

certs/Kconfig

+10 −0
Original line number Diff line number Diff line
@@ -127,4 +127,14 @@ config SYSTEM_REVOCATION_KEYS 
	  containing X.509 certificates to be included in the default blacklist

	  keyring.



config SYSTEM_BLACKLIST_AUTH_UPDATE

	bool "Allow root to add signed blacklist keys"

	depends on SYSTEM_BLACKLIST_KEYRING

	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

	  keyring.  The PKCS#7 signature of the description is set in the key

	  payload.  Blacklist keys cannot be removed.



endmenu

certs/blacklist.c

+75 −21
Original line number Diff line number Diff line
@@ -15,6 +15,7 @@ 
#include <linux/err.h>

#include <linux/seq_file.h>

#include <linux/uidgid.h>

#include <linux/verification.h>

#include <keys/system_keyring.h>

#include "blacklist.h"

#include "common.h"
@@ -26,6 +27,9 @@ 
 */

#define MAX_HASH_LEN	128



#define BLACKLIST_KEY_PERM (KEY_POS_SEARCH | KEY_POS_VIEW | \

			    KEY_USR_SEARCH | KEY_USR_VIEW)



static const char tbs_prefix[] = "tbs";

static const char bin_prefix[] = "bin";
@@ -80,19 +84,51 @@ static int blacklist_vet_description(const char *desc) 
	return 0;

}



static int blacklist_key_instantiate(struct key *key,

		struct key_preparsed_payload *prep)

{

#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE

	int err;

#endif



	/* Sets safe default permissions for keys loaded by user space. */

	key->perm = BLACKLIST_KEY_PERM;



	/*

 * The hash to be blacklisted is expected to be in the description.  There will

 * be no payload.

	 * Skips the authentication step for builtin hashes, they are not

	 * signed but still trusted.

	 */

static int blacklist_preparse(struct key_preparsed_payload *prep)

{

	if (prep->datalen > 0)

		return -EINVAL;

	return 0;

	if (key->flags & (1 << KEY_FLAG_BUILTIN))

		goto out;



#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE

	/*

	 * Verifies the description's PKCS#7 signature against the builtin

	 * trusted keyring.

	 */

	err = verify_pkcs7_signature(key->description,

			strlen(key->description), prep->data, prep->datalen,

			NULL, VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);

	if (err)

		return err;

#else

	/*

	 * It should not be possible to come here because the keyring doesn't

	 * have KEY_USR_WRITE and the only other way to call this function is

	 * for builtin hashes.

	 */

	WARN_ON_ONCE(1);

	return -EPERM;

#endif



out:

	return generic_key_instantiate(key, prep);

}



static void blacklist_free_preparse(struct key_preparsed_payload *prep)

static int blacklist_key_update(struct key *key,

		struct key_preparsed_payload *prep)

{

	return -EPERM;

}



static void blacklist_describe(const struct key *key, struct seq_file *m)
@@ -103,9 +139,8 @@ static void blacklist_describe(const struct key *key, struct seq_file *m) 
static struct key_type key_type_blacklist = {

	.name			= "blacklist",

	.vet_description	= blacklist_vet_description,

	.preparse		= blacklist_preparse,

	.free_preparse		= blacklist_free_preparse,

	.instantiate		= generic_key_instantiate,

	.instantiate		= blacklist_key_instantiate,

	.update			= blacklist_key_update,

	.describe		= blacklist_describe,

};
@@ -154,8 +189,7 @@ static int mark_raw_hash_blacklisted(const char *hash) 
				   hash,

				   NULL,

				   0,

				   ((KEY_POS_ALL & ~KEY_POS_SETATTR) |

				    KEY_USR_VIEW),

				   BLACKLIST_KEY_PERM,

				   KEY_ALLOC_NOT_IN_QUOTA |

				   KEY_ALLOC_BUILT_IN);

	if (IS_ERR(key)) {
@@ -232,8 +266,10 @@ int add_key_to_revocation_list(const char *data, size_t size) 
				   NULL,

				   data,

				   size,

				   ((KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_VIEW),

				   KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN);

				   KEY_POS_VIEW | KEY_POS_READ | KEY_POS_SEARCH

				   | KEY_USR_VIEW,

				   KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN

				   | KEY_ALLOC_BYPASS_RESTRICTION);



	if (IS_ERR(key)) {

		pr_err("Problem with revocation key (%ld)\n", PTR_ERR(key));
@@ -260,25 +296,43 @@ int is_key_on_revocation_list(struct pkcs7_message *pkcs7) 
}

#endif



static int restrict_link_for_blacklist(struct key *dest_keyring,

		const struct key_type *type, const union key_payload *payload,

		struct key *restrict_key)

{

	if (type == &key_type_blacklist)

		return 0;

	return -EOPNOTSUPP;

}



/*

 * Initialise the blacklist

 */

static int __init blacklist_init(void)

{

	const char *const *bl;

	struct key_restriction *restriction;



	if (register_key_type(&key_type_blacklist) < 0)

		panic("Can't allocate system blacklist key type\n");



	restriction = kzalloc(sizeof(*restriction), GFP_KERNEL);

	if (!restriction)

		panic("Can't allocate blacklist keyring restriction\n");

	restriction->check = restrict_link_for_blacklist;



	blacklist_keyring =

		keyring_alloc(".blacklist",

			      GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, current_cred(),

			      (KEY_POS_ALL & ~KEY_POS_SETATTR) |

			      KEY_USR_VIEW | KEY_USR_READ |

			      KEY_USR_SEARCH,

			      KEY_ALLOC_NOT_IN_QUOTA |

			      KEY_POS_VIEW | KEY_POS_READ | KEY_POS_SEARCH |

			      KEY_POS_WRITE |

			      KEY_USR_VIEW | KEY_USR_READ | KEY_USR_SEARCH

#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE

			      | KEY_USR_WRITE

#endif

			      , KEY_ALLOC_NOT_IN_QUOTA |

			      KEY_ALLOC_SET_KEEP,

			      NULL, NULL);

			      restriction, NULL);

	if (IS_ERR(blacklist_keyring))

		panic("Can't allocate system blacklist keyring\n");