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

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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: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210712170313.884724-6-mic@digikod.net
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
This commit is contained in:
Mickaël Salaün 2021-07-12 19:03:13 +02:00 committed by Jarkko Sakkinen
parent addf466389
commit 6364d106e0
2 changed files with 85 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
certs/Kconfig
View File

@ -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

96
certs/blacklist.c
View File

@ -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;
}
/*
* The hash to be blacklisted is expected to be in the description. There will
* be no payload.
*/
static int blacklist_preparse(struct key_preparsed_payload *prep)
static int blacklist_key_instantiate(struct key *key,
struct key_preparsed_payload *prep)
{
if (prep->datalen > 0)
return -EINVAL;
return 0;
#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE
int err;
#endif
/* Sets safe default permissions for keys loaded by user space. */
key->perm = BLACKLIST_KEY_PERM;
/*
* Skips the authentication step for builtin hashes, they are not
* signed but still trusted.
*/
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");