[PATCH v8 5/5] certs: Allow root user to append signed hashes to the blacklist keyring

Mickaël Salaün
2021-07-12T17:03:49Z

From: Mickaël Salaün <mic@linux.microsoft.com>

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
---

Changes since v6:
* Rebase on keys-cve-2020-26541-v3: commit ebd9c2ae369a ("integrity:
Load mokx variables into the blacklist keyring").

Changes since v5:
* Rebase on keys-next, fix Kconfig conflict, and update the asymmetric
key rights added to the blacklist keyring by the new
add_key_to_revocation_list(): align with blacklist key rights by
removing KEY_POS_WRITE as a safeguard, and add
KEY_ALLOC_BYPASS_RESTRICTION to not be subject to
restrict_link_for_blacklist() that only allows blacklist key types to
be added to the keyring.
* Change the return code for restrict_link_for_blacklist() from -EPERM
to -EOPNOTSUPP to align with asymmetric key keyrings.

Changes since v3:
* Update commit message for print-cert-tbs-hash.sh .

Changes since v2:
* Add comment for blacklist_key_instantiate().
---
certs/Kconfig | 10 +++++
certs/blacklist.c | 96 ++++++++++++++++++++++++++++++++++++-----------
2 files changed, 85 insertions(+), 21 deletions(-)

diff --git a/certs/Kconfig b/certs/Kconfig
index 0fbe184ceca5..e0e524b7eff9 100644
--- a/certs/Kconfig
+++ b/certs/Kconfig
@@ -103,4 +103,14 @@ 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
diff --git a/certs/blacklist.c b/certs/blacklist.c
index b254c87ceb3a..486ce0dd8e9c 100644
--- a/certs/blacklist.c
+++ b/certs/blacklist.c
@@ -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");

--
2.32.0