[PATCH v1 2/9] certs: Make blacklist_vet_description() more strict

Mickaël Salaün
2020-11-20T18:05:02Z

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

Before exposing this new key type to user space, make sure that only
meaningful blacklisted hashes are accepted. This is also checked for
builtin blacklisted hashes, but a following commit make sure that the
user will notice (at built time) and will fix the configuration if it
already included errors.

Check that a blacklist key description starts with a valid prefix and
then a valid hexadecimal string.

Cc: David Howells <dhowells@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
---
certs/blacklist.c | 46 ++++++++++++++++++++++++++++++++++++----------
1 file changed, 36 insertions(+), 10 deletions(-)

diff --git a/certs/blacklist.c b/certs/blacklist.c
index 4e1a58170d5c..961359885881 100644
--- a/certs/blacklist.c
+++ b/certs/blacklist.c
@@ -17,6 +17,16 @@
#include <keys/system_keyring.h>
#include "blacklist.h"

+/*
+ * According to crypto/asymmetric_keys/x509_cert_parser.c:x509_note_pkey_algo(),
+ * the size of the currently longest supported hash algorithm is 512 bits,
+ * which translates into 128 hex characters.
+ */
+#define MAX_HASH_LEN 128
+
+static const char tbs_prefix[] = "tbs";
+static const char bin_prefix[] = "bin";
+
static struct key *blacklist_keyring;

/*
@@ -25,24 +35,40 @@ static struct key *blacklist_keyring;
*/
static int blacklist_vet_description(const char *desc)
{
- int n = 0;
-
- if (*desc == ':')
- return -EINVAL;
- for (; *desc; desc++)
- if (*desc == ':')
- goto found_colon;
+ int i, prefix_len, tbs_step = 0, bin_step = 0;
+
+ /* The following algorithm only works if prefix lenghts match. */
+ BUILD_BUG_ON(sizeof(tbs_prefix) != sizeof(bin_prefix));
+ prefix_len = sizeof(tbs_prefix) - 1;
+ for (i = 0; *desc; desc++, i++) {
+ if (*desc == ':') {
+ if (tbs_step == prefix_len)
+ goto found_colon;
+ if (bin_step == prefix_len)
+ goto found_colon;
+ return -EINVAL;
+ }
+ if (i >= prefix_len)
+ return -EINVAL;
+ if (*desc == tbs_prefix[i])
+ tbs_step++;
+ if (*desc == bin_prefix[i])
+ bin_step++;
+ }
return -EINVAL;

found_colon:
desc++;
- for (; *desc; desc++) {
+ for (i = 0; *desc && i < MAX_HASH_LEN; desc++, i++) {
if (!isxdigit(*desc) || isupper(*desc))
return -EINVAL;
- n++;
}
+ if (*desc)
+ /* The hash is greater than MAX_HASH_LEN. */
+ return -EINVAL;

- if (n == 0 || n & 1)
+ /* Checks for an even number of hexadecimal characters. */
+ if (i == 0 || i & 1)
return -EINVAL;
return 0;
}
--
2.29.2