certs: Make blacklist_vet_description() more strict

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>
Cc: Eric Snowberg <eric.snowberg@oracle.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20210712170313.884724-4-mic@digikod.net
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

diff --git a/certs/blacklist.c b/certs/blacklist.c
index 1cdf309..b254c87 100644 (file)
--- a/certs/blacklist.c
+++ b/certs/blacklist.c
@@ -19,6 +19,16 @@
 #include "blacklist.h"
 #include "common.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;
 
 #ifdef CONFIG_SYSTEM_REVOCATION_LIST
@@ -32,24 +42,40 @@ extern __initconst const unsigned long revocation_certificate_list_size;
  */
 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 lengths 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 -ENOPKG;
 
-       if (n == 0 || n & 1)
+       /* Checks for an even number of hexadecimal characters. */
+       if (i == 0 || i & 1)
                return -EINVAL;
        return 0;
 }