X.509: Don't treat self-signed keys specially

Trust for a self-signed certificate can normally only be determined by
whether we obtained it from a trusted location (ie. it was built into the
kernel at compile time), so there's not really any point in checking it -
we could verify that the signature is valid, but it doesn't really tell us
anything if the signature checks out.

However, there's a bug in the code determining whether a certificate is
self-signed or not - if they have neither AKID nor SKID then we just assume
that the cert is self-signed, which may not be true.

Given this, remove the code that treats self-signed certs specially when it
comes to evaluating trustability and attempt to evaluate them as ordinary
signed certificates.  We then expect self-signed certificates to fail the
trustability check and be marked as untrustworthy in x509_key_preparse().

Note that there is the possibility of the trustability check on a
self-signed cert then succeeding.  This is most likely to happen when a
duplicate of the certificate is already on the trust keyring - in which
case it shouldn't be a problem.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: David Woodhouse <David.Woodhouse@intel.com>
cc: Mimi Zohar <zohar@linux.vnet.ibm.com>

crypto/asymmetric_keys/x509_public_key.c

@@ -255,6 +255,9 @@ static int x509_validate_trust(struct x509_certificate *cert,
 	struct key *key;
 	int ret = 1;
 
+	if (!cert->akid_id && !cert->akid_skid)
+		return 1;
+
 	if (!trust_keyring)
 		return -EOPNOTSUPP;
 
@@ -312,17 +315,21 @@ static int x509_key_preparse(struct key_preparsed_payload *prep)
 	cert->pub->algo = pkey_algo[cert->pub->pkey_algo];
 	cert->pub->id_type = PKEY_ID_X509;
 
-	/* Check the signature on the key if it appears to be self-signed */
-	if ((!cert->akid_skid && !cert->akid_id) ||
-	    asymmetric_key_id_same(cert->skid, cert->akid_skid) ||
-	    asymmetric_key_id_same(cert->id, cert->akid_id)) {
-		ret = x509_check_signature(cert->pub, cert); /* self-signed */
-		if (ret < 0)
-			goto error_free_cert;
-	} else if (!prep->trusted) {
+	/* See if we can derive the trustability of this certificate.
+	 *
+	 * When it comes to self-signed certificates, we cannot evaluate
+	 * trustedness except by the fact that we obtained it from a trusted
+	 * location.  So we just rely on x509_validate_trust() failing in this
+	 * case.
+	 *
+	 * Note that there's a possibility of a self-signed cert matching a
+	 * cert that we have (most likely a duplicate that we already trust) -
+	 * in which case it will be marked trusted.
+	 */
+	if (!prep->trusted) {
 		ret = x509_validate_trust(cert, get_system_trusted_keyring());
 		if (!ret)
-			prep->trusted = 1;
+			prep->trusted = true;
 	}
 
 	/* Propose a description */