[RFC PATCH] X.509: Don't check the signature on apparently self-signed keys [ver #2]

[David Howells <dhowells@redhat.com>]

If a certificate is self-signed, don't bother checking the validity of the
signature.  The cert cannot be checked by validation against the next one
in the chain as this is the root of the chain.  Trust for this certificate
can only be determined by whether we obtained it from a trusted location
(ie. it was built into the kernel at compile time).

This also fixes a bug whereby certificates were being assumed to be
self-signed if they had neither AKID nor SKID, the symptoms of which show
up as an attempt to load a certificate failing with -ERANGE or -EBADMSG.
This is produced from the RSA module when the result of calculating "m =
s^e mod n" is checked.

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 |   25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
index 2a44b3752471..26e1937af7f4 100644
--- a/crypto/asymmetric_keys/x509_public_key.c
+++ b/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 */