X.509: If available, use the raw subjKeyId to form the key description

Module signing matches keys by comparing against the key description exactly.
However, the way the key description gets constructed got changed to be
composed of the subject name plus the certificate serial number instead of the
subject name and the subjectKeyId.  I changed this to avoid problems with
certificates that don't *have* a subjectKeyId.

Instead, if available, use the raw subjectKeyId to form the key description
and only use the serial number if the subjectKeyId doesn't exist.
Reported-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: David Howells <dhowells@redhat.com>

crypto/asymmetric_keys/x509_cert_parser.c

@@ -435,6 +435,8 @@ int x509_process_extension(void *context, size_t hdrlen,
 		v += 2;
 		vlen -= 2;
 
+		ctx->cert->raw_skid_size = vlen;
+		ctx->cert->raw_skid = v;
 		kid = asymmetric_key_generate_id(v, vlen,
 						 ctx->cert->raw_subject,
 						 ctx->cert->raw_subject_size);

crypto/asymmetric_keys/x509_parser.h

@@ -34,6 +34,8 @@ struct x509_certificate {
 	const void	*raw_issuer;		/* Raw issuer name in ASN.1 */
 	const void	*raw_subject;		/* Raw subject name in ASN.1 */
 	unsigned	raw_subject_size;
+	unsigned	raw_skid_size;
+	const void	*raw_skid;		/* Raw subjectKeyId in ASN.1 */
 	unsigned	index;
 	bool		seen;			/* Infinite recursion prevention */
 	bool		verified;

crypto/asymmetric_keys/x509_public_key.c

@@ -279,8 +279,13 @@ static int x509_key_preparse(struct key_preparsed_payload *prep)
 
 	/* Propose a description */
 	sulen = strlen(cert->subject);
+	if (cert->raw_skid) {
+		srlen = cert->raw_skid_size;
+		q = cert->raw_skid;
+	} else {
 		srlen = cert->raw_serial_size;
 		q = cert->raw_serial;
+	}
 	if (srlen > 1 && *q == 0) {
 		srlen--;
 		q++;