[crypto] Allow an error margin on X.509 certificate validity periods

iPXE has no concept of the local time zone, mainly because there is no
viable way to obtain time zone information in the absence of local
state.  This causes potential problems with newly-issued certificates
and certificates that are about to expire.

Avoid such problems by allowing an error margin of around 12 hours on
certificate validity periods, similar to the error margin already
allowed for OCSP response timestamps.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/ocsp.c

 	/* Check OCSP response is valid at the specified time
 	 * (allowing for some margin of error).
 	 */
-	if ( response->this_update > ( time + OCSP_ERROR_MARGIN_TIME ) ) {
+	if ( response->this_update > ( time + X509_ERROR_MARGIN_TIME ) ) {
 		DBGC ( ocsp, "OCSP %p \"%s\" response is not yet valid (at "
 		       "time %lld)\n", ocsp, ocsp->cert->subject.name, time );
 		return -EACCES_STALE;
 	}
-	if ( response->next_update < ( time - OCSP_ERROR_MARGIN_TIME ) ) {
+	if ( response->next_update < ( time - X509_ERROR_MARGIN_TIME ) ) {
 		DBGC ( ocsp, "OCSP %p \"%s\" response is stale (at time "
 		       "%lld)\n", ocsp, ocsp->cert->subject.name, time );
 		return -EACCES_STALE;

src/crypto/x509.c

 	struct x509_validity *validity = &cert->validity;
 
 	/* Check validity period */
-	if ( time < validity->not_before.time ) {
+	if ( validity->not_before.time > ( time + X509_ERROR_MARGIN_TIME ) ) {
 		DBGC ( cert, "X509 %p \"%s\" is not yet valid (at time %lld)\n",
 		       cert, cert->subject.name, time );
 		return -EACCES_EXPIRED;
 	}
-	if ( time > validity->not_after.time ) {
+	if ( validity->not_after.time < ( time - X509_ERROR_MARGIN_TIME ) ) {
 		DBGC ( cert, "X509 %p \"%s\" has expired (at time %lld)\n",
 		       cert, cert->subject.name, time );
 		return -EACCES_EXPIRED;

src/include/ipxe/ocsp.h

 #define OCSP_STATUS_SIG_REQUIRED	0x05
 #define OCSP_STATUS_UNAUTHORIZED	0x06
 
-/** Margin of error allowed in OCSP response times
- *
- * We allow a generous margin of error: 12 hours to allow for the
- * local time zone being non-GMT, plus 30 minutes to allow for general
- * clock drift.
- */
-#define OCSP_ERROR_MARGIN_TIME ( ( 12 * 60 + 30 ) * 60 )
-
 /** An OCSP request */
 struct ocsp_request {
 	/** Request builder */

src/include/ipxe/x509.h

 	struct x509_time not_after;
 };
 
+/** Margin of error allowed in X.509 response times
+ *
+ * We allow a generous margin of error: 12 hours to allow for the
+ * local time zone being non-GMT, plus 30 minutes to allow for general
+ * clock drift.
+ */
+#define X509_ERROR_MARGIN_TIME ( ( 12 * 60 + 30 ) * 60 )
+
 /** An X.509 certificate public key */
 struct x509_public_key {
 	/** Raw public key information */