X.509: Fix the time validation [ver #2]

This fixes CVE-2015-5327.  It affects kernels from 4.3-rc1 onwards.

Fix the X.509 time validation to use month number-1 when looking up the
number of days in that month.  Also put the month number validation before
doing the lookup so as not to risk overrunning the array.

This can be tested by doing the following:

cat <<EOF | openssl x509 -outform DER | keyctl padd asymmetric "" @s
-----BEGIN CERTIFICATE-----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...

crypto/asymmetric_keys/x509_cert_parser.c

@@ -531,7 +531,11 @@ int x509_decode_time(time64_t *_t,  size_t hdrlen,
 	if (*p != 'Z')
 		goto unsupported_time;
 
-	mon_len = month_lengths[mon];
+	if (year < 1970 ||
+	    mon < 1 || mon > 12)
+		goto invalid_time;
+
+	mon_len = month_lengths[mon - 1];
 	if (mon == 2) {
 		if (year % 4 == 0) {
 			mon_len = 29;
@@ -543,14 +547,12 @@ int x509_decode_time(time64_t *_t,  size_t hdrlen,
 		}
 	}
 
-	if (year < 1970 ||
-	    mon < 1 || mon > 12 ||
-	    day < 1 || day > mon_len ||
+	if (day < 1 || day > mon_len ||
 	    hour > 23 ||
 	    min > 59 ||
 	    sec > 59)
 		goto invalid_time;
-	
+
 	*_t = mktime64(year, mon, day, hour, min, sec);
 	return 0;