[PATCH 4.4 082/210] X.509: Fix leap year handling again

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Howells <dhowells@xxxxxxxxxx>

commit ac4cbedfdf55455b4c447f17f0fa027dbf02b2a6 upstream.

There are still a couple of minor issues in the X.509 leap year handling:

 (1) To avoid doing a modulus-by-400 in addition to a modulus-by-100 when
     determining whether the year is a leap year or not, I divided the year
     by 100 after doing the modulus-by-100, thereby letting the compiler do
     one instruction for both, and then did a modulus-by-4.

     Unfortunately, I then passed the now-modified year value to mktime64()
     to construct a time value.

     Since this isn't a fast path and since mktime64() does a bunch of
     divisions, just condense down to "% 400".  It's also easier to read.

 (2) The default month length for any February where the year doesn't
     divide by four exactly is obtained from the month_length[] array where
     the value is 29, not 28.

     This is fixed by altering the table.

Reported-by: Rudolf Polzer <rpolzer@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Acked-by: David Woodhouse <David.Woodhouse@xxxxxxxxx>
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 crypto/asymmetric_keys/x509_cert_parser.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -494,7 +494,7 @@ int x509_decode_time(time64_t *_t,  size
 		     unsigned char tag,
 		     const unsigned char *value, size_t vlen)
 {
-	static const unsigned char month_lengths[] = { 31, 29, 31, 30, 31, 30,
+	static const unsigned char month_lengths[] = { 31, 28, 31, 30, 31, 30,
 						       31, 31, 30, 31, 30, 31 };
 	const unsigned char *p = value;
 	unsigned year, mon, day, hour, min, sec, mon_len;
@@ -540,9 +540,9 @@ int x509_decode_time(time64_t *_t,  size
 		if (year % 4 == 0) {
 			mon_len = 29;
 			if (year % 100 == 0) {
-				year /= 100;
-				if (year % 4 != 0)
-					mon_len = 28;
+				mon_len = 28;
+				if (year % 400 == 0)
+					mon_len = 29;
 			}
 		}
 	}