[added to the 4.1 stable tree] X.509: Fix leap year handling again

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From: David Howells <dhowells@xxxxxxxxxx>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit ac4cbedfdf55455b4c447f17f0fa027dbf02b2a6 ]

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>
cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
 crypto/asymmetric_keys/x509_cert_parser.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
index 80d3877..1995d6d 100644
--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -496,7 +496,7 @@ int x509_decode_time(time64_t *_t,  size_t hdrlen,
 		     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;
@@ -538,9 +538,9 @@ int x509_decode_time(time64_t *_t,  size_t hdrlen,
 		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;
 			}
 		}
 	}
-- 
2.5.0

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